Note: Descriptions are shown in the official language in which they were submitted.
CA 02542024 1999-12-17
WESTERN.006A PATENT
ELECTRICALLY SEQUENCED TRACTOR
Back rg ound
Field of the Invention
The present invention relates to downhole drilling and, in particular, to an
electrically sequenced tractor (EST) for controlling the motion of a downhole
drilling
tool in a borehole.
Description of the Related Art
The art of drilling vertical, inclined, and horizontal boreholes plays an
important
role in many industries, such as the petroleum, mining, and communications
industries.
In the petroleum industry, for example, a typical oil well comprises a
vertical borehole
which is drilled by a rotary drill bit attached to the end of a drill string.
The drill string
is typically constructed of a series of connected links of drill pipe which
extend between
ground surface equipment and the drill bit. A drilling fluid, such as drilling
mud, is
pumped from the ground surface equipment through an interior flow channel of
the drill
string to the drill bit. The drilling fluid is used to cool and lubricate the
bit, and to
remove debris and rock chips from the borehole, which are created by the
drilling
process. The drilling fluid returns to the surface, carrying the cuttings and
debris,
through the annular space between the outer surface of the drill pipe and the
inner
surface of the borehole.
The method described above is commonly termed "rotary drilling" or
"conventional drilling." Rotary drilling often requires drilling numerous
boreholes to
recover oil, gas, and mineral deposits. For example, drilling for oil usually
includes
drilling a vertical borehole until the petroleum reservoir is reached, often
at great depth.
Oil is then pumped from the reservoir to the ground surface. Once the oil is
completely
recovered from a first reservoir, it is typically necessary to drill a new
vertical borehole
from the ground surface to recover oil from a second reservoir near the first
one. Often
a large number of vertical boreholes must be drilled within a small area to
recover oil
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from a plurality of nearby reservoirs. This requires a large investment of
time and
resources.
In order to recover oil from a plurality of nearby reservoirs without
incurring the
costs of drilling a large number of vertical boreholes from the surface, it is
desirable to
drill inclined or horizontal boreholes. In particular, it is desirable to
initially drill
vertically downward to a predetermined depth, and then to drill at an inclined
angle
therefrom to reach a desired target location. This allows oil to be recovered
from a
plurality of nearby underground locations while minimizing drilling. In
addition to oil
recovery, boreholes with a horizontal component may also be used for a variety
of other
purposes, . such as coal exploration and the construction of pipelines and
communications lines.
Two methods of drilling vertical, inclined, and horizontal boreholes are the
aforementioned rotary drilling and coiled tubing drilling. In rotary drilling,
a rigid drill
string, consisting of a series of connected segments of drill pipe, is lowered
from the
ground surface using surface equipment such as a derrick and draw works.
Attached to
the lower end of the drill string is a bottom hole assembly, which may
comprise a drill
bit, drill collars, stabilizers, sensors, and a steering device. In one mode
of use, the
upper end of the drill string is connected to a rotary table or top drive
system located at
the ground surface. The top drive system rotates the drill string, the bottom
hole
assembly, and the drill bit, allowing the rotating drill bit to penetrate into
the formation.
In a vertically drilled hole, the drill bit is forced into the formation by
the weight of the
drill string and the bottom hole assembly. The weight on the drill bit can be
varied by
controlling the amount of support provided by the derrick to the drill string.
This
allows, for example, drilling into different types of formations and
controlling the rate at
which the borehole is drilled.
The inclination of the rotary drilled borehole can be gradually altered by
using
known equipment such as a downhole motor with an adjustable bent housing to
create
inclined and horizontal boreholes. Downhole motors with bent housings allow
the
ground surface operator to change drill bit orientation, for example, with
pressure pulses
from the surface pump. Typical rates of change of inclination of the drill
string are
relatively small, approximately 3 degrees per 100 feet of borehole depth.
Hence, the
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drill string inclination can change from vertical to horizontal over a
vertical distance of
about 3000 feet. The ability of the substantially rigid drill string to turn
is often too
limited to reach desired locations within the earth. In addition, friction of
the drilling
assembly on the casing or open hole frequently limits the distance that can be
achieved
with this drilling method.
As mentioned above, another type of drilling is coiled tubing drilling. In
coiled
tubing drilling, the drill string is a non-rigid, generally compliant tube.
The tubing is
fed into the borehole by an injector assembly at the ground surface. The
coiled tubing
drill string can have specially designed drill collars located proximate the
drill bit that
apply weight to the drill bit to penetrate the formation. The drill string is
not rotated.
Instead, a downhole motor provides rotation to the bit. Because the coiled
tubing is not
rotated or not normally used to force the drill bit into the formation, the
strength and
stiffness of the coiled tubing is typically much less than that of the drill
pipe used in
comparable rotary drilling. Thus, the thickness of the coiled tubing is
generally less
than the drill pipe thickness used in rotary drilling, and the coiled tubing
generally
cannot withstand the same rotational, compression, and tension forces in
comparison to
the drill pipe used in rotary drilling.
One advantage of coiled tubing drilling over rotary drilling is the potential
for
greater flexibility of the drilling assembly, to permit sharper turns to more
easily reach
desired locations within the earth. The capability of a drilling tool to turn
from vertical
to horizontal depends upon the tool's flexibility, strength, and the load
which the tool is
carrying. At higher loads, the tool has less capability to turn, due to
friction between the
borehole and the drill string and drilling assembly. Furthermore, as the angle
of turning
increases, it becomes more difficult to deliver weight on the drill bit. At
loads of only
2000 pounds or less, existing coiled tubing tools, which are pushed through
the hole by
the gravity of weights, can turn as much as 90 per 100 feet of travel but are
typically
capable of horizontal travel of only 2500 feet or less. In comparison, at
loads up to
3000 pounds, existing rotary drilling tools, whose drill strings are thicker
and more rigid
than coiled tubing, can only turn as much as 30 -40 per 100 feet of travel
and are
typically limited to horizontal distances of 5000-6000 feet. Again, such
rotary tools are
pushed through the hole by the gravity force of weights.
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In both rotary and coiled tubing drilling, downhole tractors have been used to
apply
axial loads to the drill bit, bottom hole assembly, and drill string, and
generally to move the
entire drilling apparatus into and out of the borehole. The tractor may be
designed to be
secured between the lower end of the drill string and the upper end of the
bottom hole
assembly. The tractor may have anchors or grippers adapted to grip the
borehole wall just
proximal the drill bit. When the anchors are gripping the borehole, hydraulic
power from
the drilling fluid may be used to axially force the drill bit into the
formation. The anchors
may advantageously be slidably engaged with the tractor body, so that the
drill bit, body,
and drill string (collectively, the "drilling tool") can move axially into the
formation while
the anchors are gripping the borehole wall. The anchors serve to transmit
axial and
torsional loads from the tractor body to the borehole wall. One example of a
downhole
tractor is disclosed in U.S. Patent No. 6,003,606 to Moore ("Moore `606").
Moore `606
teaches a highly effective tractor design as compared to existing
alternatives.
It is known to have two or more sets of anchors (also referred to herein as
"grippers") on the tractor, so that the tractor can move continuously within
the borehole.
For example, Moore `606 discloses a tractor having two grippers. Longitudinal
(unless
otherwise indicated, the terms "longitudinal" and "axial" are hereinafter used
interchangeably and refer to the longitudinal axis of the tractor body) motion
is achieved
by powering the drilling tool forward with respect to a first gripper which is
actuated (a
"power stroke"), and simultaneously moving a retracted second gripper forward
with
respect to the drilling tool ("resetting"), for a subsequent power stroke. At
the completion
of the power stroke, the second gripper is actuated and the first gripper is
retracted. Then,
the drilling tool is powered forward while the second gripper is actuated, and
the retracted
first gripper is simultaneously reset for a subsequent power stroke. Thus,
each gripper is
operated in a cycle of actuation, power stroke, retraction, and reset,
resulting in
longitudinal motion of the drilling tool.
It has been proposed that the power required for actuating the anchors,
axially
thrusting the drilling tool, and axially resetting the anchors may be provided
by the
drilling fluid. For example, in the tractor disclosed by Moore `606, the
grippers
comprise inflatable engagement bladders. The Moore tractor uses hydraulic
power from
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the drilling fluid to inflate and radially expand the bladders so that they
grip the borehole walls.
Hydraulic power is also used to power forward cylindrical pistons residing
within propulsion
cylinders slidably engaged with the tractor body. Each such cylinder is
rigidly secured to a
bladder, and each piston is axially fixed with respect to the tractor body.
When a bladder is
inflated to grip the borehole, drilling fluid is directed to the proximal side
of the piston in the
cylinder that is secured to the inflated bladder, to power the piston forward
with respect to the
borehole. The forward hydraulic thrust on the piston results in forward thrust
on the entire
drilling tool. Further, hydraulic power is also used to reset each cylinder
when its associated
bladder is deflated, by directing drilling fluid to the distal side of the
piston within the cylinder.
Tractors may employ a system of pressure-responsive valves for sequencing the
distribution of hydraulic power to the tractor's anchors, thrust, and reset
sections. For example,
the Moore `606 tractor includes a number of pressure-responsive valves which
shuttle between
their various positions based upon the pressure of the drilling fluid in
various locations of the
tractor. In one configuration, a valve can be exposed on both sides to
different fluid streams.
The valve position depends on the relative pressures of the fluid streams. A
higher pressure in a
first stream exerts a greater force on the valve than a lower pressure in a
second stream, forcing
the valve to one extreme position. The valve moves to the other extreme
position when the
pressure in the second stream is greater than the pressure in the first
stream. Another type of
valve is spring-biased on one side and exposed to fluid on the other, so that
the valve will be
actuated against the spring only when the fluid pressure exceeds a threshold
value. The Moore
`606 tractor uses both of these types of pressure-responsive valves.
It has also been proposed to use solenoid-controlled valves in tractors. In
one
configuration, solenoids electrically trigger the shuttling of the valves from
one extreme
position to another. Solenoid-controlled valves are not pressure-actuated.
Instead, these
valves are controlled by electrical signals sent from an electrical control
system at the ground
surface.
Various types of radially expanding anchors have been used in downhole
tractors,
such as rigid friction blocks, flexible beams, and engagement bladders. Some
advantages
of bladders are that they are more radially expandable and thus can operate
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within certain voids in the earth. Also, bladders can conform to various
different
geometries of the borehole wall. One known bladder configuration comprises a
combination of fiber and rubber. Previous designs utilized Nylon fibers and
Nitrile
Butadiene Rubber (NBR). The fatigue life of cunrent bladder designs is such
that the
bladders are able to achieve as much as 7400 cycles of inflation.
One problem with bladders is that they do not resist torque in the tractor
body.
As the drill bit rotates into the formation, the earth transmits a reactive
torque to the bit,
which is transmitted proximally through the tractor body. When an engagement
bladder
is inflated to grip the borehole wall, the compliant bladder tends to permit
the tractor
body to twist to some degree due to the torque therein. Such rotation can
confuse tool
direction sensors, requiring an approximation of such reverse twist in the
drill direction
control algorithm.
Prior art tractors have utilized anchors which permit at least some degree of
rotation of the tractor body when the anchor is engaged with an underground
borehole
wall. A disadvantage of this configuration is that it causes the drill string
to absorb
reaction torque from the formation. When drilling, the drill bit exerts a
drilling torque
onto the formation. Simultaneously, the formation exerts an equal and opposite
torque
to the tractor body. This torque is absorbed partially by the drill string,
since the
configuration allows rotation of the tractor body when the anchor is actuated.
This
causes the drill string to twist. If all of the anchors are retracted, which
may occur when
the tool is to be retrieved, the drill string tends to untwist, which can
result in
inconsistent advance during walking.
Thus, there is a need for a downhole drilling tractor which overcomes the
above-
mentioned limitations of the prior art.
Summary of the Invention
Accordingly, it is a principle advantage of the present invention to overcome
some or all of these limitations and to provide an improved downhole drilling
tractor.
The structural configuration of the tractor, which allows it to work within
the
harsh environment and limited space within the bore of an oil well,, is,an
important
aspect of the invention. An important aspect of the invention is the
structural
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configuration that permits the tractor to fit within an envelope no more than
8.5 inches
in diameter and, preferably, no more than 2.875 inches in diameter. This
relatively
small diameter permits the tractor to work with standard oil well equipment
that is
designed for 2.875-8.5 inch diameter well bores. Another important aspect of
the
present invention is the structural configuration that permits the tractor to
make
relatively sharp turns. Specifically, the tractor desirably has a length of no
more than
150 feet, more desirably no more than 100 feet, more desirably no more than 75
feet,
more desirably no more than 50 feet, and even more desirably no more than 40
feet.
Preferably the length of the tractor is approximately 32 feet. Advantageously,
the
tractor can turn at least 60 per 100 feet of travel. Yet another important
aspect of the
invention is a structure that permits the tractor to operate at downhole
pressures up to
16,000 psi and, preferably, 5,000-10,000 psi, and downhole temperatures up to
300 F
and, preferably, 200-250 F. Preferably, the tractor can operate at
differential pressures
of 200-2500 psi, and more preferably within a range of 500-1600 psi (the
pressure
differential between the inside and outside of the EST, thus across the
internal flow
channel and the annulus surrounding the tractor).
One limitation of prior art tractors that have valves whose positions control
fluid
flow providing thrust to the tractor body is that such valves tend to operate
only at
extreme positions. These valves can be characterized as having distinct
positions in
which the valve is either on or off, open or closed, etc. As a result, these
valves fail to
provide fine-tuned control over the position, speed, thrust, and direction of
the tractor.
In another aspect, the present invention provides a tractor for moving within
a
borehole, which is capable of an exceptionally fast response to variations in
load exerted
on the tractor by the borehole or by external equipment such as a bottom hole
assembly
or drill string. The tractor comprises a tractor body sized and shaped to move
within a
borehole, a valve on the tractor body, a motor on the tractor body, and a
coupler. The
valve is positioned along a flowpath between a source of fluid and a thrust-
receiving
portion of the body. The valve comprises a fluid port and a flow restrictor.
The flow -37'
restrictor has a first position in which the restrictor completely blocks
fluid flow through
the fluid port, a range of second positions in which the restrictor permits a
first leyel of
fluid flow through the fluid port, a third position in which the restrictor
permits a second
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level of fluid flow through the fluid port. The second level of fluid flow is
greater than
the first level of fluid flow. The coupler connects the motor and the flow
restrictor, such
that movement of the motor causes the restrictor to move between the first
position, the
range of second positions, and the third position. The restrictor is movable
by the motor
such that the net thrust received by the thrust receiving portion can be
altered by 100
pounds within 0.5 seconds.
One, goal of the present invention is to provide a downhole tractor which
provides an exceptional level of control over position, speed, thrust, and
change of
direction of the tractor within a borehole, compared to prior art tractors.
Accordingly, in
one aspect the present invention provides a tractor for moving within a hole,
comprising
a tractor body having a plurality of thrust receiving portions, at least one
valve on the
tractor body, and a plurality of grippers. The valves are positioned along at
least one of
a plurality of fluid flow paths between a source of fluid and the thrust
receiving
portions. Each of the plurality of grippers is longitudinally movably engaged
with the
body and has an actuated position in which the gripper limits movement of the
gripper
relative to an inner surface of the borehole and a retracted position in which
the gripper
permits substantially free relative movement of the gripper relative to the
inner surface.
The plurality of grippers, the plurality of thrust receiving portions, and the
valves are
configured such the tractor can propel itself at a sustained rate of less than
50 feet per
hour and at a sustained rate of greater than 100 feet per hour.
In other embodiments, the tractor can propel itself at sustained rates of less
than
feet per hour and greater than 100 feet per hour, less than 10 feet per hour
and greater
than 100 feet per hour, less than 5 feet per hour and greater than 100 feet
per hour, less
than 50 feet per hour and greater than 250 feet per hour, and less than 50
feet per hour
25 and greater than 500 feet per hour. In another embodiment, the source of
fluid has a
differential pressure in the range of 200-2500 psi. In another embodiment, the
source of
fluid has a differential pressure in the range of 500-1600 psi. In another
embodiment,
the tractor can change the rate at which it propels itself without a change in
differential
pressure of the fluid. In various embodiments, the tractor has a length
preferably less
30, than 150 feet, more preferably less than 100 feet, even more preferably
less than 75 feet;
even more preferably less than 50 feet, and most preferably less than 40 feet.
In various
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embodiments, the tractor has a maximum diameter preferably less than eight
inches,
more preferably less than six inches, and even more preferably less than four
inches.
In another aspect the present invention provides a tractor comprising a
tractor
body sized and shaped to move within a borehole, and a valve on the tractor
body. The
valve is positioned along a fluid flow path between a source of fluid and a
thrust=
receiving portion of the tractor body, such as a tubular piston. The thrust-
receiving
portion is sized and configured to receive hydraulic thrust from the fluid.
The configuration of the valve facilitates improved control over the
aforementioned properties. In particular, the valve permits precise control
over the fluid
flowrate along the fluid flow path to the thrust-receiving portion. The valve
comprises a
valve body and an elongated valve spool. The valve body has an elongated spool
passage defining a spool axis, and at least a first fluid port which
communicates with the
spool passage. The fluid flow path passes through the spool passage and
through at
least the first fluid port. The valve spool is received within the spool
passage and
movable along the spool axis. The spool has a flow-restricting segment
defining a first
chamber within the spool passage on a first end of the flow-restricting
segment and a
second chamber within the spool passage on a second end of the flow-
restricting
segment. The flow-restricting segment has an outer radial surface configured
to slide
along inner walls of the spool passage so as to fluidly seal the first chamber
from the
second chamber. The flow-restricting segment also has one or more recesses on
one of
its ends and on its outer radial surface.
The spool has first, second, and third ranges of positions as follows: In the
first
range of positions, the flow-restricting segment of the spool completely
blocks fluid
flow through the first fluid port. In the second range of positions, the flow-
restricting
segment permits fluid flow through the first fluid port only through the
recesses. In the
third range of positions, the flow-restricting segment penmits fluid flow
through the first
fluid port at least partially outside of the recesses. Advantageously, the
flowrate of fluid
flowing along the fluid flow path is controllable by controlling the position
of the valve
spool within the first, second, and third ranges of positions.
In another embodiment, the valve controls the flowrates of fluid to a
plurality of
different surfaces of the thrust-receiving portion, thereby controlling the
net thrust on
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the tractor body. In yet another embodiment, the tractor body has a second
thrust-
receiving portion, and a second valve controls the flowrate of fluid flowing
thereto.
In another embodiment, the tractor comprises a tractor body, a spool valve, a
motor, a coupler, and a gripper. The tractor body has a thrust-receiving
portion having a
first surface and a second opposing surface. The first surface may be a rear
surface, and
the second surface may be a front surface. The spool valve comprises a valve
body and
an elongated spool. The valve body has a spool passage defining a spool axis,
and fluid
ports which communicate with the spool passage.
Received within the spool passage, the spool is movable along the spool axis
to
control flowrates along fluid flow paths through the fluid ports and the spool
passage.
The spool has a first position range in which the valve permits fluid flow
from a fluid
source to the first surface of the thrust-receiving portion and blocks fluid
flow to the
second surface. The flowrate of the fluid flow to the first surface varies
depending upon
the position of the spool within the first position range. The fluid flow to
the first
surface delivers thrust to the body to propel the body in a first direction in
the borehole.
The magnitude of the thrust in the first direction depends on the flowrate of
the fluid
flow (with its associated pressure) to the first surface. The spool also has a
second
position range in which the valve pennits fluid flow from the fluid source to
the second
surface of the thrust-receiving portion and blocks fluid flow to the first
surface. The
flowrate of the fluid flow to the second surface varies depending upon the
position of
the spool within the second position range. The fluid flow to the second
surface
delivers thrust to the body to propel the body in a second direction in the
borehole. The
first direction may be downhole, and the second direction may be uphole. The
magnitude of the thrust in the second direction depends on the flowrate of the
fluid flow
to the second surface.
The motor is within the tractor body. The coupler connects the motor and the
spool so that operation of the motor causes the spool to move along the spool
axis. The
gripper is longitudinally movably engaged with the tractor body. The gripper
has an
actuated position in which the gripper limits movement of the gripper relative
to an
inner surface of the borehole, and a retracted position in which the gripper
permits
substantially free relative movement of the gripper relative to the inner
surface.
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Advantageously, the motor is operable to move the spool along the spool axis
sufficiently fast to alter the net thrust received by the thrust-receiving
portion by 100
pounds within 2 seconds, and preferably within 0.1-0.2 seconds.
In one embodiment, the tractor further comprises one or more sensors and an
electronic logic component on the tractor body. The sensors are configured to
generate
electrical feedback signals which describe one or more of: fluid pressure in
the tractor,
the position of the tractor body with respect to the gripper, longitudinal
load exerted on
the tractor body by equipment external to the tractor or by inner walls of the
borehole,
and the rotational position of an output shaft of the motor. The output shaft
controls the
position of the spool along the spool axis. The logic component is configured
to receive
and process the electrical feedback signals, and to transmit electrical
command signals
to the motor. The motor is configured to be controlled by the electrical
command
signals. The conunand signals control the position of the spool.
In another aspect, the present invention provides a tractor having a valve
whose.
position controls the position, speed, and thrust of the tractor body, and in
which fluid
pressure resistance to valve motion is minimized. Accordingly, the tractor
comprises a
body and a valve, motor, coupler, and pressure compensation piston all within
the body.
The valve is positioned along a fluid flow path from a source of a first fluid
to a thrust-
receiving portion of the body. The valve is movable generally along a valve
axis. The
valve has a first position in which the valve completely blocks fluid flow
along the flow
path, and a second position in which the valve permits fluid flow along the
flow path.
The coupler connects the motor and the valve so that operation of the motor
causes the
valve to move along the valve axis. The pressure compensation piston is
exposed on a
first side to the first fluid and on a second side to a second fluid. The
first and second
fluids are fluidly separate. The compensation piston is configured to move in
response
to pressure forces from the first and second fluids so as to effectively
equalize the
pressure of the first and second fluids. The valve is exposed to the first
fluid, and the
motor is exposed to the second fluid. Advantageously, the compensation piston
acts to
minimize the net fluid pressure force acting on the valve along the valve
axis, thereby
- minimizing resistance to valve movement and permitting improved, control
over the
position, speed, thrust, and change of direction of the tractor.
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Since the tractor is electric and the motion is controlled electrically, the
present
invention permits the use of multiple tractors connected in series and
simultaneous or
non-simultaneous sequencing of the tractors' packerfeet for various functions.
In other
words, any number of the tractors can operate simultaneously as a group. Also,
some
tractors can be deactivated while others are operating. In one example, one
tractor can
be used for normal drilling with low speeds (0.25-750 feet per hour), and a
second
tractor in the drill string can be designed for high speeds (750-5000 feet per
hour) for
faster tripping into the borehole. In another example, two or more tractors
can be used
with similar perfoimance characteristics. This type of assembly would be
useful for
applications of pulling long and heavy assemblies into long or deep boreholes.
Another
example is the use of two or more tractors performing different functions.
This type of
assembly can have one tractor set up for milling and a second tractor for
drilling after
the milling job is complete, thus requiring fewer trips to the ground surface.
Any
combination of different or similar types of tractors is possible.
In another design variation, the tractor can be fonned from less expensive
materials, such as steel, resulting in decreased performance capability of the
tractor.
Such a low cost tractor can be used for specialized applications, such as
pulling
specialty oil production apparatus into the borehole and then leaving it in
the hole.
Sliding sleeve sand filter production casing can be installed in this manner.
Another goal of the present invention is to provide a downhole tractor for
drilling or moving within a borehole, which is capable of turning at
significantly high
angles while pulling or pushing a large load andlor while minimizing twisting
of the
tractor body. Accordingly, in another aspect the present invention provides a
tractor for
moving within a borehole, comprising an elongated body, a gripper, and a
propulsion
system on the body. The body is configured to push or pull equipment within
the
borehole, the equipment exerting a longitudinal load on the body. The gripper
is
longitudinally movably engaged with the body. The gripper has an actuated
position in
which the gripper limits movement between the gripper and an inner surface of
the
borehole, and a retracted position in which the gripper permits substantially
free relative
30, movement between the gripper and the inner surface. The propulsion system
is
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configured to propel the body through the borehole while the gripper is in its
actuated
position.
Advantageously, the body is sufficiently flexible such that the tractor can
preferably turn up to 30 , more preferably 45 , and even more preferably 60
per 100
feet of travel, while pushing or pulling a longitudinal load. The particular
load which
the body can push or pull while exhibiting this turning capability depends
upon the
body diameter. Various embodiments of the invention include tractors having
diameters
of 2.175 inches, 3.375 inches, 4.75 inches, and 6.0 inches. Note that other
embodiments
are also conceived. A tractor having a diameter of 2.175 inches desirably has
the above-
mentioned turning capability while pushing or pulling loads up to 1000 pounds,
and
more desirably up to 2000 pounds. The same information for other embodiments
is
summarized in the following table:
EST Diameter Load at which tractor can tum up to 30 . 45 . or 600 per 100 feet
2.175 inches Preferably 1000 pounds, and more preferably 2000 pounds
3.375 inches Preferably 5250 pounds, and more preferably 10,500 pounds
4.75 inches Preferably 13,000 pounds, and more preferably 26,000 pounds
6.0 inches Preferably 22,500 pounds, and more preferably 45,000 pounds
It should be noted that as the maximum diameter of the tractor's pistons,
shafts,
and control assembly increase so also shall the maximum thrust-pull and speed.
These
and other design considerations can be adjusted for optimum performance with
respect
to maximum and minimum speed, maximum and minimum pull-thrust, control
response
times, turning radius, and other desirable performance characteristics.
In one embodiment, the tractor has large diameter segments and small diameter
segments. The large diameter segments include one or more of (1) a
valve.housing
having valves configured to control the flow of fluid to components of the
propulsion
system, (2) a motor housing having motors configured to control the valves,
(3) an
electronics housing having logic componentry configured to control the motors,
(4) one
or more propulsion chambers configured to receive fluid to propel the body,
(5) pistons
axially movable within the propulsion chambers, and (6) the gripper. For the
tractor
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CA 02542024 1999-12-17
having a diameter of 3.375 inches, the large diameter segments have a diameter
of at
least 3.125 inches. The small diameter segments have a diameter of 2.05 inches
or less
and a modulus of elasticity of 19,000,000 or more. Substantially all of the
bending of
the tractor occurs in the small diameter segments.
In another aspect, the present invention provides a tractor for moving within
a
borehole, comprising an elongated body, at least a first gripper, and a
propulsion system
on the body: The body defines a longitudinal axis and is configured to
transmit torque
through the body. In particular, the body is configured so that when the body
is
subjected to a torque about the longitudinal axis below a certain value,
twisting of the
body is limited to no more than 5 per movement of a gripper, i.e., per on
stroke length
of a propulsion cylinder. These values vary depending upon the tractor
diameter, and
are summarized in the table below:
EST Diameter Torque below which body twists less than 5 per stroke
2.175 inches 250 ft-lbs
3.375 inches 500 ft-lbs
4.75 inches 1000 ft-lbs
6.0 inches 3000 ft-lbs
The first gripper is axially movably engaged with the body. The first gripper
has
an actuated position in which the first gripper limits movement of the first
gripper
relative to an inner surface of the borehole, and a retracted position in
which the first
gripper permits substantially free relative movement between the first gripper
and the
inner surface. The first gripper is rotationally fixed with respect to the
body so that the
first gripper resists rotation of the body with respect to the borehole when
the first
gripper is in the actuated position. A second gripper may also be provided,
which is
configured identically to the first gripper and is also axially movably
engaged with the
body. The propulsion system is configured to propel the body when at least one
of the
grippers is in its actuated position. Advantageously, the body is sufficiently
flexible
30- such that the tractor can turn up to 60 per 100 feet of longitudinal
travel.
-14-
CA 02542024 1999-12-17
Another goal of the present invention is to provide an improved gripper for a
downhole tractor used for moving within a borehole. Accordingly, in yet
another aspect
the invention provides a tractor for moving within a borehole, comprising an
elongated
body and a packerfoot configured to provide enhanced radial expansion compared
to the
prior art. The packerfoot comprises an elongated mandrel longitudinally
movably
engaged on the body, and a generally tubular bladder assembly concentrically
engaged
on the mandrel. The bladder assembly comprises a generally tubular inflatable
bladder
having a radial exterior, a first mandrel engagement member attached to a
first end of
the bladder and engaged with the mandrel, a second mandrel engagement member
,
attached to a second end of.the bladder and engaged with the mandrel, a
plurality of
longitudinally oriented flexible beams on the radial exterior of the bladder,
a first band
securing the first ends of the beams against the first mandrel engagement
member, and a
second band securing the second ends of the beams against the second mandrel
engagement member. The beams have first ends at the first end of the bladder
and
second ends at the second end of the bladder. The beams are configured to flex
and grip
onto a borehole when the bladder is inflated.
In one embodiment, the mandrel is non-rotatably engaged on the body. In
another embodiment, the first mandrel engagement member is fixed to the
mandrel, the
second mandrel engagement member is longitudinally slidably engaged with the
mandrel, and the second tube portion is non-rotatable with respect to the
mandrel. In
another embodiment, the tractor of the present invention can be fitted with
different
sizes of packerfeet, which allows the tractor to enter and operate in a range
of hole sizes.
In another aspect, the present invention provides a downhole tractor having a
"flextoe packerfoot," in which separate components provide outward radial
force for
gripping a borehole and torque transmission from the tractor body to the
borehole.
Accordingly, a tractor for moving within a borehole comprises an elongated
body, an
elongated mandrel longitudinally movably engaged with the body, and a gripper
assembly. The gripper assembly comprises one or more inflatable bladders on
the
mandrel, and one or more elongated beams. The beams have first ends fixed to
the
,. . 3,0,. mandrel on a first end of the bladder, and second ends
longitudinally movably engaged
with the mandrel on a second end of the bladder. The bladder has an inflated
position in
-15-
CA 02542024 1999-12-17
which the bladder or the beams litrtit movement of the gripper assembly
relative to an
inner surface of the borehole, and a deflated position in which the bladder or
the beams
permit substantially free relative movement between the gripper assembly
and.the inner
surface. The beams are configured to flex radially outward to grip the inner
surface of
the borehole when the bladder is in the inflated position. The beams are also
configured
to transmit torque from within the body to the inner surface of the borehole.
In one embodiment, the bladder is configured to apply a radially outward force
onto the beams when the bladder is in the inflated position, which causes the
beams to
flex outward and grip the inner surface of the borehole. In another
embodiment, the
mandrel is non-rotatably engaged with the body so that the body is prevented
from
rotating with respect to the inner surface of the borehole when the bladder is
in the
inflated position. In another embodiment, the first ends of the beams are
hingedly
secured to the mandrel, and the second ends of said beams are hingedly secured
to a
shuttle configured to slide longitudinally on the mandrel. The shuttle is non-
rotatable
with respect to the mandrel.
Another goal of the present invention is to provide a downhole tractor having
an
improved, longer-lasting inflatable bladder for gripping onto the inner
surface of a
borehole. In particular, the bladder has a higher fatigue life than prior art
bladders.
Accordingly, the present invention provides a tractor for moving within a
borehole,
comprising an elongated body defining a longitudinal axis, and an inflatable
bladder
longitudinally movably engaged with the body. The bladder is formed from an
elastomeric material reinforced by fibers oriented in two general directions
crossing one
another at an angle of between 0 and 90 woven together, more preferably
between 14
and 60 , and even more preferably between approximately 30 and 40 . The
bladder has
an inflated position in which the bladder limits movement of the bladder
relative to an
inner surface of the borehole, and a deflated position in which the bladder
permits
substantially free relative movement between the bladder and the inner
surface.
The above-described embodiments of the invention, which utilize the drilling
fluid to provide power for the tool, have specific design considerations to
optimize tool
operational life. Experiments have shown that drilling fluids can rapidly
erode many
metals, including Stabaloy and Copper-Beryllium if drilling fluid velocities
within the
-16-
CA 02542024 2009-03-09
tool are exceeded. It is another aspect of this invention to limit fluid
velocities on
straight sections within the tool to less than 35 feet per second, unless high
abrasion
resistant materials are used or other geometrical flow path considerations are
used. It
is known that at higher velocities erosion occurs within the tool, which
limits the
operational life of tractor components. Operational life is significant in
that downhole
failures and tool retrievals are extremely costly.
For purposes of summarizing the invention and the advantages achieved over
the prior art, certain objects and advantages of the invention have been
described
herein above. Of course, it is to be understood that not necessarily all such
objects or
advantages may be achieved in accordance with any particular embodiment of the
invention. Thus, for example, those skilled in the art will recognize that the
invention
may be embodied or carried out in a manner that achieves or optimizes one
advantage
or group of advantages as taught herein without necessarily achieving other
objects or
advantages as may be taught or suggested herein.
In an aspect of the present invention there is provided a tractor for moving
within a passage, comprising: an elongate tractor body configured to pull
equipment
within the passage; a first gripper assembly having a first mode in which the
first
gripper assembly limits movement of the first gripper assembly with respect to
an
inner surface of the passage and a second mode in which the first gripper
assembly
permits substantially free relative movement between the first gripper
assembly and
the passage, the first gripper assembly comprising: at least one gripper
defining a
gripping surface, said gripper having a first end, a second end, a first
connection
location, and a second connection location, said at least one gripper
supported by the
tractor body at the first connection location and the second connection
location; and
an actuator operatively coupled to the gripper, the actuator movable between a
first
position in which the first gripper assembly is in the first mode and a second
position
in which the first gripper assembly is in the second mode,wherein application
of an
expansion force by the actuator to the gripper between the first connection
location
and the second connection location causes the first gripper assembly to assume
the
first mode.
In a further aspect of the present invention there is provided a tractor for
moving within a passage, comprising: an elongate tractor body; one or more
gripper
assemblies, comprising a first gripper assembly having a first mode in which
the first
-17-
CA 02542024 2009-03-09
gripper assembly is in contact with an inner surface of the passage and a
second
mode, the first gripper assembly comprising: at least one gripper defining a
gripping
surface, said gripper having a first end and a second end, said at least one
gripper
supported by the tractor body, wherein the at least one gripper defines an
elongate
beam having a length extending between the first end of the gripper and the
second
end of the gripper and said at least one gripper bows outward in said first
mode; and
an actuator operatively coupled to the gripper, the actuator movable between a
first
position in which the first gripper assembly is in the first mode and a second
position
in which the first gripper assembly is in the second mode, where in normal
operation
said one or more gripper assemblies exert sufficient force on the inner
surface of the
passage to permit the tractor to move itself longitudinally relative to the
inner surface
of the passage.
All of these embodiments are intended to be within the scope of the invention
herein disclosed. These and other embodiments of the present invention will
become
readily apparent to those skilled in the art from the following detailed
description of
the preferred embodiments having reference to the attached figures, the
invention not
being limited to any particular preferred embodiment(s) disclosed.
Brief Description of the Drawings
Figure 1 is a schematic diagram of the major components of one embodiment
of a coiled tubing drilling system of the present invention;
Figure 2 is a front perspective view of the electrically sequenced tractor of
the
present invention (EST);
Figure 3 is a rear perspective view of the control assembly of the EST;
Figures 4A-F are schematic diagrams illustrating an operational cycle of the
EST;
Figure 5 is a rear perspective view of the aft transition housing of the EST;
Figure 6 is a front perspective view of the aft transition housing of Figure
5;
-17a-
CA 02542024 1999-12-17
Figure 7 is a sectional view nf the aft transition housing, taken along line 7-
7 of
Figure 5;
Figure 8 is a rear perspective view of the electronics housing of the EST;
Figure 9 is a front perspective view of the forward end of the electronics
housing
of Figure 8;
Figure 10 is a front view of the electronics housing of Figure 8;
Figure 11 is a longitudinal sectional view of the electronics housing, taken
along
line 11-11 of Figure 8;
Figure 12 is a cross-sectional view of the electronics housing, taken along
line
12-12 of Figure 8;
Figure 13 is a rear perspective view of the pressure transducer manifold of
the
EST;
Figure 14 is a front perspective view of the pressure transducer manifold of
Figure 13;
Figure 15 is a cross-sectional view of the pressure transducer manifold, taken
along line 15-15 of Figure 13;
Figure 16 is a cross-sectional view of the pressure transducer manifold, taken
along line 16-16 of Figure 13;
Figure 17 is a rear perspective view of the motor housing of the EST;
Figure 18 is a front perspective view of the motor housing of Figure 17;
Figure 19 is a rear perspective view of the motor mount plate of the EST;
Figure 20 is a front perspective view of the motor mount plate of Figure 19;
Figure 21 is a rear perspective view of the valve housing of the EST;
Figure 22 is a front perspective view of the valve housing of Figure 21;
Figure 23 is a front view of the valve housing of Figure 21;
Figure 24 is a side view of the valve housing, showing view 24 of Figure 23;
Figure 25 is a side view of the valve housing, showing view 25 of Figure 23;
Figure 26 is a side view of the valve housing, showing view 26 of Figure 23;
Figure 27 is a side view of the valve housing, showing view 27 of Figure 23;
Figure 28 is a rear perspective view of the forward transition housing of the
EST;
-18-
CA 02542024 1999-12-17
Figure 29 is a front perspective view of the forward transition housing of
Figure
28;
Figure 30 is a cross-sectional view of the forward transition housing, taken
along
line 30-30 of Figure 28;
Figure 31 is a rear perspective view of the diffuser of the EST;
Figure 32 is a sectional view of the diffuser, taken along line 32-32 of
Figure 31;
Figure 33 is a rear perspective view of the failsafe valve spool and failsafe
valve
body of the EST;
Figure 34 is a side view of the failsafe valve spool of Figure 33;
Figure 35 is a bottom view of the failsafe valve body;
Figure 36 is a longitudinal sectional view of the failsafe valve in a closed
position;
Figure 37 is a longitudinal sectional view of the failsafe valve in an open
position;
Figure 38 is a rear perspective view of the aft propulsion valve spool and aft
propulsion valve body of the EST;
Figure 39 is a cross-sectional view of the aft propulsion valve spool, taken
along
line 39-39 of Figure 38;
Figure 40 is a longitudinal sectional view of the aft propulsion valve in a
closed
position;
Figure 41 is a longitudinal sectional view of the aft propulsion valve in a
first
open position;
Figure 42 is a longitudinal sectional view of the aft propulsion valve in a
second
open position;
Figures 43A-C are exploded longitudinal sectional views of the aft propulsion
valve, illustrating different flow-restricting positions of the valve spool;
Figure 44A is a longitudinal partially sectional view of the EST, showing the
leadscrew assembly for the aft propulsion valve;
Figure 44B is an exploded view of the leadscrew assembly of Figure 44A;
Figure 45 is a longitudinal partially sectional view of the EST, showing the
failsafe valve spring and pressure compensation piston;
-19-
CA 02542024 1999-12-17
Figure 46 is a longitudinal sectional view of the relief valve poppet and
relief
valve body of the EST;
Figure 47 is a rear perspective view of the relief valve poppet of Figure 46;
Figure 48 is a longitudinal sectional view of the EST, showing the relief
valve
assembly;
Figure 49A is a front perspective view of the aft section of the EST, shown
disassembled;
Figure 49B is an exploded view of the forward end of the aft shaft shown in
Figure 49A
Figure 50 is a side view of the aft shaft of the EST;
Figure 51 is a front view of the aft shaft of Figure 50;
Figure 52 is a rear view of the aft shaft of Figure 50;
Figure 53 is a side view of the aft shaft of Figure 50, shown rotated 180
about
its longitudinal axis;
Figure 54 is a front view of the aft shaft of Figure 53;
Figure 55 is a cross-sectional view of the aft shaft, taken along line 55-55
shown
in Figures 49 and 50;
Figure 56 is a cross-sectional view of the aft shaft, taken along line 56-56
shown
in Figures 49 and 50;
Figure 57 is a cross-sectional view of the aft shaft, taken along line 57-57
shown
in Figures 49 and 50;
Figure 58 is a cross-sectional view of the aft shaft, taken along line 58-58
shown
in Figures 49 and 50;
Figure 59 is a cross=sectional view of the aft shaft, taken along line 59-59
shown
in Figures 49 and 50;
Figure 60 is a rear perspective view of the aft packerfoot of the EST, shown
disassembled;
Figure 61 is a side view of the aft packerfoot of the EST;
Figure 62 is a longitudinal sectional view of the aft packerfoot of Figure 61;
Figure 63 is an exploded view of the aft end of the,aft packerfoo,t of Figure
62;
-20-
;,..~
CA 02542024 1999-12-17
Figure 64 is an exploded view of the forward end of the aft packerfoot of
Figure
62;
Figure 65 is a rear perspective view of an aft flextoe packerfoot of the
present
invention, shown disassembled;
Figure 66 is a rear perspective view of the mandrel of the flextoe packerfoot
of
Figure 65;
Figure 67 is a cross-sectional view of the bladder of the flextoe packerfoot
of
Figure 65;
Figure 68 is a cross-sectional view of a shaft of the EST, formed by diffusion-
bonding;
Figure 69 schematically illustrates the relationship of Figures 69A-D;
Figures 69A-D are a schematic diagram of one embodiment of the electronic
configuration of the EST;
Figure 70 is a graph illustrating the speed and load-carrying capability range
of
the EST;
Figure 71 is an exploded longitudinal sectional view of a stepped valve spool;
Figure 72 is an exploded longitudinal sectional view of a stepped tapered
valve
spool;
Figure 73A is a chord illustrating the tuming ability of the EST;
Figure 73B is a schematic view illustrating the flexing characteristics of the
aft
shaft assembly of the EST;
Figure 74 is a rear perspective view of an inflated packerfoot of the present
invention;
Figure 75 is a cross-sectional view of a packerfoot of the present invention;
Figure 76 is a side view of an inflated flextoe packerfoot of the present
invention;
Figure 77A is a front perspective view of a Wiegand wheel assembly, shown
disassembled;
Figure 77B is a front perspective view of the Wiegand wheel assembly of Figure
77A, shown assembled;
-21-
CA 02542024 1999-12-17
Figure 77C is front perspective view of a piston having a Wiegand displacement
sensor;
Figure 78 is a graph illustrating the relationship between longitudinal
displacement of a propulsion valve spool of the EST and flowrate of fluid
admitted to
the propulsion cylinder; and
Figure 79 is a perspective view of a notch of a propulsion valve spool of the
EST.
Detailed Description of the Preferred Embodiment
It must be emphasized that the following describes one configuration of the
EST. However, numerous variations are possible. These variations in structure
result in
various ranges of perforraance characteristics. Several physical constraints
require the
EST to be innovative with respect to the use of available space within the
borehole. The
physical constraints are the result of the drilling environment. First, the
maximum
diameter of the tool is restricted by the diameter of the drilled hole and the
amount and
pressure of the drilling fluid pumped through the intemal bore of the tool and
returning
to the ground surface with drill cuttings. Next, the physical length of the
tractor is
restricted by the size of surface handling equipment and rig space. The
temperature and
pressure downhole are the result of rock formation conditions. The desired
thrust
capacity of the EST is defined by the size of the drill bit, the downhole
motor thrust
capacity, and rock characteristics. The desired pull capacity of the tool is
defined by the
weight of the drill string and the bottom hole assembly in the drilling fluid
considering
the friction of the components against the borehole wall or casing wall or by
the desired
functional requirements, such as the amount of force required to move a
sliding sleeve
in a casing. The desired maximum speed is influenced by rig economics that
include
the associated costs of drilling labor, material, facilities, cost of money,
risk, and other
economic factors. The lowest desired speed is defined by the type of
operation, such as
rate of penetration in a particular formation or rate of milling casing. In
addition,
drilling convention has resulted in numerous default sizes used in drilling.
These size
constraints are generally a function of the size of drill bit available, the
size of casing
available, the size of ground surface equipment, and other parameters.
-22-
CA 02542024 1999-12-17
For example, the EST design described herein has a maximum diameter of 3.375
inches for use in a 3.75-inch hole. However, several other designs are
conceived,
including a 2.125 inch diameter tool for use in a 2.875 inch hole, a 4.75 inch
diameter
tool for use in a 6.0 inch hole, and a 6.0 inch diameter tool for use in a 8.5
inch hole.
It is believed, however, that for a given set of operating criteria, such as a
requirement that the tool operate within a 3.75 inch diameter borehole and
have a given
maximum length, that the present invention has numerous advantages over prior
art
tractors. For example, having a single tractor which can fit within a given
borehole and
which can sustain both slow speeds' for activities such as milling an d high
speeds for
1'0 activities such as tripping out of a borehole is extremely valuable, in
that it saves both
the expense of having another tractor and the time which would otherwise be
required to
change tractors.
Figure l shows an electrically sequenced tractor (EST) 100 for moving
equipment within a passage, configured in accordance with a preferred
embodiment of
the present invention. In the embodiments shown in the accompanying figures,
the
electrically sequenced tractor (EST) of the present invention may be used in
conjunction
with a coiled tubing drilling system 20 and a bottom hole assembly 32. System
20 may
include a power supply 22, tubing reel 24, tubing guide 26, tubing injector
28, and
coiled tubing 30, all of which are well known in the art. Assembly 32 may
include a
measurement while drilling (MWD) system 34, downhole motor 36, and drill bit
38, all
of which are also known in the art. The EST is configured to move within a
borehole
having an inner surface 42. An annulus 40 is defined by the space between the
EST and
the inner surface 42.
It will be appreciated that the EST may be used to move a wide variety of
tools
and equipment within a borehole. Also, the EST can be used in conjunction with
numerous types of drilling, including rotary drilling and the like.
Additionally, it will
be understood that the EST may be used in many areas including petroleum
drilling,
mineral deposit drilling, pipeline installation and maintenance,
communications, and the
like. Also, it will be understood that the apparatus and method for moving
equipment
within a passage may be used in many applications in addition to drilling. For
example,
these other applications include well completion and production work for
producing oil
-23-
CA 02542024 1999-12-17
from an oil well, pipeline work, and communications activities. It will be
appreciated
that these applications may require the use of other equipment in conjunction
with an
EST according to the present invention. Such equipment, generally referred to
as a
working unit, is dependent upon the specific application undertaken.
For example, one of ordinary skill in the art will understand that oil and gas
well
completion typically requires that the reservoir be logged using a variety of
sensors.
These sensors may operate using resistivity, radioactivity, acoustics, and the
like. Other
logging activities include measurement of formation dip and borehole geometry,
formation sampling, and productiou logging. These completion activities can be
accomplished in inclined and horizontal boreholes using a preferred embodiment
of the
EST. For instance, the EST can deliver these various types of logging sensors
to
regions of interest. The EST can either place the sensors in the desired
location, or the
EST may idle in a stationary position to allow the measurements to be taken at
the
desired locations. The EST can also be used to retrieve the sensors from the
well.
Examples of production work that can be performed with a preferred
embodiment of the EST include sands and solids washing and acidizing. It is
known
that wells sometimes become clogged with sand, hydrocarbon debris, and other
solids
that prevent the free flow of oil through the borehole 42. To remove this
debris,
specially designed washing tools known in the industry are delivered to. the
region, and
fluid is injected to wash the region. The fluid and debris then return to the
surface.
Such tools include acid washing tools. These washing tools can be delivered to
the
region of interest for performance of washing activity and then returned to
the ground
surface by a preferred embodiment of the EST.
In another example; a preferred embodiment of the EST can be used to retrieve
objects, such as damaged equipment and debris, from the borehole. For example,
equipment may become separated from the drill string, or objects may fall into
the
borehole. These objects must be retrieved, or the borehole must be abandoned
and
plugged. Because abandonment and plugging of a borehole is very expensive,
retrieval
of the object is usually attempted. A variety of retrieval tools known to the
industry are
.30 ,, available to capture these lost objects. The EST can be used to
transport retrieving tools
-24-
CA 02542024 1999-12-17
to the appropriate location, retrieve the object, and return the retrieved
object to the
surface.
In yet another example, a preferred embodiment of the EST can also be used for
coiled tubing completions. As known in the art, continuous-completion drill
string
deployment is becoming increasingly important in areas where it is undesirable
to
damage sensitive formations in order to run production tubing. These
operations
require the installation and retrieval of fully assembled completion drill
string in
boreholes with surface pressure. The EST can be used in conjunction with the
deployment of conventional velocity string and simple primary production
tubing
installations. The EST can also be used with the deployment of artificial lift
devices
such as gas lift and downhole flow control devices.
In a further example, a preferred embodiment of the EST can be used to service
plugged 'pipelines or other similar passages. Frequently, pipelines are
difficult to
service due to physical constraints such as location in deep water or
proximity to
metropolitan areas. Various types of cleaning devices are currently available
for
cleaning pipelines. These various types of cleaning tools can be attached to
the EST so
that the cleaning tools can be moved within the pipeline.
In still another example, a preferred embodiment of the EST can be used to
move communication lines or equipment within a passage. Frequently, it is
desirable to
run or move various types of cables or conununication lines through various
types of
conduits. The EST can move these cables to the desired location within a
passage.
Overview of EST Components
Figure 2 shows a preferred embodiment of an electrically sequenced tractor
(EST) of the present invention. The EST 100 comprises a central control
assembly 102,
an uphole or aft packerfoot 104, a downhole or forward packerfoot 106, aft
propulsion
cylinders 108 and 110, forward propulsion cylinders 112 and 114, a drill
string
connector 116, shaf3s 118 and 124, flexible connectors 120, 122, 126, and 128,
and a
bottom hole assembly connector 129. Drill string connector 116 connects a
drill string,
such as coiled tubing, to shaft 118. Aft packerfoot 104, aft propulsion
cylinders 108 and
110, and connectors 120 and 122 are assembled together end to end and are all
axially
-25-
CA 02542024 1999-12-17
slidably engaged with shaft 118. Similarly, forward packerfoot 106, forward
propulsion
cylinders 112 and 114, and connectors 126 and 128 are assembled together end
to end
and are slidably engaged with shaft 124. Connector 129 provides a connection
between
EST 100 and downhole equipment such as a bottom hole assembly. Shafts 118 and
124
and control assembly 102 are axially fixed with respect to one another and are
sometimes referred to herein as the body of the EST. The body of the EST is
thus
axially fixed with respect to the drill string and the bottom hole assembly.
EST Schematic Confip-uration and Operation
Figures 4A-4F schematically illustrate a preferred configuration and operation
of
the EST. Aft propulsion cylinders 108 and 110 are axially slidably engaged
with shaft
118 and form annular chambers surrounding the shaft. Annular pistons 140 and
142
reside within the annular chambers formed by cylinders 108 and 110,
respectively, and
are axially fixed to shaft 118. Piston 140 fluidly divides the annular chamber
formed by
cylinder 108 into a rear chamber 166 and a front chamber 168. Such rear and
front
chambers are fluidly sealed to substantially prevent fluid flow between the
chambers or
leakage to annulus 40. Similarly, piston 142 fluidly divides the annular
chamber
formed by cylinder I 10 into a rear chamber 170 and a front chamber 172.
The forward propulsion cylinders 112 and 114 are configured similarly to the
aft
propulsion cylinders. Cylinders 112 and 114 are axially slidably engaged with
shaft
124. Annular pistons 144 and 146 are axially fixed to shaft 124 and are
enclosed within
cylinders 112 and 114, respectively. Piston 144 fluidly divides the chamber
formed by
cylinder 112 into a rear chamber 174 and a front chamber 176. Piston 146
fluidly
divides the chamber formed by cylinder 114 into a rear chamber 178 and a front
chamber 180. Chambers 166, 168, 170, 172, 174, 176, 178, and 180 have varying
volumes, depending upon the positions of pistons 140, 142, 144, and 146
therein.
Although two aft propulsion cylinders and two forward propulsion cylinders
(along with two corresponding aft pistons and forward pistons) are shown in
the
illustrated embodiment, any number of aft cylinders and forward cylinders may
be
. 30. provided, which includes only a single aft cylinder and a single forward
cylinder. As
described below, the hydraulic thrust provided by the EST increases as the
number of
-26-
CA 02542024 1999-12-17
propulsion cylinders increases. In other words, the hydraulic force provided
by the
cylinders is additive. Four propulsion cylinders are used to provide the
desired thrust of
approximately 10,500 pounds for a tractor with a maximum outside diameter of
3.375
inches. It is believed that a configuration having four propulsion cylinders
is preferable,
because it permits relatively high thrust to be generated, while limiting the
length of the .
tractor. Alternatively, fewer cylinders can be used, which will decrease the
resulting
maximum tractor pull-thrust. Alternatively, more cylinders can be used, which
will
increase the maximum output force from the tractor. The number of cylinders is
selected to provide sufficient force tq provide sufficient force for the
anticipated loads
for a given hole size.
The EST is hydraulically powered by a fluid such as drilling mud or hydraulic
fluid. Unless otherwise indicated, the tenns "fluid" and "drilling fluid" are
used
interchangeably hereinafter. In a preferred embodiment, the EST is powered by
the
same fluid which lubricates and cools the drill bit. Preferably, drilling mud
is used in an
open system. This avoids the need to provide additional fluid channels in the
tool for
the fluid powering the EST. Alternatively, hydraulic fluid may be used in a
closed
system, if desired. Referring to Figure 1, in operation, drilling fluid flows
from the drill
string 30 through EST 100 and down to drill bit 38. Referring again to Figures
4A-F,
diffuser 148 in control assembly 102 diverts a portion of the drilling fluid
to power the
EST. Preferably, diffuser 148 filters out larger fluid particles which can
damage
internal components of the control assembly, such as the valves.
Fluid exiting diffuser 148 enters a spring-biased failsafe valve 150. Failsafe
valve 150 serves as an entrance point to a central galley 155 (illustrated as
a flow path in
Figures 4A-F) in the control assembly which communicates with a relief valve
152,
packerfoot valve 154, and propulsion valves 156 and 158. When the differential
pressure (unless otherwise indicated, hereinafter "differential pressure" or
"pressure" at
a particular location refers to the difference in pressure at that location
from the pressure
in annulus 40) of the drilling fluid approaching failsafe valve 150 is below a
threshold
value, failsafe valve 150 remains in an off position, in which fluid within
the central
= galley vents out to exhaust line E, i.e., to annulus 40. When the
differential pressure
rises above the threshold value, the spring force is overcome and failsafe
valve 150
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CA 02542024 1999-12-17
opens to permit drilling fluid to enter central galley 155. Failsafe valve 150
prevents
premature starting of the EST and provides a fail-safe means to shut down the
EST by
pressure reduction of the drilling fluid in the coiled tubing drill string.
Thus, valve 150
operates as a system on/off valve. The threshold value for opening failsafe
valve 150,
i.e., for tuming the system on, is controlled by the stiffness of spring 151
and can be any
value within the expected operational drilling pressure range of the tool. A
preferred
threshold pressure is about 500 psig.
Drilling fluid within central galley 155 is exposed to all of the valves of
EST
100. A spring-biased relief valve 152 protects over-pressurization of the
fluid within
the tool. Relief valve 152 operates similarly to failsafe valve 150. When the
fluid
pressure in central galley 155 is below a threshold value, the valve remains
closed.
When the fluid pressure exceeds the threshold, the spring force of spring 153
is
overcome and relief valve 152 opens to permit fluid in galley 155 to vent out
to annulus
40. Relief valve 152 protects pressure-sensitive components of the EST, such
as the
bladders of packerfeet 104 and 106, which can rupture at high pressure. In the
illustrated embodiment, relief valve 152 has a threshold pressure of about
1600 psig.
Packerfoot valve 154 controls the inflation and deflation of packerfeet 104
and
106. Packerfoot valve 154 has three positions. In a first extreme position
(shown in
Figure 4A), fluid from central galley 155 is permitted to flow through passage
210 into
aft packerfoot 104, and fluid from forward packerfoot 106 is exhausted through
passage
260 to annulus 40. When valve 154 is in this position aft packerfoot 104 tends
to inflate
and forward packerfoot 106 tends to deflate. In a second extreme position
(Figure 4D),
fluid from the central galley is permitted to flow through passage 260 into
forward
packerfoot 106, and fluid from aft packerfoot 104 is exhausted through passage
210 to
annulus 40. When valve 154 is in this position aft packerfoot 104 tends to
deflate and
forward packerfoot 106 tends to inflate. A central third position of valve 154
permits
restricted flow from galley 155 to both packerfeet. In this position, both
packerfeet can
be inflated for a double-thrust stroke, described below.
In normal operation, the aft and forward packerfeet are alternately actuated.
As
aft packerfoot 104 is inflated, forward packerfoot 106 is deflated, and vice-
versa. The
position of packerfoot valve 154 is controlled by a packerfoot motor 160. In a
preferted
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CA 02542024 1999-12-17
embodiment, motor 160 is electrically controllable and can be operated by a
progrannnable logic component on EST 100, such as in electronics housing 130
(Figures 8-12), to sequence the inflation and deflation of the packerfeet.
Although the
illustrated embodiment utilizes a single packerfoot valve controlling both
packerfeet,
two valves could be provided such that each valve controls one of the
packerfeet. An
advantage of a single packerfoot valve is that it requires less space than two
valves. An
advantage of the two-valve configuration is that each packerfoot can be
independently
controlled. Also, the packerfeet can be more quickly simultaneously inflated
for a
double thrust stroke. I
Propulsion valve 156 controls the flow of fluid to and from the aft propulsion
cylinders 108 and 110. In one extreme position (shown in Figure 4B), valve 156
permits fluid from central galley 155 to flow through passage 206 to rear
chambers 166
and 170. When valve 156 is in this position, rear chambers 166 and 170 are
connected
to the drilling fluid, which is at a higher pressure than the rear chambers.
This causes
pistons 140 and 142 to move toward the downhole ends of the cylinders due to
the
volume of incoming fluid. Simultaneously, front chambers 168 and 172 reduce in
volume, and fluid is forced out of the front chambers through passage 208 and
valve
156 out to annulus 40. If packerfoot 104 is inflated to grip borehole wall 42,
the pistons
move downhole relative to wall 42. If packerfoot 104 is deflated, then
cylinders 108
and 110 move uphole relative to wall 42.
In its other extreme position (Figure 4E), valve 156 permits fluid from
central
galley 155 to flow through passage 208 to front chambers 168 and 172. When
valve
156 is in this position, front chambers 168 and 172 are connected to the
drilling fluid,
which is at a higher pressure than the front chambers. This causes pistons 140
and 142
to move toward the uphole ends of the cylinders due to the volume of incoming
fluid.
Simultaneously, rear chambers 166 and 170 reduce in volume, and fluid is
forced out of
the rear chambers through passage 206 and valve 156 out to annulus 40. In a
central
position propulsion valve 156 blocks any fluid communication between cylinders
108
and 110, galley 155, and annulus 40. If packerfoot 104 is inflated to grip
borehole wall
42, the pistons move uphole relative to wall 42. If packerfoot 104 -is
deflated, then
cylinders 108 and 110 move downhole relative to wall 42.
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CA 02542024 1999-12-17
Propulsion valve 158 is configured similarly to valve 156. Propulsion valve
158
controls the flow of fluid to and from the forward propulsion cylinders 112
and 114. In
one extreme position (Figure 4E), valve 158 permits fluid from central galley
155 to
flow through passage 234 to rear chambers 174 and 178. When valve 156 is in
this
position, rear chambers 174 and 178 are connected to the drilling fluid, which
is at a
higher pressure than the rear chambers. This causes the pistons 144 and 146 to
move
toward the 'downhole ends of the cylinders due to the volume of incoming
fluid.
Simultaneously, front chambers 176 and 180 reduce in volume, and fluid is
forced out
of the front chambers through passage 236 and valve 158 out to annulus 40. If
packerfoot 106 is inflated to'grip borehole wall 42, the pistons move downhole
relative
to wall 42. If packerfoot 106 is deflated, then cylinders 108 and 110 move
uphole
relative to wall 42.
In its other extreme position (Figure 4B), valve 158 permits fluid from
central
galley 155 to flow through passage 236 to front chambers 176 and 180 are
connected to
the drilling fluid, which is at a higher pressure than rear chambers 174 and
178. This
causes the pistons 144 and 146 to move toward the uphole ends of the cylinders
due to
the volume of incoming fluid. Simultaneously, rear chambers 174 and 178 reduce
in
volume, and fluid is forced out of the rear chambers through passage 234 and
valve 158
out to annulus 40. If packerfoot 106 is inflated to grip borehole wall 42, the
pistons
move uphole relative to wall 42. If packerfoot 106 is deflated, then cylinders
108 and
l 10 move downhole relative to wall 42. In a central position, propulsion
valve 158
blocks any fluid communication between cylinders 112 and 114, galley 155, and
annulus 40.
In a preferred embodiment, propulsion valves 156 and 158 are configured to
form a controllable variable flow restriction between cer_tral galley 155 and
the
chambers of the propulsion cylinders. The physical configuration of valves 156
and 158
is described below. To illustrate the advantages of this feature, consider
valve 156. As
valve 156 deviates slightly from its central position, it permits a limited
volume
flowrate from central galley 155 into the aft propulsion cylinders. The volume
flowrate
can be precisely increased or decreased by varying the flow restriction, i.e.,
by opening
further or closing further the valve. By carefully positioning the valve, the
volume
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CA 02542024 1999-12-17
flowrate of fluid into the aft propulsion cylinders can be controlled. The
flow-
restricting positions of the valves are indicated in Figures 4A-F by flow
lines which
intersect X's. The flow-restricting positions permit precise control over (1)
the
longitudinal hydraulic force received by the pistons; (2) the longitudinal
position of the
pistons within the aft propulsion cylinders; and (3) the rate of longitudinal
movement of
the pistons between positions. Propulsion valve 158 may be similarly
configured, to
permit the same degree of control over the forward propulsion cylinders and
pistons.
As will be shown below, controlling these attributes facilitates enhanced
control of the
thrust and speed of the EST and, hence, the drill bit.
In a preferred embodiment, the position of propulsion valve 156 is controlled
by
an aft propulsion motor 162, and the position of propulsion valve 158 is
controlled by a
forward propulsion motor 164. Preferably, these motors are electrically
controllable and
can be operated by a programmable logic component on EST 100, such as in
electronics
unit 92 (Figure 3), to precisely control the expansion and contraction of the
rear and
front chambers of the aft and forward propulsion cylinders.
,The above-described configuration of the EST permits greatly improved control
over tractor thrust, speed, and direction of travel. EST 100 can be moved
downhole
according to the cycle illustrated in Figures 4A-F. As shown in Figure 4A,
packerfoot
valve 154 is shuttled to a first extreme position, pennitting fluid to flow
from central
galley 155 to aft packerfoot 104, and also permitting fluid to be exhausted
from forward
packerfoot 106 to annulus 40. Aft packerfoot 104 inflates and grips borehole
wall 42,
anchoring aft propulsion cylinders 108 and 110. Forward packerfoot 106
deflates, so
that forward propulsion cylinders 112 and 114 are free to move axially with
respect to
borehole wall 42. Next, as shown in Figure 4B, propulsion valve 156 is moved
toward
its first extreme position, permitting fluid to flow from central galley 155
into rear
chambers 166 and 170, and also pennitting fluid to be exhausted from front
chambers
168 and 172 to annulus 40. The incoming fluid causes rear chambers 166 and 170
to
expand due to hydraulic force. Since cylinders 108 and 110 are fixed with
respect to
borehole wall 42, pistons 140 and 142 are forced downhole to the forward ends
of the
' 30' pistons, as shown in Figure 4C. Since the pistons are fixed to shaft -
118 of the EST
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CA 02542024 1999-12-17
body, the forward movement of the pistons propels the EST body downhole. This
is
known as a power stroke.
Simultaneously or independently to the power stroke of the aft pistons 140 and
142, propulsion valve 158 is moved to its second extreme position, shown in
Figure 4B. =
This permits fluid to flow from central galley 155 into front chambers 176 and
180, and
from rear chambers 174 and 178 to annulus 40. The incoming fluid causes front
chambers 176 and 180 to expand due to, hydraulic force. Accordingly, forward
propulsion cylinders 112 and 114 move downhole with respect to the pistons 144
and
146, as shown in Figure 4C. This is known as a reset stroke.
After the aft propulsion cylinders complete a power stroke and the forward
propulsion cylinders complete a reset stroke, packerfoot valve 154 is shuttled
to its
second extreme position; shown in Figure 4D. This causes forward packerfoot
106 to
inflate and grip borehole wa1142, and also causes aft packerfoot 104 to
deflate. Then,
propulsion valves 156 and 158 are reversed, as shown in Figure 4E. This causes
cylinders 112 and 114 to execute a power stroke and also causes the cylinders
108 and
110 to execute a reset stroke, shown in Figure 4F. Packerfoot valve 154 is
then shuttled
back to its first extreme position, and the cycle repeats.
Those skilled in the art will understand that EST 100 can move in reverse,
i.e.,
uphole, simply by reversing the sequencing of packerfoot valve 154 or
propulsion
valves 156 and 158. When packerfoot 104 is inflated to grip borehole wall 42,
propulsion valve 156 is positioned to deliver fluid to front chambers 168 and
172. The
incoming fluid imparts an uphole hydraulic force on pistons 140 and 142,
causing
cylinders 108 and 110 to execute an uphole power stroke. Simultaneously,
propulsion
valve 158 is positioned to deliver fluid to rear chambers 174 and 178, so that
cylinders
112 and 114 execute a reset stroke. Then, packerfoot valve 154 is moved to
inflate
packerfoot 106 and deflate packerfoot 104. Then the propulsion valves are
reversed so
that cylinders 112 and 114 execute an uphole power stroke while cylinders 108
and 110
execute a reset stroke. Then, the cycle is repeated.
Advantageously, the EST can reverse direction prior to reaching the end of any
particular power or reset stroke. The tool can be reversed simply, by
reversing the
positions of the propulsion valves so that hydraulic power is provided on the
opposite
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CA 02542024 1999-12-17
sides of the annular pistons in the propulsion cylinders. This feature
prevents damage to
the drill bit which can be caused when an obstruction is encountered in the
formation.
The provision of separate valves controlling (1) the inflation of the
packerfeet,
(2) the delivery of hydraulic power to the aft propulsion cylinders, and (3)
the delivery
of hydraulic power to the forward propulsion cylinders penmits a dual power
stroke
operation and, effectively, a doubling of axial thrust to the EST body. For
example,
packerfoot valve 154 can be moved to its central position to inflate both
packerfeet 104
and 106. Propulsion valves 156 and 158 can then be positioned to deliver fluid
to the
rear chambers of their respective propulsion cylinders. This would result in a
doubling
of downhole thrust to the EST body. Similarly, the propulsion valves can also
be
positioned to deliver fluid to the front chambers of the propulsion cylinders,
resulting in
double uphole thrust. Double thrust may be useful when penetrating harder
formations.
As mentioned above, packerfoot valve motor 160 and propulsion valve motors
162 and 164 may be controlled by an electronic control system. In one
embodiment, the
control system of the EST includes a surface computer, electric cables (fiber
optic or
wire), and a programmable logic component 224 (Figure 69) located in
electronics
housing 130. Logic component 224 may comprise electronic circuitry, a
microprocessor, EPROM and/or tool control software. The tool control software
is
preferably provided on a programmable integrated chip (PIC) on an electronic
control
board. The control system operates as follows: An operator places commands at
the
surface, such as desired EST speed, direction, thrust, etc. Surface software
converts the
operator's commands to electrical signals that are conveyed downhole through
the
electric cables to logic component 224. The electric cables are preferably
located within
the composite coiled tubing and connected to electric wires within the EST
that run to
logic component 224. The PIC converts the operator's electrical commands into
signals
which control the motors.
As part of its control algorithm, logic component 224 can also process various
feedback signals containing information regarding tool conditions. For
example, logic
component 224 can be configured to process pressure and position signals from
pressure
30, , transducers and position sensors throughout the EST, a weight on bit
(WOB) signal
from a sensor measuring the load on the drill bit, and/or a pressure signal
from a sensor
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CA 02542024 1999-12-17
measuring the pressure difference across the drill bit. In a preferred
embodiment, logic
component 224 is programmed to intelligently operate valve motors 160, 162,
and 164
to control the valve positions, based at least in part upon one or both of two
different
properties - pressure and displacement. 'From pressure information the control
system
can determine and control the thrust acting upon the EST body. From
displacement
information, the control system can determine and control the speed of the
EST. In
particular, logic component 224 can control the valve motors in response to
(1) the
differential pressure of fluid in the rear and front chambers of the
propulsion cylinders
and in the entrance to the failsafe valve, (2) the positions of the annular
pistons with
respect to the propulsion cylinders, or (3) both.
The actual command logic and software for controlling the tractor will depend
on the desired performance characteristics of the tractor and the environment
in which
the tractor is to be used. Once the performance characteristics are
determined, it is
believed that one skilled in the art can readily deterniine the desired
logical sequences
and software for the controller. It is believed that the structure and methods
disclosed
herein offer numerous advantages over the prior art, regardless of the
performance
characteristics and software selected. Accordingly, while a prototype of the
invention
uses a particular software program (developed by Halliburton Company of
Dallas,
Texas), it is believed that a wide variety of software could be used to
operate the
system.
Pressure transducers 182, 184, 186, 188, and 190 may be provided on the tool
to
measure the differential fluid pressure in (1) rear chambers 166 and 170, (2)
front
chambers 168 and 172, (3) rear chambers 174 and 178, (4) front chambers 176
and 180,
and (5) in the entrance to failsafe valve 150, respectively. These pressure
transducers
send electrical signals to logic component 224, which are proportional to the
differential
fluid pressure sensed. In addition, as shown in Figures 4A-F, displacement
sensors 192
and 194 may be provided on the tool to measure the positions of the annular
pistons
with respect to the propulsion cylinders. In the illustrated embodiment,
sensor 192
measures the axial position of piston 140 with respect to cylinder 110, and
sensor 194
measures the axial position of piston 144 with respect to cylinder 112.
Sensors 192 and
.,, . . , -34-
CA 02542024 1999-12-17
194 can also be positioned on pistons 140 and 146, or additional displacement
sensors
can be provided if desired.
Rotary accelerometers or potentiometers are preferably provided to measure the
rotation of the motors. By monitoring the rotation of the motors, the
positions of the
motorized valves 154, 156, and 158 can be determined. Like the signals from
the
pressure transducers and displacement sensors, the signals from the rotary
accelerometers or potentiometers are fed back to logic component 224 for
controlling
the valve positions.
Detailed Structure of the EST
The major subassemblies of the EST are the aft section, the control assembly,
and the forward section. Referring to Figure 2, the major components of the
aft section
comprise shaft 118, aft packerfoot 104, aft propulsion cylinders 108 and 110,
connectors 120 and 122, and aft transition housing 131. The aft section
includes a
central conduit for transporting drilling fluid supply from the drill string
to control
assembly 102 and to the drill bit. The aft section also includes passages for
fluid flow
between control assembly 102 and aft packerfoot 104 and aft propulsion
cylinders 108
and 110. The aft section further includes at least one passage for wires for
transmission
of electrical signals between the ground surface, control assembly 102, and
the bottom
hole assembly. A drill string connector 116 is attached to the aft end of the
aft section,
for fluidly connecting a coiled tubing drill string to shaft 118, as known in
the art.
The forward section is structurally nearly identical to the aft section, with
the
exceptions that the components are inverted in order and the forward section
does not
include an aft transition housing. The forward section comprises shaft 124,
forward
propulsion cylinders 112 and 114, connectors 126 and 128, and forward
packerfoot 106.
The forward section includes a central conduit for transporting drilling fluid
supply to
the drill bit. The forward section also includes passages for fluid flow
between control
assembly 102 and forward packerfoot 106 and forward propulsion cylinders 112
and
114. The forward section further includes at least one passage for wires for
transmission of electrical signals between the ground surface,
control,assembly 102, and .
the bottom hole assembly. A connector 129 is attached to the forward end of
the
-35-
CA 02542024 1999-12-17
forward section, for connecting shaft 124 to downhole components such as the
bottom
hole assembly, as known in the art.
Control Assembly
Referring to Figures 2 and 3, control assembly 102 comprises an aft transition
housing 131 (Figure 2), electronics unit 92, motor unit 94, valve unit 96, and
forward
transition unit 98. Electronics unit 92 includes an electronics housing 130
which
contains electronic components, such as logic component 224, for controlling
the EST.
Motor unit 94 includes a motor housing 132 which contains motors 160, 162, and
164.
These motors control packerfoot valve 154 and propulsion valves 156 and 158,
respectively. Valve unit 96 includes a valve housing 134 containing these
valves, as
well as failsafe valve 150. Forward transition unit 98 includes a forward
transition
housing'136 which contains diffuser 148 (not shown) and relief valve 152.
The first component of control assembly 102 is aft transition unit 90. Aft
transition housing 131 is shown in Figures 5-7. Housing 131 is connected to
and is
supplied with drilling fluid from shaft 118. Housing 131 shifts the drilling
fluid supply
from the center of the tool to a side, to provide space for an electronics
package 224 in
electronics unit 92. Figure 5 shows the aft end of housing 131, and Figure 6
shows its
forward end. The aft end of housing 131 attaches to flange 366 (Figures 49A-B)
on
shaft 118. In particular, housing 131 has pentagonally arranged threaded
connection
bores 200 which align with similar bores 365 in flange 366. High strength
connection
studs or bolts are received within bores 365 and bores 200 to secure the
flange and
housing 131 together. Flange 366 has recesses 367 through which nuts can be
fastened
onto the aft ends of the connection studs, against surfaces of recesses 367.
Suitable
connection bolts are MP33 non-magnetic bolts, which are high in strength,
elongation,
and toughness. At its forward end, housing 131 is attached to electronics
housing 130
in a similar manner, which therefore need not be described in detail.
Furthermore, all of
the adjacent housings may be attached to each other and to the shafts in a
like or similar
manner, and, therefore, also need not be described in detail.
30- It will be appreciated that the components of the EST include numerous
passages for transporting drilling fluid and electrical wires through the
tool. Aft
-36-
CA 02542024 1999-12-17
transition housing 131 includes several longitudinal bores which comprise a
portion of
these passages. Lengthwise passage 202 transports the drilling fluid supply
(from the
drill string) downhole. As shown in Figure 7, passage 202 shifts from the
center axis of
the tool at the aft end of housing 131 to an offcenter position at the forward
end.
Longitudinal wire passage 204 is provided for electrical wires. A longitudinal
wire
passage 205 is provided in the forward end of housing 131, extending about
half of the
length of the housing. Passages 204 and 205 communicate through an elongated
opening 212 in housing 131. In a preferred embodiment, wires from the surface
are
separated at opening 212 and connected to a 7-pin boot 209 (Figure 69) and a
10-pin
boot 211. Boots 209 and 211 fit into passages 204 and 205, respectively, at
the forward
end of housing 131 and connect to corresponding openings in electronics
housing 132.
Passage 206 permits fluid communication between aft propulsion valve 156 and
rear
chambers 166 and 170 of aft propulsion cylinders 108 and 110. Passage 208
permits
fluid communication between valve 156 and front chambers 168 and 172 of
cylinders
108 and 110. Passage 210 permits fluid communication between packerfoot valve
154
and aft packerfoot 104.
Figures 8-12 show electronics housing 130 of electronics unit 92, which
contains an electronic logic component or package 224. Housing 130 includes
longitudinal bores for passages 202, 204, 205, 206, 208, and 210. Electronics
package
224 resides in a large diameter portion of passage 205 inside housing 130. The
above-
mentioned 10-pin boot 211 at the forward end of aft transition housing 131 is
connected
to electronics package 224. Passage 205 is preferably sealed at the aft and
forward ends
of electronics housing 130 to prevent damage to electronics package 224 caused
by
exposure to high pressure from annulus 40, which can be as high as 16,000 psi.
A
suitable seal, rated at 20,000 psi, is sold by Green Tweed, Inc., having
offices in
Houston, Texas. Preferably, housing 130 is constructed of a material which is
sufficiently heat-resistant to protect electronics package 224 from damage
which can be
caused by exposure to high downhole temperatures. A suitable material is
Stabaloy AG
17.
As shown in Figures 9 and 11, a recess 214 is provided in the forward end,of .
electronics housing 130, for receiving a pressure transducer manifold 222
(Figures 13-
-37-
CA 02542024 1999-12-17
16) which includes pressure transducers 182, 184, 186, 188, and 190 (Figure
3).
Passages 206, 208, and 210 are shifted transversely toward the central axis of
electronics housing 130 to make room for the pressure transducers. Referring
to Figure
12, transverse shift bores 216, 218, and 220 are provided to shift passages
206, 208, and
210, respectively, to their forward end positions shown in Figures 9 and 10.
Shift bores
216, 218, and 220 are plugged at the radial exterior of housing 130 to prevent
leakage of
fluid to annulus 40.
Figures 13-16 show pressure transducer manifold 222, which is configured to
house pressure transducers for measuring the differential pressure' of
drilling fluid
passing through various manifold passages. Pressure transducers 182, 184, 186,
188,
and 190 are received within transducer bores 225, 226, 228, 230, and 232,
respectively,
which extend radially inward from the outer surface of manifold 222 to
longitudinal
bores therein. Longitudinal bores for passages 205, 206, 208, and 210 extend
through
the length of manifold 222 and align with corresponding bores in electronics
housing
130. In addition, longitudinal bores extend rearward from the forward end of
manifold
222 without reaching the aft end, forming passages 234, 236, and 238. Passage
234
fluidly communicates with rear chambers 174 and 178 of forward propulsion
cylinders
112 and 114. Passage 236 fluidly communicates with front chambers 176 and 180
of
cylinders 112 and 114. Passage 238 fluidly communicates with forward
packerfoot 106.
As shown in Figures 15 and 16, transducer bores 225, 226, 228, 230, and 232
communicate with passages 206, 208, 234, 236, and 238, respectively. As will
be
described below, the pressure transducers are exposed to drilling fluid on
their inner
sides and to oil on their outer sides. The oil is maintained at the pressure
of annulus 40
via a pressure compensation piston 248 (Figure 45), in order to produce the
desired
differential pressure measurements.
Figures 17 and 18 show motor housing 132 of motor unit 94. Attached to the
forward end of electronics housing 130, housing 132 includes longitudinal
bores for
passages 202, 204, 206, 208, 210, 234, 236, and 238 which align with the
corresponding
bores in electronics housing 130 and pressure transducer manifold 222. Housing
132
also includes longitudinal bores for passages 240; 242, and 244, which
,respectively ..
house packerfoot motor 160, aft propulsion motor 162, and forward propulsion
motor
-38-
CA 02542024 1999-12-17
164. In addition, a longitudinal bore for a passage 246 houses a pressure
compensation
piston 248 on its aft end and failsafe valve spring 151 (Figure 45) on its
forward end.
The assembly and operation of the motors, valves, pressure compensation
piston, and
failsafe valve spring are described below.
A motor mount plate 250, shown in Figures 19 and 20, is secured between the
forward end of motor housing 132 and the aft end of valve housing 134. The
motors are
enclosed within leadscrew housings 318 (described below) which are secured to
mount
plate 250. Plate 250 includes bores for passages 202, 204, 206, 208, 210, 234,
236, 238,
240, 242, 244, and 246 which align vvith corresponding bores in motor housing
132 and
valve housing 134. As shown in Figure 20, on the forward side of plate 250 the
bores
for passages 240 (packerfoot motor), 242 (aft propulsion motor), and 244
(forward
propulsion motor) are countersunk to receive retaining bolts 334 (Figure 44).
Bolts 334
secure leadscrew housings 318 to the aft side of plate 250.
Figures 21-27 show valve housing 134 of valve unit 96. Attached to the forward
end of motor mount plate 250, housing 134 has longitudinal recesses 252, 254,
256, and
258 in its outer radial surface which house failsafe valve 150, packerfoot
valve 154, aft
propulsion valve 156, and forward propulsion valve 158, respectively. Housing
134 has
bores for passages 202, 204, 206, 208, 210, 234, 236, 238, 240, 242, 244, and
246,
which align with corresponding bores in motor mount plate 250. At the forward
end of
housing 134, a central longitudinal bore is provided which forms an aft
portion of galley
155. Galley 155 does not extend to the aft end of housing 134, since its
purpose is to
feed fluid from the exit of failsafe valve 150 to the other valves. In
addition, a
longitudinal bore is provided at the forward end of housing 134 for a passage
260.
Passage 260 permits fluid communication between packerfoot valve 154 and
forward
packerfoot 106.
As shown in Figures 24-27, valve housing 134 includes various transverse bores
which extend from the valve recesses to the longitudinal fluid passages, for
fluid
communication with the valves. As described below, valves 150, 154, 156, and
158 are
spool valves, each comprising a spool configured to translate inside of a
valve body.
During operation, the spools translate longitudinally within the bores in the
valve bodies
and communicate with the fluid passages to produce the behavior schematically
shown
-39-
CA 02542024 1999-12-17
in Figures 4A-F. Figure 24 shows the openings of transverse bores within
failsafe valve
recess 252 which houses failsafe valve 150. The bores form passages 262, 264,
266,
and 268 which extend inward between failsafe valve 150 and various internal
passages.
In particular, passages 262 and 266 extend inward to passage 238 (the exit of
diffuser
148), and passages 264 and 268 extend to galley 155. As will be described
below,
failsafe valve 150 distributes fluid from passage 238 to galley 155 when the
fluid
pressure in passage 238 exceeds the desired "on/off' threshold.
Figure 25 shows the openings of transverse bores within forward propulsion
valve recess 258. The bores form passages 270, 272, and 274 which extend from
forward propulsion valve 158 to passage 236, galley 155, and passage 234,
respectively.
Figure 26 shows the openings of transverse bores within aft propulsion valve
recess
256. The bores form passages 276, 278, and 280 which extend from aft
propulsion
valve 156 to passage 208, galley 155, and passage 206, respectively. Figure 27
shows
the openings of transverse bores within packerfoot valve recess 254. The bores
form
passages 282, 284, and 286 which extend from packerfoot valve 154 to passage
260,
galley 155, and passage 210, respectively. As mentioned above, propulsion
valves 156
and 158 distribute fluid from galley 155 to the rear and front chambers of aft
and
forward propulsion cylinders 108, 110, 112, and 114. Packerfoot valve 154
distributes
fluid from galley 155 to aft and forward packerfeet 104 and 106.
Figures 28-30 show forward transition housing 136 of forward transition unit
98,
which connects valve housing 134 to forward shaft 124 and houses relief valve
152 and
diffuser 148. To simplify manufacturing of the tool, aft and forward shafts
118 and 124
are preferably identical. Thus, housing 136 repositions the various passages
passing
through the tool, via transverse shift bores (Figure 30) as described above,
to align with
corresponding passages in forward shaft 124. Note that the shift bores are
plugged on
the exterior radial surface of housing 136, to prevent leakage of fluid to
annulus 40. As
seen in the figures, the aft end of housing 136 has longitudinal bores for
passages 155,
202, 204, 234, 236, 238, and 260, which align with the corresponding bores in
valve
housing 134. Supply passage 202 transitions from the lower portion of the
housing at
the aft end to the central axis of the housing at the.forward end, to align,
with a central
bore in forward shaft 124. Wire passage 204 is enlarged at the forward end of
housing
-40-
CA 02542024 1999-12-17
136, to facilitate connection with wire passages in forward shaft 124. Also,
note that
passage 238 does not extend to the forward end of housing 136. The purpose of
passage
238 is to feed fluid from the diffuser to failsafe valve 150.
Referring still to Figures 28-30, diffuser 148 (Figures 31 and 32) is received
in
passage 202, at the forward end of housing 136. Fluid passing through the
diffuser wall
enters passage 238 and flows back toward valve housing 134 and to failsafe
valve 150.
An additional passage 238A fluidly communicates with passage 238 via a
transverse
shift bore. Fluid in passage 238A exerts an uphole axial force on the failsafe
spool and
hence on spring 151 (Figure 45), to open the valve. Galley 155 extends forward
to
upper orifice 288 of housing 136, within which relief valve 152 (Figures 46-
48) is
received. The configuration and operation of diffuser 148 and the valves of
the tool are
described below.
One embodiment of diffuser 148 is shown in Figures 31 and 32. As shown,
diffuser 148 is a cylindrical tube having a flange at its forward end and
rearwardly
angled holes 290 in the tube. The majority of the drilling fluid flowing
through passage
202 of forward transition housing 136 flows through the tube of diffuser 148
down to
the bottom hole assembly. However, some of the fluid flows back uphole through
holes
.290 and into passage 238 which feeds failsafe valve 150. It is believed that
the larger
fluid particles will generally not make a reversal in direction, but will be
forced
downhole by the current. Holes 290 form an angle of approximately 135 with
the flow
of fluid, though an angle of at least 110 with the flow of fluid is believed
sufficient to
reduce blockage. Further, rear angled holes 290 are sized to restrict the flow
of larger
fluid particles to valve housing 134. Preferably, holes 290 have a diameter of
0.125
inch or less. Those skilled in the art will appreciate that a variety of
different types of
diffusers or filters may be used, giving due consideration to the goal of
preventing
larger fluid particles from entering and possibly plugging the valves. Of
course, if the
valves are configured so that pluggage is not a significant concern, or if the
fluid is
sufficiently devoid of harmful larger fluid particles, then diffuser 148 may
be omitted
from the EST.
Referring to Figures 33-37, failsafe valve 150 comprises vslvc spool, 292,
received within valve body 294. Spool 292 has segments 293 of larger diameter.
Body
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CA 02542024 1999-12-17
294 has a central bore 298 which receives spool 292, and fluid ports in its
lower wall for
fluid passages 262, 264, 266, and 268, described above. The diameter of bore
298 is
such that spool 292 can be slidably received therein, and so that segments 293
of spool
298 can slide against the inner wall of bore 298 in an effectively fluid-
sealing
relationship. Central bore 298 has a slightly enlarged diameter at the axial
positions of
passages 264 and 268. These portions are shown in the figures as regions 279.
Regions
279 allow entering fluid to move into or out of the valve with less erosion to
the valve
body or valve spool. Body 294 is sized to fit in a fluid-tight axially
slidable manner in
failsafe valve recess 252 in valve housing 134. Body 294 has angled end faces
296
which are compressed between similarly angled portions of valve housing 134
and
forward transition housing 136 which define the ends of recess 252. Such
compression
keeps body 294 tightly secured to the outer surface of valve housing 134.
Further, a
spacer, such as a flat plate, may be provided in recess 252 between the
forward end of
valve body 294 and forward transition housing 136. The spacer can be sanded to
absorb
tolerances in construction of such mating parts. In an EST having a diameter
of 3.375
inches, ports 262, 264, 266, and 268 of valve body 294 have a diameter of
preferably
0.1 inches to 0.5 inches, and more preferably of 0.2 inches to 0.25 inches. In
the same
embodiment, passage 298 preferably has a diameter of 0.4 inches to 0.5 inches.
Vent 300 of valve body 294 permits fluid to be exhausted from passage 298 to
annulus 40. The ports of valve body 294 fluidly communicate with one another
depending upon the position of spool 292. Figures 36 and 37 are longitudinal
sectional
views of failsafe valve 150. Note that ports 264 and 268 are shown in phantom
because
these ports do not lie on the central axis of body 294. Nevertheless, the
positions of
ports 264 and 268 are indicated in the figures. In a closed position, shown in
Figure 36,
spool 292 permits fluid flow from passage 268 (which communicates with galley
155)
to vent 300 (which communicates with annulus 40). In an open position, shown
in
Figure 37, spool 292 permits fluid flow from passages 264 and 268 (which
communicates with galley 155) to passages 262 and 266 (which communicates with
diffuser exit 238).
. As mentioned above, failsafe valve 150 permits fluid to flow into the galley
155
of valve unit 96. The desired volume flowrate into galley 155 depends upon the
tractor
-42-
CA 02542024 1999-12-17
size and activity to be performed, and is summarized in the table below. The
below-
listed ranges of values are the flowrates (in gallons per minute) through
valve 150 into
galley 155 for milling, drilling, tripping into an open or cased borehole, for
various EST
diameters. The flowrate into galley 155 depends upon the dimensions of the
failsafe
valve body and ports.
EST Diameter Milling Drillin Tripping
2.175 inches 0.003-1 0-6 8-100
3.375 inches 0.006-1 0-12 8-200
4.75 inches 0.06-3 0-25 8-350
6.0 inches 0.6-10 0-55 10-550
If desired, the stroke length of failsafe valve 150 may be limited to a 1/8
inch
stroke (from its closed to open positions), to minimize the burden on relief
valve 152.
The failsafe valve spool's stroke is limited by the compression of spring 151.
For an
EST having a diameter of 3.375 inches, this stroke results in a maximum volume
flowrate of approximately 12 gallons per minute from diffuser exit 238 to
galley 155,
with an average flowrate of approximately 8 gallons per minute. The volume
flowrate
capacity of failsafe valve 150 is preferably significantly more than, and
preferably
twice, that of propulsion valves 154 and 156, to assure sufficient flow to
operate the
tool.
In the illustrated embodiment, propulsion valves 156 and 158 are identical,
and
packerfoot valve 154 is structurally similar. In particular, as shown in
Figures 23-28,
the locations of the fluid ports of packerfoot valve 154 are slightly
different from those
of propulsion valves 156 and 158, due to space limitations which limit the
positioning
of the internal fluid passages of valve housing 134. However, it will be
understood that
packerfoot valve 154 operates in a substantially similar manner to those of
propulsion
valves 156 and 158. Thus, only aft propulsion valve 156 need be described in
detail
herein.
Figures 38-42 show aft propulsion valve 156, which is configured substantially
similarly to failsafe valve 150. Valve 156 is a 4-way valve comprising spool
304 and
-43-
CA 02542024 1999-12-17
valve body 306. Spool 304 has larger diameter segments 309 and smaller
diameter
segments 311. As shown in Figure 39, segments 309 include one or more notches
312
which permit a variable flow restriction between the various flow ports in
valve body
306. Valve body 306 has a configuration similar to that of failsafe valve body
294, with
the exception that body 306 has three ports in its lower wall for fluid
passages 276, 278,
and 280, described above, and two vents 308 and 310 which fluidly communicate
with
annulus 40. . A central bore 307 has a diameter configured to receive spool
304 so that
segments 309 slide along the inner wall of bore 307 in an effectively fluid-
sealing
relationship. Since the positions of the notches 312 along the circumference
of the
segments 309 may or may not be adjacent to the fluid ports in the valve body,
bore 307
preferably has a slightly enlarged diameter at the axial positions of passages
276 and
280, so that the ports can communicate with all of the notches. That is, the
inner radial
surface of the valve body 306 defining bore 307 has a larger diameter than the
other
inner radial surfaces constraining the path of movement of segments 309 of
spool 304.
These enlarged diameter portions are shown in the figures as regions 279.
Valve body
306 is sized to fit tightly in aft propulsion valve recess 256 in valve
housing 134. A
spacer may also be provided as described above in connection with failsafe
valve body
294.
Figures 40-42 are longitudinal sectional views of the aft propulsion valve
156.
Note that ports 276 and 280 are shown in phantom because these ports do not
lie on the
central axis of valve body 306. Nevertheless, the positions of ports 276 and
280 are
indicated in the figures. The ports of body 306 fluidly communicate with one
another
depending upon the axial position of spool 304. In a closed position of afft
propulsion
valve 156, shown in Figure 40, spoo1304 completely restricts fluid flow to and
from the
aft propulsion cylinders. In another position, shown in Figure 41, spool 304
permits
fluid flow from passage 278 (which communicates with galley 155) to passage
280
(which communicates with rear chambers 166 and 170 of aft propulsion cylinders
108
and 110), and from passage 276 (which communicates with front chambers 168 and
172
of cylinders 108 and 110) to vent 310 (which communicates with annulus 40). In
this
position, valve 156 supplies hydraulic power for a forward thrust stroke of
the aft
propulsion cylinders, during which fluid is supplied to rear chambers 166 and
170 and
-44-
CA 02542024 1999-12-17
exhausted from front chambers 168 and 172. In another position, shown in
Figure 42,
spool 304 permits fluid flow from passage 278 (which communicates with galley
155)
to passage 276 (which conununicates with front chambers 168 and 172), and from
passage 280 (which communicates with rear chambers 166 and 170) to vent 308
(which
communicates with annulus 40). In this position, valve 156 supplies hydraulic
power
for a reset stroke of the aft propulsion cylinders, during which fluid is
supplied to front
chambers 168 and 172 and exhausted from rear chambers 166 and 170.
It will be appreciated that the volume flowrate of drilling fluid into aft
propulsion cylinders 108 and 110 can be precisely controlled by controlling
the axial
position of valve spool 304 within valve body 306. The volume flowrate of
fluid
through any given fluid port of body 306 depends upon the extent to which a
large
diameter segment 309 of spoo1.304 blocks the port.
Figures 43A-C illustrate this concept. Figure 43A shows the spool 304 having a
position such that a segment 309 completely blocks a fluid port of body 306.
In this
position, there is no flow through the port. As spool 304 slides a certain
distance in one
direction, as shown in Figure 43B, some fluid flow is pennitted through the
port via the
notches 312. In other words, segment 309 permits fluid flow through the port
only
through the notches. This means that all of the fluid passing through the port
passes
through the regions defined by notches 312. The volume flowrate through the
port is
relatively small in this position, due to the small opening through the
notches. In
general, the flowrate depends upon the shape, dimensions, and number of the
notches
312. Notches 312 preferably have a decreasing depth and width as they extend
toward
the center of the length of the segment 309. This permits the flow
restriction, and hence
the volume flowrate, to be very finely regulated as a function of the spool's
axial
position.
In Figure 43C, spool 304 is moved further so that the fluid is free to flow
past
- segment 309 without necessarily flowing through the notches 312. In other
words,
segment 309 permits fluid flow through the port at least partially outside of
the notches.
This means that some of the fluid passing through the port does not flow
through the
30,. regions defined by notches 312. In this position the flow restriction is
significantly
decreased, resulting in a greater flowrate through the port. Thus, the valve
-45-
CA 02542024 1999-12-17
configuration of the EST permits more precise control over the fluid flowrate
to the
annular pistons in the propulsion cylinders, and hence the speed and thrust of
the tractor.
Figure 78 graphically illustrates how the fluid flowrate to either the rear or
front
chambers of the propulsion cylinders varies as a function of the axial
displacement of
the propulsion valve spool. Section A of the curve corresponds to the valve
position
shown in Figure 43B, i.e., when the fluid flows only through the notches 312.
Section
B corresponds to the valve position shown in Figure 43C, i.e., when the fluid
is free to
flow past the edge of the large diameter segment 309 of the spool. As shown,
the
flowrate gradually increases in Section A and then increases much more
substantially in
Section B. Thus, Section A is a region which corresponds to fine-tuned control
over
speed, thrust, and position of the EST.
Valve spool 304 preferably includes at least two, advantageously between two
and eight, and more preferably three, notches 312 on the edges of the large
diameter
segments 309. As shown in Figure 79, each notch 79 has an axial length L
extending
inward from the edge of the segment 309, a width W at the edge of the segment
309,
and depth D. For an EST having a diameter of 3.375 inches, L is preferably
about
0.055-0.070 inches, W is preferably about 0.115-0.150 inches, and D is
preferably about
0.058-0.070 inches. For larger sized ESTs, the notch sizes are preferably
larger, and/or
more notches are provided, so as to produce larger flowrates through the
notches. The
notch size significantly affects the ability for continuous flow of fluid into
the pistons,
and hence continuous motion of the tractor at low speeds. In fact, the notches
allow
significantly improved control over the tractor at low speeds, compared to the
prior art.
However, some drilling fluids (especially barite muds) have a tendency to stop
flowing
at low flow rates and bridge shut small channels such as those in these
valves. Greater
volume of the notches allows more mud to flow before bridging occurs, but also
results
in less control at lower speeds. As an alternative means of controlling the
tractor at very
low speeds, the spool can be opened for a specified interval, then closed and
reopened in
a "dithering" motion, producing nearly continuous low speed of the tractor.
The valve spools can also have alternative configurations. For example, the
segments 309 may have a single region of smaller diameter at theiT, axial
ends, ,to,
provide an annular flow conduit for the drilling fluid. In other embodiments,
the spools
-46-
CA 02542024 1999-12-17
can be provided with a multiplicity of steps and shapes that would allow
different
mudflow rates through the EST. For example, multiple steps 550 can be provided
as
shown in Figure 71. Altematively, multiple tapered steps 552 may provided as
shown
in Figure 72. The spool configurations shown in Figures 71 and 72 allow the
spool to
more quickly "dither" into and out of different positions. Dithering would add
surges of
pressure to the propulsion cylinders, which may provide a more responsive tool
advance, but less fine-tuned control. A stepwise formation of tapers on the
spool also
tends to prevent drilling mud from plugging gaps between the spool and valve
body.
Although the above-described spool configurations can be used to provide
different flowrate regulation capabilities, the notched configuration of
Figure 38 is
preferred. Notches 312 have a larger minimum dimension than steps or tapered
steps as
shown in Figures 71 and 72. Thus, notches 312 are less likely to become
plugged by
larger fluid particles, which could render the spool ineffective. Also, the
notches are
less affected by fluid boundary layers on the spools because the fluid
boundary layer
represents a smaller percentage of the total cross-sectional area defined by
the notches.
Of significance in the design for the spool valves is the radial clearance
between
the valve body and spool. The clearance is preferably made sufficiently large
to resist
potential plugging by large particles in the drilling fluid, but sufficiently
small to
prevent leakage which could inhibit control of the EST. This behavior is
attributable to
the tendency of some muds (especially those containing barite) to bridge or
seal small
openings. The clearance is sized within the typical operational
characteristics of most
drilling fluids. Preferably, the clearance is about 0.0006-0023 inches.
As mentioned above, the configuration of valves 154, 156, and 158 permits
precise control over the volume flowrate of fluid to propulsion cylinders 108,
110, 112,
and 114 and packerfeet 104 and 106. In the illustrated embodiment of the EST,
the
volume flowrate of fluid to the propulsion cylinders can be more precisely
controlled
and maintained at any flowrate to a minimum of preferably 0.6 gallons per
minute, more
preferably 0.06 gallons per minute, and even more preferably 0.006 gallons per
minute,
corresponding to fluid flow only through the notches 312. The ability to
control and
maintain a substantially constant volume flowrate at such small flow levels
permits the
EST to operate at slow speeds. For an EST having a diameter of 3.375 inches,
the
-47-
CA 02542024 1999-12-17
stroke length of the propulsion valve spools is preferably limited so that the
maximum
volume flowrate into the propulsion cylinders is approximately 0-9 gallons per
minute.
Preferably, the maximum stroke length from the closed position shown in Figure
40 is
0.25 inches.
As mentioned above, packerfoot valve 154 and aft and forward propulsion
valves 156 and 158 are controlled by motors. In a preferred embodiment, the
structural
configuration which permits the motors to communicate with the valves is
similar for
each motorized valve. Thus, only that of aft propulsion valve 156 is described
herein.
Figures 44A and B illustrate the structural configuration of the EST which
permits aft
propulsion motor 162 to control valve 156. This configuration transforms
torque output
from the motor into axial translation of valve spool 304. Motor 162 is
cylindrical and is
secured within a tubular leadscrew housing 318. Motor 162 and leadscrew
housing 318
reside in bore 242 of motor housing 132. The forward end of leadscrew housing
318 is
retained in abutment with motor mount plate 250 via a retaining bolt 334 which
extends
through mount plate 250 and is threadingly engaged with the internal surface
of housing
318.
Inside leadscrew housing 318, motor 162 is coupled to a leadscrew 322 via
motor coupling 320, so that torque output from the motor causes leadscrew 322
to
rotate. A bearing 324 is provided to maintain leadscrew 322 along the center
axis of
housing 318, which is aligned with aft propulsion valve spool 304 in valve
housing 134.
Leadscrew 322 is threadingly engaged with a leadscrew nut 326. A longitudinal
key
325 on leadscrew nut 326 engages a longitudinal slot 328 in leadscrew housing
318.
This restricts nut 326 from rotating with respect to leadscrew housing 318,
thereby
causing nut 326 to rotate along the threads of leadscrew 322. Thus, rotation
of
leadscrew 322 causes axial translation of nut 326 along leadscrew 322. A stem
330 is
attached to the forward end of nut 326. Stem 330 extends forward through
annular
restriction 333, which separates oil in motor housing 132 from drilling fluid
in valve
housing 134. The drilling fluid is sealed from the oil via a tee seal 332 in
restriction
333. The forward end of stem 330 is attached to valve spool 304 via a spool
bolt 336
and split retainer 338. Stem 330 is preferably relatively thin and flexible so
that it can
compensate for any misalignment between the stem and the valve spool.
-48-
CA 02542024 1999-12-17
Thus, it can be seen that torque output from the motors is converted into
axial
translation of the valve spools via leadscrew assemblies as described above.
The
displacement of the valve spools is monitored by constantly measuring the
rotation of
the motors. Preferably, rotary accelerometers or potentiometers are built into
the motor
cartridges to measure the rotation of the motors, as known in the art. The
electrical
signals from the accelerometers or potentiometers can be transmitted back to
logic
component 224 via electrical wires 536 and 538 (Figure 69).
Preferably, motors 160, 162, and 164 are stepper motors, which require fewer
wires. Advantageously, stepper motors are brushless. If, in contrast, brush-
type motors
are used, filaments from the breakdown of the metal brushes may render the oil
electrically conductive. Importantly, stepper motors can be instructed to
rotate a given
number of steps, facilitating precise control of the valves. Each motor
cartridge may
include a gearbox to generate enough torque and angular velocity to turn the
leadscrew
at the desired rate. The motor gear box assembly should be able to generate
desirably at
least 5 pounds, more desirably at least 10 pounds, and even more desirably at
least 50
pounds of force and angular velocity of at least 75-180 rpm output. The motors
are
preferably configured to rotate 12 steps for every complete revolution of the
motor
output shafts. Further, for an EST having a diameter of 3.375 inches, the
motor, gear
box, and accelerometer assembly desirably has a diameter no greater than 0.875
inches
(and preferably 0.75 inches) and a length no longer than 3.05 inches. A
suitable motor
is product no. DF7-A sold by CD Astro Intercorp , Inc. of Deerfield, Florida.
In order to optimally control the speed and thrust of the EST, it is desirable
to
know the relationships between the angular positions of the motor shafts and
the
flowrates through the valves to the propulsion cylinders. Such relationships
depend
upon the cross-sectional areas of the flow restrictions acting on the fluid
flows through
the valves, and thus upon the dimensions of the spools, valve bodies, and
fluid ports of
the valve bodies. Such relationships also depend upon the thread pitch of the
leadscrews. In a preferred embodiment, the leadscrews have about 8-32 threads
per
inch.
,, Inside motor housing 132, bores 240, 242, and 244 contain the motors as
well as
electrical wires extending rearward to electronics unit 92. For optimal
performance,
-49-
CA 02542024 1999-12-17
these bores are preferably filled with an electrically nonconductive fluid, to
reduce the
risk of ineffective electrical transmission through the wires. Also, since the
pressure of
the motor chambers is preferably equalized to the pressure of annulus 40 via a
pressure
compensation piston (as described below), such fluid preferably has a
relatively low
compressibility, to minimize the longitudinal travel of the compensation
piston. A
preferred fluid is oil, since the compressibility of oil is much less than
that of air. At the
aft end of motor housing 132, these bores are fluidly open to the space
surrounding
pressure transducer manifold 222. Thus, the outer ends of pressure transducers
182,
184, 186, 188, and 190 are also exposed to oil.
Figure 45 illustrates the assembly and operation of failsafe valve 150. The
aft
end of failsafe valve spool 292 abuts a spring guide 340 that slides inside
passage 246
within motor housing 132, motor mount plate 250, and valve housing 134. Inside
motor
housing =132 passage 246 has an annular spring stop 342 which is fixed with
respect to
housing 132. Guide 340 has an annular flange 344. Failsafe valve spring 151,
preferably a coil spring, resides within passage 246 so that its ends abut
stop 342 and
flange 344. Fluid within passage 238A (from the exit of diffuser 148) exerts
an axial
force on the forward end of spool 292, which is countered by spring 151. As
shown, a
spacer having a passage 238B may be provided to absorb tolerances between the
mating
surfaces of valve housing 134 and forward transition housing 136. Passage 238B
fluidly communicates with passage 238A and with spool passage 298 of failsafe
valve
body 294. When the fluid pressure in passage 238A exceeds a particular
threshold, the
spring force is overcome to open failsafe valve 150 as shown in Figure 37.
Spring 151
can be carefully chosen to compress at a desired threshold fluid pressure in
passage
238A.
When the EST is removed from a borehole, drilling fluid residue is likely to
remain within passage 246 of motor housing 132. As shown in Figures 17-18, a
pair of
cleaning holes 554 may be provided which extend into passage 246. Such holes
permit
passage 246 to be cleaned by spraying water through the passage, so that
spring 153
operates properly during use. During use, holes 554 may be plugged so that the
drilling
fluid does not escape to annulus 40.
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CA 02542024 1999-12-17
Referring to Figures 44A-B, the leadscrew assemblies for the motorized valves
contain drilling fluid from annulus 40. Such fluid enters the leadscrew
assemblies via
the exhaust vents in the valve bodies, and surrounds portions of the valve
spools and
stems 330 forward of annular restrictions 333. As mentioned above, the
chambers
rearward of restrictions 333 are filled with oil. In order to move the valve
spools, the
motors must produce sufficient torque to overcome (1) the pressure difference
between
the drilling fluid and the oil, and (2) the seal friction caused by tee seals
332. Since the
fluid pressure in annulus 40 can be as high as 16,000 psi, the oil pressure is
preferably
equalized with the fluid pressure in annulus 40 so that the pressure
difference across
seals 332 is zero. Absent such oil pressure compensation, the motors would
have to
work extremely hard to advance the spools against the high pressure drilling
fluid. A
significant pressure difference can cause the motors to stall. Further, if the
pressure
difference across seals 332 is sufficiently high, the seals would have to be
very tight to
prevent fluid flow across the seals. However, if the seals were very tight
they would
hinder and, probably, prevent movement of the stems 330 and hence the valve
spools.
With reference to Figure 45, a pressure compensation piston 248 is preferably
provided to avoid the above-mentioned problems. Preferably, piston 248 resides
in
passage 246 of motor housing 132. Piston 248 seals drilling fluid on its
forward end
from oil on its aft end, and is configured to slide axially within passage
246. As the
pressure in annulus 40 increases, piston 248 slides rearward to equalize the
oil pressure
with the drilling fluid pressure. Conversely, as the pressure in annulus 40
decreases,
piston 248 slides forward. Advantageously, piston 248 effectively neutralizes
the net
longitudinal fluid pressure force acting on each of the valve spools by the
drilling fluid
and oil. Piston 248 also creates a zero pressure difference across seals 332
of the
leadscrew assemblies of the valves.
Figures 46-48 illustrate the configuration and operation of relief valve 152.
Relief valve 152 comprises a valve body 348, poppet 350, and coil spring 153.
Body
348 is generally tubular and has a nose 351 and an internal valve seat 352.
Poppet 350
has a rounded end 354 configured to abut valve seat 352 to close the valve.
Poppet 350
also has a plurality of longitudinal ribs 356 between which fluid may flow out
to
annulus 40. Inside forward transition housing 136, relief valve body 348
resides within
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CA 02542024 1999-12-17
a diagonal portion 349 of galley 155 which extends to orifice 288 and out to
annulus 40.
Body 348 is tightly and securely received within the aft end of diagonal bore
349. A
tube 351 resides forward of body 348. Tube 351 houses relief valve spring 153.
Poppet
350 is slidably received within body 348. The forward end of poppet 350 abuts
the aft
end of spring 153. The forward end of spring 153 is held by an intemal annular
flange
of tube 351. In operation, the drilling fluid inside galley 155 exerts a force
on rounded
end 354 of poppet 350, which is countered by spring 153. As the fluid pressure
rises,
the force on end 354 also rises. If the fluid pressure in galley 155 exceeds a
threshold
pressure, the spring force is overcome, forcing end 354 to unseat from valve
seat 352.
This permits fluid from galley 155 to exhaust out to annulus 40 through bore
349 and
between the ribs 356 of poppet 350.
In a preferred embodiment, control assembly 102 is substantially cylindrical
with a diameter of about 3.375 inches and a length.of about 46.7 inches.
Housings 130,
131, 132, 134, and 136 are preferably constructed of a high strength material,
to prevent
erosion caused by exposure to harsh drilling fluids such as calcium bromide or
cesium
formate muds. In general, the severity and rate of erosion depends on the
velocity of the
drilling fluid to which the material is exposed, the solid material within the
fluid, and
the angle at which the fluid strikes a surface. In operation, the control
assembly
housings are exposed to drilling mud velocities of 0 to 55 feet per second,
with typical
mean operating speeds of less than 30 feet per second (except within the
valves). Under
these conditions, a suitable material for the control assembly housings is
Stabaloy,
particularly Stabaloy AG 17. In the valves, mud flow velocities can be as high
as 150
feet per second. Thus, the valves and valve bodies are preferably formed from
an even
more erosion-resistant material, such as tungsten carbide, Ferro-Tec (a
proprietary steel
formed of titanium carbide and available from Alloy Technologies
International, Inc. of
West Nyack, New York), or similar materials. The housings and valves may be
constructed from other materials, giving due consideration to the goal of
resisting
erosion.
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CA 02542024 1999-12-17
Shaft Assemblies
In a preferred embodiment, the aft and forward shaft assemblies are
structurally
similar. Thus, only the aft shaft assembly is herein described in detail.
Figure 49 shows
the configuration of the aft shaft assembly. Aft packerfoot 104, flexible
connector 120,
cylinder 108, flexible connector 122, and cylinder 110 are connected together
end to end
and are collectively slidably engaged on aft shaft 118. Annular pistons 140
and 142 are
attached to shaft 118 via bolts secured into bolt holes 360 and 362,
respectively. 0-
rings or specialized elastomeric seals may be provided between the pistons and
the shaft
to prevent flow of fluid under the pistons. Cylinders 108 and 110 enclose
pistons 140
and 142, respectively. The forward and aft ends of each propulsion cylinder
are sealed,
via tee-seals, 0-rings, or otherwise, to prevent the escape of fluid from
within the
cylinders to annulus 40. Also, seals are provided between the outer surface of
the
pistons 140 and 142 and the inner surface of the cylinders 108 and 110 to
prevent fluid
from flowing between the front and rear chambers of the cylinders.
Connectors 120 and 122 may be attached to packerfoot 104 and cylinders 108
and 110 via threaded engagement, to provide high-pressure integrity and avoid
using a
multiplicity of bolts or screws. Tapers may be provided on the leading edges
of
connectors 120 and 122 and seal cap 123 attached to the forward end of
cylinder 110.
Such tapers help prevent the assembly from getting caught against sharp
surfaces such
as milled casing passages.
A plurality of elongated rotation restraints 364 are preferably attached onto
shaft
118, which prevent packerfoot 104 from rotating with respect to the shaft.
Restraints
364 are preferably equally spaced about the circumference of shaft 118, and
can be
attached via bolts as shown. Preferably four restraints 364 are provided.
Packerfoot
104 is configured to engage the restraints 364 so as to prevent rotation of
the packerfoot
with respect to the shaft, as described in greater detail below.
Figures 50-59 illustrate in greater detail the configuration of shaft 118. At
its
forward end, shaft 118 has a flange 366 which is curved for more even stress
distribution. Flange 366 includes bores for fluid passages 202, 206, 208, and
210,
which align with corresponding bores in aft transition housing 131. Note that
the sizes
of these passages may be varied to provide different flowrate and speed
capacities of the
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CA 02542024 1999-12-17
EST. In addition, a pair of wire passages 204A is provided, one or both of the
passages
aligning with wire bore 204 of housing 131. Electrical wires 502, 504, 506,
and 508
(Figure 69), which run up to the surface and, in one embodiment, to a position
sensor on
piston 142, reside in passages 204A. As shown in Figure 52, only wire passages
204A
and supply passage 202 extend to the aft end of shaft 118.
As shown in Figure 55, within shaft 118 fluid passages 206, 208, and 210 each
comprise a pair of passages 206A, 208A, and 210A, respectively. Preferably,
the
passages split into pairs inside of flange 366. In the illustrated embodiment,
pairs of
gun-drilled passages are provided instead of single larger passages because
larger
diameter passages could jeopardize the structural integrity of the shaft. With
reference
to Figure 53, passages 206A deliver fluid to rear chambers 166 and 170 of
propulsion
cylinders 108 and 110 via fluid ports 368 and 370, respectively. Figure 58
shows ports
370 which communicate with rear chamber 170 of cylinder 110. These ports are
transverse to the longitudinal axis of shaft 118. Ports 368 are configured
similarly to
ports 370. With reference to Figure 50, passages 208A deliver fluid to front
chambers
168 and 172 of cylinders 108 and 110 via fluid ports 372 and 374,
respectively. Ports
374 are shown in Figure 56. Ports 372 are configured similarly to ports 374.
Passages
206A and 208A are provided for the purpose of delivering fluid to the
propulsion
cylinders. Hence, passages 206A and 208A do not extend rearwardly beyond
longitudinal position 380.
With reference to Figure 53, passages 210A deliver fluid to aft packerfoot
104,
via a plurality of fluid ports 378. Ports 378 are preferably arranged linearly
along shaft
118 to provide fluid throughout the interior space of packerfoot 104. In the
preferred
embodiment, nine ports 378 are provided. Figure 59 shows one of the ports 378,
which
fluidly communicates with each of passages 210A. Since passages 210A are
provided
for the purpose of delivering fluid to aft packerfoot 104, such passages do
not extend
rearwardly beyond longitudinal position 382.
With reference to Figure 50, a wire port 376 is provided in shaft 118. Port
376
pennits electrical communication between control assembly 102 and position
sensor
.
192 (Figures 4A-F) on piston 142. For example, a Wiegand sensor~ or
magnetometer,
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CA 02542024 1999-12-17
device (described below) may be located on piston 142. Port 376 is also shown
in
Figure 57.
In a preferred embodiment, some of the components of the EST are formed from
a flexible material, so that the overall flexibility of the tool is increased.
Also, the
components of the tool are preferably non-magnetic, since magnetic materials
can
interfere with the performance of magnetic displacement sensors. Of course, if
magnetic displacement sensors are not used, then magnetic materials are not
problematic. A preferred material is copper-beryllium (CuBe) or CuBe alloy,
which has
trace amounts of nickel and iron. This material is non-magnetic and has high
strength
and a low tensile modulus. With reference to Figure 2, shafts 118 and 124,
propulsion
cylinders 109, 110, 112, and 114, and connectors 120, 122, 126, and 128 may be
formed
from CuBe. Pistons 140 and 142 may also be fonned from CuBe or CuBe alloy. The
cylinders are preferably chrome-plated for maximum life of the seals therein.
In a preferred embodiment, each shaft is about 12 feet long, and the total
length
of the EST is about 32 feet. Preferably, the propulsion cylinders are about
25.7 inches
long and 3.13 inches in diameter. Connectors 120, 122, 126, and 128 are
preferably
smaller in diameter than the propulsion cylinders and packerfeet at their
center. The
connectors desirably have a diameter of no more than 2.75 inches and,
preferably, no
more than 2.05 inches. This results in regions of the EST that are more
flexible than the
propulsion cylinders and control assembly 102. Consequently, most of the
flexing of
the EST occurs within the connectors and shafts. In one embodiment, the EST
can turn
up to 60 per 100 feet of drilled arc. Figure 73A shows an arc curved to
schematically
illustrate the turning capability of the tool. Figure 73B schematically shows
the flexing
of the aft shaft assembly of the EST. The degree of flexing is somewhat
exaggerated for
clarity. As shown, the flexing is concentrated in aft shaft 118 and connectors
120 and
122.
Shafts 118 and 124 can be constructed according to several different methods.
One method is diffusion bonding, wherein each shaft comprises an inner shaft
and an
outer shaft, as shown in Figure 68. Inner shaft 480 includes a central bore
for fluid
. 30 , supply passage 202, and ribs 484 along its length. The outer diameter
of inner shaft 480
at the ribs 484 is equal to the inner diameter of outer shaft 482, so that
inner shaft 480
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CA 02542024 1999-12-17
fits tightly into outer shaft 482. Substantially the entire outer surface of
ribs 484 mates
with the inner surface of shaft 482. Longitudinal passages are formed between
the
shafts. In aft shaft 118, these are passages 204 (wires), 206 (fluid to rear
chambers of
aft propulsion cylinders), 208 (fluid to front chambers of aft propulsion
cylinders), and
210 (fluid to aft packerfoot).
The inner and outer shafts 480 and 482 may be formed by a co-extrusion
process. Shafts 480 and 482 are preferably made from CuBe alloy and annealed
with a
"drill string" temper process (annealing temper and thermal aging) that
provides
excellent mechanical properties (tensile modulus of 110,000 - 130,000 psi, and
elongation of 8-10% at room temperature). The inner and outer shafts are then
diffusion
bonded together. Accordingly, the shafts are coated with silver, and the inner
shaft is
placed inside the outer shaft. The assembly is internally pressurized,
externally
constrained, and heated to approximately 1500 F. The CuBe shafls expand under
heat
to form a tight fit. Heat also causes the silver to diffuse into the CuBe
material, fonning
the diffusion bond. Experiments on short pieces of diffusion-bonded shafts
have
demonstrated pressure integrity within the several passages. Also, experiments
with
short pieces have demonstrated diffusion bond shear strengths of 42,000 to
49,000 psi.
After the shafts are bonded together, the assembly is electrolitically chrome-
plated to increase the life of the seals on the shaft. Special care`is made to
minimize the
thickness of the chrome to allow both long life and shaft flexibility. The use
of
diffusion bonding permits the unique geometry shown in Figure 68, which
maximizes
fluid flow channel area and simultaneously maximizes the torsional rigidity of
the shaft.
In a similar diffusion bonding process, the flange portion 366 (Figures 49A-B)
can be
bonded to the end of the shaft.
Alternatively, other materials and constructions can be used. For example,
Monel or titanium alloys can be used with appropriate welding methods. Monel
is an
acceptable material because of its non-magnetic characteristics. However,
Monel's high
modulus of elasticity or Young's Modul~us tends to restrict turning radius of
the tractor
to less than 40 per 100 feet of drilled arc. Titanium is an acceptable
material because
of its non-magnetic characteristics, such,as high, tensile strength and low
Young's
, , .
-56-
CA 02542024 1999-12-17
modulus (compared to steel). However, titanium welds are known to have
relatively
short fatigue life when subjected to drilling environments.
In another method of constructing shafts 118 and 124, the longitudinal wire
and
fluid passages are formed by "gun-drilling," a well-known process used for
drilling long
holes. Advantages of gun-drilling include moderately lower torsional and
bending
stiffness than the diffusion-bonded embodiment, and lower cost since gun-
drilling is a
more developed art. When gun-drilling a hole, the maximum length and accuracy
of the
hole depends upon the hole diameter. The larger the hole diameter, the longer
and more
accurately the hole can be gun-drilled. However, since the shafts have a
relatively small
diameter and have numerous internal passages, too great a hole diameter may
result in
inability of the shafts to withstand operational bending and torsion loads.
Thus, in
selecting an appropriate hole diameter, the strength of the shaft must be
balanced
against the ability to gun-drill long, accurate holes.
The shaft desirably has a diameter of 1-3.5 inches and a fluid supply passage
of
preferably 0.6-1.75 inches in diameter, and more preferably at least 0.99
inches in
diameter. In a preferred embodiment of the EST, the shaft diameter is 1.746-
1.748
inches, and the diameter of fluid supply passage 202 is 1 inch. For an EST
having a
diameter of 3.375 inches, the shafts are designed to survive the stresses
resulting from
the combined loads of 1000 R-lbs of torque, pulling-thrusting load up to 6500
pounds,
and bending of 60 per 100 feet of travel. Under these constraints, a suitable
configuration is shown in Figure 55, which shows aft shaft 118. Passages 204A,
206A,
208A, and 210A comprise pairs of holes substantially equally distanced between
the
inner surface of passage 202 and the outer surface of shaft 118. For each
passage, a pair
of holes is provided so that the passages have sufficient capacity to
accommodate
required operational drilling fluid flowrates. This configuration is chosen
instead of a
single larger hole, because a larger hole may undesirably weaken the shaft.
Each hole
has a diameter of 0.188 inch. The holes of each individual pair are spaced
apart by
approximately one hole diameter. For a hole diameter of 0.188 inch, it may not
be
possible to gun-drill through the entire length of each shaft 118 and 124. In
that case,
,. each shaft can be made by gun-drilling the holes into two or more shorter
shafts and
then electron beam (EB) welding them together end to end.
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CA 02542024 1999-12-17
The welded shaft is then preferably thermally annealed to have desired
physical
properties, which include a tensile modulus of approximately 19,000,000 psi,
tensile
strength of approximately 110,000 - 130,000 psi, and elongation of about 8-
12%. The
shaft can be baked at 1430 F for 1-8 hours depending upon the desired
characteristics.
Details of post-weld annealing methods are found in literature about CuBe.
After the
thermal annealing step, the welded shaft is then finished, machined, ground,
and
chrome-plated.
Packerfeet
Figures 60-64 and 74-75 show one embodiment of aft packerfoot 104. The
major components of packerfoot 104 comprise a mandrel 400, bladder assembly
404,
end clamp 414, and connector 420. Mandrel 400 is generally tubular and has
internal
grooves 402 sized and configured to slidably engage rotation restraints 364 on
aft shaft
118 (Figure 49A). Thus, mandre1400 can slide longitudinally, but cannot
rotate, with
respect to shaft 118. Bladder assembly 404 comprises generally rigid tube
portions 416
and 417 attached to each end of a substantially tubular inflatable engagement
bladder
406. Assembly 404 generally encloses mandre1400. On the aft end of packerfoot
104,
assembly 404 is secured to mandrel 400 via eight bolts 408 received within
bolt holes
410 and 412 in assembly 404 and mandrel 400, respectively. An end clamp 414 is
used
as armor to protect the leading edge of the bladder 406 and is secured via
bolts onto end
417 of assembly 404. If desired, an additional end clamp can be secured onto
end 416
of assembly 404 as well. Connector 420 is secured to mandrel 400 via eight
bolts 422
received within bolt holes 424 and 426. Connector 420 provides a connection
between
packerfoot 104 and flexible, connector 120 (Figure 49A).
The ends of bladder assembly 404 are preferably configured to move
longitudinally toward each other to enhance radial expansion of bladder 406 as
it is
inflated. In the illustrated embodiment, aft end 416 of assembly 404 is fixed
to mandrel
400, and forward end 417 is slidably engaged with segment 418 of mandrel 400.
This
permits forward end 417 to slide toward aft end 416 as the packerfoot is
inflated,
thereby increasing the radial expansion of bladder 406. The EST's packerfeet
are
, ,.
designed to traverse holes up to 10% larger than the drill bit without losing
traction. For
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. . . . .. . .:._ ,...... . , . . .. .
CA 02542024 1999-12-17
example, a typical drill bit size, and the associated drilled hole, is 3.75
inches in diameter. A
correspondingly sized packerfoot can traverse a 4.1 inch diameter hole.
Similarly, a 4.5-inch
diameter hole will be traversed with a packerfoot that has an expansion
capability to a minimum
of 5.0 inches. Further, the slidable connection of bladder assembly 404 with
segment 418 tends
to prevent the fibers in bladder 406 from overstraining, since the bladder
tends not to stretch as
much. Alternatively, the bladder assembly can be configured so that its
forward end is fixed to
the mandrel and its aft can slide toward the forward end. However, this may
cause the bladder to
undesirably expand when pulling the tractor upward out of a borehole, which
can cause the
tractor to "stick" to the borehole walls. Splines 419 on the forward end of
assembly 404 engage
grooves inside connector 420 so that end 417 cannot rotate with respect to
mandrel 400.
One or more fluid ports 428 are provided along a length of mandre1400, which
communicate with the interior of bladder 406. Ports 428 are preferably
arranged about the
circumference of mandrel 400, so that fluid is introduced uniformly throughout
the bladder
interior. Fluid from aft packerfoot passage 210 reaches bladder 406 by flowing
through ports
378 in shaft 118 (Figures 53 and 59) to the interior of mandre1400, and then
through ports 428 to
the interior of bladder 406. Suitable fluid seals, such as 0-rings, are
provided at the ends of
packerfoot 104 between mandrel 400 and bladder assembly 404 to prevent fluid
within the
bladder from leaking out to annulus 40.
In a preferred embodiment, bladder 406 is constructed of high strength fibers
and
rubber in a special orientation that maximizes strength, radial expansion, and
fatigue life.
The rubber component may be nitrile butadiene rubber (NBR) or a tetra-fluor-
ethylene
(TFE) rubber, such as the rubber sold under the trade name AFLASTM. NBR is
preferred
for use with invert muds (muds that have greater diesel oil content by volume
than water).
AFLASTM material is preferred for use with some specialized drilling fluids,
such as
calcium formate muds. Other additives may be added to the rubber to improve
abrasion
resistance or reduce hysterisis, such as carbon, oil, plasticizers, and
various coatings
including bonded TeflonTM type materials.
High strength fibers are included within the bladder, such as S-glass, E-
glass,
KevlarTM (polyamides), and various graphites. The preferred material is S-
glass because
of its high strength (530,000 psi) and high elongation (5-6%), resulting in
greatly
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CA 02542024 1999-12-17
improved fatigue life compared to previous designs. For instance, if the
fatigue life
criterion for the bladders is that the working strain will remain below
approximately 25-
35% of the ultimate strain of the fibers, previous designs were able to
achieve about
7400 cycles of inflation. In contrast, the expected life of the bladders of
the present
invention under combined loading is estimated to be over 25,000 cycles.
Advantageously, more inflation cycles results in increased operational
downhole time
and lower rig costs.
The fibers are advantageously arranged in multiple layers, a cross-ply
pattern.
The fibers are preferably oriented at angles of a relative to the
longitudinal axis of the
tractor, where a is preferably between 0 and 45 , more preferably between 7
and 30 ,
even more preferably between 15 and 20 , and most preferably about 15 . This
allows
maximal radial expansion without excessive bulging of the bladder into the
regions
between the packerfoot toes, described below. It also allows optimal fatigue
life by the
criterion described above.
When bladder 406 is inflated to engage a borehole wal142, it is desirable that
the
bladder not block the uphole return flow of drilling fluid and drill cuttings
in annulus
40. To prevent this, elongated toes 430 are bonded or otherwise attached to
the outer
surface of the rubber bladder 406, as shown in Figures 60 and 75. Toes 430 may
have a
triangular or trapezoidal cross-section and are preferably arranged in a rib-
like manner.
When the bladder engages the borehole wall, crevices are formed between the
toes 430
and the wall, permitting the flow of drilling fluid and drill cuttings past
the packerfoot.
Toes 430 are preferably designed to be (1) sufficiently large to provide
traction against
the hole wall, (2) sufficiently small in cross-section to maximize uphole
return flow of
drilling fluid past the packerfoot in annulus 40, (3) appropriately flexible
to deform
during the inflation of the bladder, and (4) elastic to assist in the
expulsion of drilling
fluid from the packerfoot during deflation. Preferably, each toe has an outer
radial
width of 0.1-0.6 inches, and a modulus of elasticity of about 19,000,000. Toes
430 may
be constructed of CuBe alloy. The ends of toes 430 are secured onto ends 416
and 417
of bladder assembly 404 by bands of material 432, preferably a high-strength
non-
- magnetic material such as Stabaloy. Bands 432 prevent toes 430 from
separating from
the bladder during unconstrained expansion, thereby preventing formation of
"fish-
-60-
CA 02542024 1999-12-17
hooks" which could undesirably restrict the extraction of the EST from the
borehole.
Figure 74 shows packerfoot 104 inflated.
A protective shield of plastic or metal may be placed in front of the leading
edge
of the packerfoot, to channel the annulus fluid flow up onto the inflated
packerfoot and
thereby protect the leading edge of the bladder from erosion by the fluid and
its
particulate contents.
Figures 65-67 and 76 illustrate an alternative embodiment of an aft
packerfoot,
referred to herein as a "flextoe packerfoot." Aft and forward flextoe
packerfeet can be
provided in place of the previously described packerfeet 104 and 106. Unlike
prior art
bladder-type anchors, the flextoe packerfoot - of the invention utilizes
separate
components for radial expansion force and torque transmission of the anchors.
In
particular, bladders provide force for radial expansion to grip a borehole
wall, while
"flextoes" transmit torque from the EST body to the borehole. The flextoes
comprise
beams which elastically bend within a plane parallel to the tractor body the
tractor body.
Advantageously, the flextoes substantially resist rotation of the body while
the
packerfoot is engaged with the borehole wall. Other advantages of the flextoe
packerfoot include longer fatigue life, greater expansion capability, shorter
length, and
less operational costs.
The figures show one embodiment of an aft flextoe packerfoot 440. Since the
forward flextoe packerfoot is structurally similar to aft flextoe packerfoot
440, it is not
described herein. The major components of aft flextoe packerfoot 440 comprise
a
mandrel 434, fixed endpiece 436, two dowel pin assemblies 438, two jam nuts
442,
shuttle 444, spline endpiece 446, spacer tube 448, connector 450, four
bladders 452,
four bladder covers 454, and four flextoes 456.
With reference to Figure 66, mandrel 434 is substantially tubular but has a
generally rectangular bladder mounting segment 460 which includes a plurality
of
elongated openings 462 an anged about the sides of segment 460. In the EST,
bladders
452 are clamped by bladder covers 454 onto segment 460 so as to cover and seal
shut
openings 462. In operation, fluid is delivered to the interior space of
mandrel 434 via
,. ports 378 in shaft 118 (Figures 53 and 59) to, inflate the bladders.,
Although four
bladders are shown in the drawings, any number of bladders can be provided. In
an
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CA 02542024 1999-12-17
alternative embodiment, shown in Figure 76, one continuous bladder 452 is
used. This
configuration prevents stress concentrations at the edges of the multiple
bladders and
allows greater fatigue life of the bladder.
Referring to Figure 65, bladder covers 454 are mounted onto mandrel 434 via
bolts 468 which pass through holes on the side edges of covers 454 and extend
into
threaded holes 464 in mandrel 434. Bolts 468 fluidly seal bladders 452 against
mandrel
434, and prevent the bladders from separating from mandrel 434 due to the
fluid
pressure inside the bladders. Since the pressure inside the bladders can be as
high as
2400 psi, a large number of bolts 468 are preferably provided to enhance the
strength of
the seal. In the illustrated embodiment, 17 bolts 468 are arranged linearly on
each side
of the covers 454. Jam nuts 442 clamp the aft and forward ends of bladder
covers 454
onto mandrel 434, to fluidly seal the aft and forward ends of the bladders.
The
individual bladders can easily be replaced by removal of the associated
bladder cover
454, substantially reducing replacement costs and time compared to prior art
configurations. Bladder covers 454 are preferably constructed of CuBe or CuBe
alloy.
Referring to Figure 65, fixed endpiece 436 is attached to the aft end of
mandrel
434 via bolts extending into holes 437. Forward of the bladders, shuttle 444
is slidably
engaged on mandrel 434. One dowel pin assembly 438 is mounted onto endpiece
436,
and another assembly 438 is mounted onto shuttle 444. In the illustrated
embodiment,
assemblies 438 each comprise four dowel pin supports 439 which support the
ends of
the dowel pins 458. The dowel pins hingedly support the ends of flextoes 456.
Endpiece 436 and shuttle 444 each have four hinge portions 466 which have
holes that
receive the dowel pins 458. During operation, inflation of the bladders 452
causes
bladder covers 454 to expand radially. This causes the flextoes 456 to hinge
at pins 458
and bow outward to engage the borehole wall. Figure 76 shows an inflated
flextoe
packerfoot (having a single continuous bladder), with flextoes 456 gripping
borehole
wall 42. Shuttle 444 is free to slide axially toward fixed endpiece 436,
thereby
enhancing radial expansion of the flextoes. Those skilled in the art will
understand that
either end of the flextoes 456 can be permitted to slide along mandrel 434.
However, it
is preferred that the forward ends of the flextoes be permitted to slide,
while the aft ends
are fixed to the mandrel. This prevents the slidable end of the flextoes from
being
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CA 02542024 1999-12-17
axially displaced by the borehole wall during tool removal, which could cause
the
flextoes to flex outwardly and interfere with removal of the tractor.
Spline end piece 446 is secured to mandrel 434 via bolts extending into
threaded
holes 472. At the point of attachment, the inner diameter of end piece 446 is
approximately equal to the outer diameter of mandrel 434. Rear of the point of
attachment, the inner diameter of end piece 446 is slightly larger, so that
shuttle 444 can
slide within,end piece 446. End piece 446 also has longitudinal grooves in its
inner
diameter, which receive splines 470 on the outer surface of shuttle 444. This
prevents
shuttle 470, and hence the forward ends of the flextoes 456, from rotating
with respect
to mandrel 434. Thus, since both the forward and aft ends of flextoes 456 are
prevented
from rotating with respect to mandrel 434, the flextoes substantially prevent
the tool
from rotating or twisting when the packerfoot is engaged with the borehole
wall.
In the same manner as described above with regard to mandrel 400 of packerfoot
104, mandrel 434 of flextoe packerfoot 440 has grooves on its internal surface
to
slidably engage rotation restraints 364 on aft shaft 118. Thus, mandrel 434
can slide
longitudinally, but cannot rotate, with respect to shaft 118. Restraints 364
transmit
torque from shaft 118 to a borehole wall 42. The components of packerfoot 440
are
preferably constructed of a flexible, non-magnetic material such as CuBe.
Flextoes 456
may include roughened outer surfaces for improved traction against a borehole
wall.
The spacer tube 448 is used as an adapter to allow interchangeability of the
Flextoe packerfoot 440 and the previous described packerfoot 104 (Figure 60).
The
connector 450 is connected to the mandrel via the set screws. Connector 450
connects
packerfoot 440 with flexible connector 120 (Figure 49A) of the EST.
Figure 67 shows the cross-sectional configuration of one of the bladders 452
utilized in flextoe packerfoot 440. In its uninflated state, bladder 452 has a
multi-folded
configuration as shown. This allows for greater radial expansion when the
bladder is
inflated, caused by the unfolding of the bladder. Also, the bladders do not
stretch as
much during use, compared to prior bladders. This results in longer life of
the bladders.
The bladders are made from fabric reinforced rubber, and may be constructed in
several
30,, configurations. From the inside to the outside of the bladder, a typical
construction is
rubber/fiber/rubber/fiber/rubber. Rubber is necessary on the inside to
maintain pressure.
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CA 02542024 1999-12-17
Rubber is necessary on the outside to prevent fabric damage by cuttings
passing the
bladder. The rubber may be NBR or AFLAS (TFE rubber). Suitable fabrics include
S-
glass, E-glass, Kevlar 29, Kevlar 49, steel fabric (for ESTs not having
magnetic
sensors), various types of graphite, polyester-polarylate fiber, or metallic
fibers.
Different fiber reinforcement designs and fabric weights are acceptable. For
the
illustrated embodiment, the bladder can withstand inflation pressure up to
1500 psi.
This inflation strength is achieved with a 400 denier 4-tow by 4-tow basket
weave
Kevlar 29 fabric. The design includes consideration for fatigue by a maximum
strain
criterion of 25% of the maximum elongation of the fibers. It has been
experimentally
determined that a minimum thickness of 0.090 inches of rubber is required on
the inner
surface to assure pressure integrity.
For both the non-flextoe and flextoe embodiments, the packerfeet are
preferably
positioned near the extreme ends of the EST, to enhance the tool's ability to
traverse
underground voids. The packerfeet are preferably about 39 inches long. The
metallic
parts of the packerfeet are preferably made of CuBe alloy, but other non-
magnetic
materials can be used.
During use, the packerfeet (all of the above-described embodiments, i.e.,
Figures
60 and 65) can desirably grip an open or cased borehole so as to prevent
slippage at high
longitudinal and torsional loads. In other words, the normal force of the
borehole
against each packerfoot must be high enough to prevent slippage, giving due
consideration to the coefficient of friction (typically about 0.3). The normal
force
depends upon the surface area of contact between the packerfoot and the
borehole and
the pressure inside the packerfoot bladder, which will normally be between 500-
1600
psi, and can be as high as 2400 psi. When inflated, the surface area of
contact between
each packerfoot and the borehole is preferably at least 6 inZ, more preferably
at least 9
inZ, even more preferably at least 13 in2, and most preferably at least 18
in2.
Those in the art will understand that fluid seals are preferably provided
throughout the EST, to prevent drilling fluid leakage that could render the
tool
inoperable. For example, the propulsion cylinders and packerfeet are
preferably sealed
to prevent leakage to annulus 40. Annular pistons 140, 142, 144, and 146 are
preferably
sealed to prevent fluid flow between the rear and front chambers of the
propulsion
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CA 02542024 1999-12-17
cylinders. The interfaces between the various housings of control assembly 1D2
and the
flanges of sha.fts 118 and 124 are preferably sealed to prevent leakage.
Compensation
piston 248 is sealed to fluidly separate the oil in electronics housing 130
and motor
housing 132 from drilling fluid in annulus 40. Various other seals are also
provided
throughout the tractor. Suitable seals include rubber 0-rings, tee seals, or
specialized
elastomeric seals. Suitable seal materials include AFLAS or NBR rubber.
Sensors
As mentioned above, the control algorithm for controlling motorized valves
154,
156, and 158 is preferably based at least in part upon (1) pressure signals
from pressure
transducers 182, 184, 186, 188, and 190 (Figures 3 and 4A-F), (2) position
signals from
displacement sensors 192 and 194 (Figures 4A-F) on the annular pistons inside
the aft
and forward propulsion cylinders, or (3) both.
The pressure transducers measure differential pressure between the various
fluid
passages and annulus 40. When pressure information from the above-listed
pressure
transducers is combined with the differential pressure across the differential
pressure
sub for the downhole motor, the speed can be controlled between 0.25-2000 feet
per
hour. That is, the tractor can maintain speeds of 0.25 feet per hour, 2000
feet per hour,
and intermediate speeds as well. In a preferred embodiment, such speeds can be
maintained for as long as required and, essentially, indefinitely so long as
the tractor
does not encounter an obstruction which will not permit the tractor to move at
such
speeds. Differential pressure information is especially useful for control of
relatively
higher speeds such as those used while tripping into and out of a borehole
(250-1000
feet per hour), fast controlled drilling (5-150 feet per hour), and short
trips (30-1000 feet
per hour). The EST can sustain speeds within all of these ranges. Suitable
pressure
transducers for the EST are Product No. 095A201A, manufactured and sold by
Industrial Sensors and Instruments Incorporated, located in Roundrock, Texas.
These
pressure transducers are rated for 15000 psi operating pressure and 2500 psid
differential pressure.
The position of the annular pistons of the propulsion cylinders can be
measured
using any of a variety of suitable sensors, including Hall Effect transducers,
MIDIM
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CA 02542024 1999-12-17
(mirror image differential induction-amplitude magnetometer, sold by Dinsmore
Instrument Co., Flint, MI) devices, conventional magnetometers, Wiegand
sensors, and
other magnetic and distance-sensitive devices. If magnetic displacement
sensors are
used, then the components of the EST are preferably constructed of non-
magnetic
materials which will not interfere with sensor performance. Suitable materials
are CuBe
and Stabaloy. Magnetic materials can be used if non-magnetic sensors are
utilized.
For example, displacement of aft piston 142 can be measured by locating a
MIDIM in connector 122 and a small magnetic source in piston 142. The MIDIM
transmits an electrical signal to logic component 224 which is inversely
proportional to
the distance between the MIDIM and the magnetic source. As piston 142 moves
toward
the MIDIM, the signal increases, thus providing an indication of the relative
longitudinal positions of piston 142 and the MIDIM. Of course, this provides
an
indication of the relative longitudinal positions of aft packerfoot 104 and
the tractor
body, i.e., the shafts and control assembly 102. In addition, displacement
information is
easily converted into speed information by measuring displacement at different
time
intervals.
Another type of displacement sensor which can be used is a Wiegand sensor. In
one embodiment, a wheel is provided on one of the annular pistons in a manner
such
that the wheel rotates as the piston moves axially within one of the
propulsion cylinders.
The wheel includes two small oppositely charged magnets positioned on opposite
sides
of the wheel's outer circumference. In other words, the magnets are separated
by 180 .
The Wiegand sensor senses reversals in polarity of the two magnets, which
occurs every
time the wheel rotates 180 . For every reversal in polarity, the sensor sends
an electric
pulse signal to logic component 224. When piston 142 moves axially within
cylinder
110, causing the wheel to rotate, the Wiegand sensor transmits a stream of
electric
pulses for every 180 rotation of the wheel. The position of the piston 142
with respect
to the propulsion cylinder can be determined by monitoring the number of
pulses and
the direction of piston travel. The position can be calculated from the wheel
diameter,
since each pulse corresponds to one half of the wheel circumference.
,. Figures 77A-C illustrate one embodiment of a Wiegand sensor assembly. As
shown, annular piston 142 includes recesses 574 and 576 in its outer surface.
Recess
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CA 02542024 1999-12-17
574 is sized and configured to receive a wheel assembly 560, shown in Figures
77A and
77B. Wheel assembly 560 comprises a piston attachment member 562, anns 564, a
wheel holding member 572, axle 570, and wheel 566. Wheel 566 rotates on axle
570
which is received within holes 569 in wheel holding member 572. Members 562
and
572 have holes for receiving arms 564. Wheel assembly 560 can be secured
within
recess 574 via a screw received within a hole in piston attachment member 562.
Arms
564 are preferably somewhat flexible to bias wheel 566 against the inner
surface of
propulsion cylinder 110, so that the wheel rotates as piston 142 moves within
cylinder
110. Whee1566 has oppositely charged magnets 568 separated by 180 about the
center
of the wheel. Recess 576 is sized and configured to receive a Wiegand sensor
578
which senses reversals of polarity of magnets 568, as described above. The
figures do
not show the electric wires through which the electric signals flow.
Preferably, the
wires are twisted to prevent electrical interference from the motors or other
components
of the EST.
Those skilled in the art will understand that the relevant displacement
information can be obtained by measuring the displacement of any desired
location on
the EST body (shafts 118, 124, control assembly 102) with respect to each of
the
packerfeet 104 and 106. A convenient method is to measure the displacement of
the
annular pistons (which are fixed to shafts 118 and 124) with respect to the
propulsion
cylinders or connectors (which are fixed with respect to the packerfeet). In
one
embodiment, the displacement of piston 142 is measured with respect to
connector 122.
Alternatively, the displacement of piston 142 can be measured with respect to
an
internal wall of propulsion cylinder 110 or to control assembly 102. The same
infonnation is obtained by measuring the displacement of piston 140. Those
skilled in
the art will understand that it is sufficient to measure the position of only
one of pistons
140 and 142, and only one of pistons 144 and 146, relative to packerfeet 104
and 106,
respectively.
Electronics Confi ur~ ation
.30 Figures 69 illustrates one embodiment of the electronic configuration of
the
EST. All of the wires shown reside within wire passages described above. As
shown,
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CA 02542024 1999-12-17
five wires extend uphole to the surface, including two 30 volt power wires
502, an RS
232 bus wire 504, and two 1553 bus wires 506 (MIL-STD-1553). Wires 502 provide
power to the EST for controlling the motors, and electrically communicate with
a 10-pin
connector that plugs into electronics package 224 of electronics housing 130.
Wire 504
also communicates with electronics package 224. Desired EST parameters, such
as
speed, thrust, position, etc., may be sent from the surface to the EST via
wire 504.
Wires 506 transmit signals downhole to the bottom hole assembly. Commands can
be
sent from the surface to the bottom hole assembly via wires 506, such as
commands to
the motor controlling the drill bit.
A pair of wires 508 permits electrical communication between electronics
package 224 and the aft displacement sensor, such as a Wiegand sensor as
shown.
Similarly, a pair of wires 510 permits communication between package 224 and
the
forward displacement sensor as well. Wires 508 and 510 transmit position
signals from
the sensors to package 224. Another RS 232 bus 512 extends from package 224
downhole to communicate with the bottom hole assembly. Wire 512 transmits
signals
from downhole sensors, such as weight on bit and differential pressure across
the drill
bit, to package 224. Another pair of 30 volt wires 514 extend from package 224
downhole to communicate with and provide power to the bottom hole assembly.
A 29-pin connector 213 is provided for communication between electronics
package 224 and the motors and pressure transducers of control assembly 102.
The
signals from the five pressure transducers may be calibrated by calibration
resistors 515.
Alternatively, the calibration resistors may be omitted. Wires 516 and 518 and
wire
pairs 520, 522, 524, 526, and 528 are provided for reading electronic pressure
signals
from the pressure transducers, in a manner known in the art. Wires 516 and 518
extend
to each of the resistors 515, each of which is connected via four wires to one
pressure
transducer. Wire pairs 520, 522, 524, 526, and 528 extend to the resistors 515
and
pressure transducers.
Wire foursomes 530, 532, and 534 extend to motors 164, 162, and 160,
respectively, which are controlled in a manner known to those skilled in the
art. Three
wires 536 and a wire 538 extend to the rotary accelerometers 531 of the motors
for
transmitting motor feedback to electronics package 224 in a manner known to
those
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CA 02542024 1999-12-17
skilled in the art. In particular, each wire 536 extends to one accelerometer,
for a
positive signal. Wire 538 is a common ground and is connected to all of the
accelerometers. In an alternative embodiment, potentiometers may be provided
in place
of the rotary accelerometers. Note that potentiometers measure the rotary
displacement
of the motor output.
EST Performance
A particular advantage of the EST is that it can sustain both high and low
speeds. Thus, the EST can be used fpr a variety of different activities, such
as drilling,
milling into a casing, tripping into a hole, and tagging bottom (all described
below).
The EST can sustain any speed preferably within a range of 0.25-2000 feet per
hour,
more preferably within a range of 10-750 feet per hour, and even more
preferably within
a range of 35-700 feet per hour. More importantly, the EST can sustain both
fast and
slow speeds, desirably less than 0.25 feet per hour and more than 2000 feet
per hour.
The table below lists pairs of speeds (in feet per hour), wherein a single EST
or a
"string" of connected ESTs (any number of which may be operating) can
desirably
sustain speeds less than the smaller speed of the pair and can desirably
sustain speeds,
greater than the larger speed of the pair.
Less than Greater than
0.25 2000
0.25 750
0.25 250
0.25 150
0.25 100
0.25 75
0.25 50
0.5 75
2 1500
30, 2 2000
15 75
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CA 02542024 1999-12-17
15 100
25 75
25 100
100
5 5 250
5 500
5 750
5 1500
5 2000
10 100
10 125
10 250
10 500
10 750
10 1500
10 2000
30 100
30 250
30 750
30 1500
2000
50 100
50 250
50 500
25 50 750
50 1500
50 2000
Movement of a tractor into and out of an open hole (non-cased section) at high
30 speeds is referred to in the art as "tripping" into the, hole. Tripping
speeds,tend to have
a significant effect on the overall costs of the drill process. Faster speeds
result in less
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CA 02542024 1999-12-17
operational time and less costs. Tripping speeds generally depend upon the
amount of
load that the tractor carries. The higher the load, the slower the maximum
speed of the
tractor. For example, one embodiment of an EST has a diameter of 3.375 inches
and,
while canrying a 9,000 pound load, can travel up to speeds preferably within a
range of
0-180 feet per hour, and more preferably within a range of 120-150 feet per
hour. While
carrying a 3,700 pound load the same EST can travel up to speeds preferably
within a
range of 450-575 feet per hour, and more preferably within a range of 500-525
feet per
hour. These speeds constitute a significant improvement over prior art
tractors.
As mentioned above, a string of multiple tractors can be connected end to end
to
provide greater overall capability. For example, one tractor may be more
suited for
tripping, another for drilling, and another for milling. Any number and
combination of
tractors may be provided. Any number of the tractors may be operating, while
others
are deactivated. In one embodiment, a set of tractors includes a first tractor
configured
to move at speeds within 600-2000 feet per hour, a second tractor configured
to move at
speeds within 10-250 feet per hour, and a third tractor configured to move at
speeds
within 1-10 feet per hour. On the other hand, by providing multiple processors
or a
processor capable of processing the motors in parallel, a single tractor of
the illustrated
EST can move at speeds roughly between 10-750 feet per hour.
Figure 70 shows the speed performance envelope, as a function of load, of one
embodiment of the EST, having a diameter of 3.375 inches. Curve B indicates
the
performance limits imposed by failsafe valve 150, and curve A indicates the
performance limits imposed by relief valve 152. Failsafe valve 150 sets a
minimum
supply pressure, and hence speed, for tractor operation. Relief valve 152 sets
a
maximum supply pressure, and hence speed.
The EST is capable of moving continuously, due to having independently
controllable propulsion cylinders and independently inflatable packerfeet.
When drilling a hole, it is desirable to drill continuously as opposed to
periodically. Continuous drilling increases bit life and maximizes drilling
penetration
rates, thus lower drilling costs. It is also desirable to maintain a constant
load on the bit.
However, the physical mechanics of the drilling process make it difficult to
maintain a
constant load on the bit. The drill string (coiled tubing) behind the tractor
tends to get
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CA 02542024 1999-12-17
caught against the hole wall in some portions of the well and then suddenly
release,
causing large fluctuations in load. Also, the bit may encounter variations in
the
hardness of the formation through which it is drilling. These and other
factors may
contribute to create a time-varying load on the tractor. Prior art tractors
are not
equipped to respond effectively to such load variations, often causing the
drill bit to
become damaged. This is partly because prior art tractors have their control
systems
located at the surface. Thus, sensor signals must travel from the tool up to
the surface to
be processed, and control signals must travel from the surface back down to
the tool.
For example, suppose a prior art drilling tool is located 15,000 feet
underground.
While drilling, the tool may encounter a load variation -due to a downhole
obstruction
such as a hard rock. In order to prevent damage to the drill bit, the tool
needs to reduce
drilling thrust to an acceptable level or perhaps stop entirely. With the tool
control
system at the surface, the time required for the tool to communicate the load
variation to
the control system and for the control system to process the load variation
and transmit
tool command signals back to the tool would likely be too long to prevent
damage to the
drill bit.
In contrast, the unique design of the EST permits the tractor to respond very
quickly to load variations. This is partly because the EST includes electronic
logic
components on the tool instead of at the surface, reducing communication time
between
the logic, sensors, and valves. Thus, the feedback control loop is
considerably faster
than in prior art tools. The EST can respond to a change of weight on the bit
of 100
pounds preferably within 2 seconds, more preferably within 1 second, even more
preferably within 0.5 seconds, even more preferably within 0.2 seconds, and
most
preferably within 0.1 seconds. That is, the weight on the drill bit can
preferably be
changed at a rate of 100 pounds within 0.1 seconds. If that change is
insufficient, the
EST can continue to change the weight on the bit at a rate of 100 pounds per
0.1
seconds until a desired control setting is achieved (the differential pressure
from the
drilling motor is reduced, thus preventing a motor stall). For example, if the
weight on
the drill bit suddenly surges from 2000 lbs to 3000 lbs due to external
conditions, the
,, EST can compensate, i.e. reduce the load on the bit from 3000 lbs to 2000
lbs, in one
second.
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CA 02542024 1999-12-17
Typically, the drilling process involves placing casings in boreholes. It is
often
desirable to mill a hole in the casing to initiate a borehole having a
horizontal
component. It is also desirable to mill at extremely slow speeds, such as 0.25-
4 feet per
hour, to prevent sharp ends from forming in the milled casing which can damage
drill
string components or cause the string to get caught in the milled hole. The
unique
design of propulsion valves 156 and 158 coupled with the use of displacement
sensors
allows a single EST to mill at speeds less than 1 foot per hour, and more
preferably as
low as or even less than 0.25 feet per hour. Thus, appropriate milling ranges
for an EST
~
are 0.25-25 feet per hour, 0.25-10 feet per hour, and 0.25-6 feet per hour
with
appropriate non-barite drilling fluids.
After milling a hole in the casing, it is frequently desirable to exit the
hole at a
high angle turn. The EST is equipped with flexible connectors 120, 122, 126,
and 128
between the packerfeet and the propulsion. cylinders, and flexible shafts 118
and 124.
These components have a smaller diameter than the packerfeet, propulsion
cylinders,
and control assembly, and are formed from a flexible material such as CuBe.
Desirably,
the connectors and shafts are formed from a material having a modulus of
elasticity of
preferably at least 29,000,000 psi, and more preferably at least 19,000,000
psi. This
results in higher flexibility regions of the EST that act as hinges to allow
the tractor to
perform high angle turns. In one embodiment, the EST can turn at an angle up
to 60
per 100 feet of drilled arc, and can then traverse horizontal distances of up
25,000-
50,000 feet.
The tractor design balances such flexibility against the desirability of
having
relatively long propulsion cylinders and packerfeet. It is desirable to have
longer
propulsion cylinders so that the stroke length of the pistons is greater. The
stroke length
of pistons of an EST having a diameter of 3.375 inches is preferably at least
10-20
inches, and more preferably at least 12 inches. In other embodiments, the
stroke length
can be as high as 60 inches. It is also desirable to have packerfeet of an
appropriate
length so that the tool can more effectively engage the inner surface of the
borehole.
The length of each packerfoot is preferably at least 15 inches, and more
preferably at
least 40 inches depending upon design type. As the length of the propulsion
cylinders
and packerfeet increase, the ability of the tool to turn at high angles
decreases. The EST
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CA 02542024 1999-12-17
achieves the above-described turning capability in a design in which the total
length of
the propulsion chambers, control assembly, and packerfeet comprises preferably
at least
50% of the total length of the EST and, in other design variations, 50%-80%,
and more
preferably at least 80% of the total length of the EST. Despite such
flexibility, a 3.375
inch diameter EST is sufficiently strong to push or pull longitudinal loads
preferably as
high as 10,500 pounds.
Advantageously, one aspect of the invention is that a single EST can generate
a
thrust to push and/or pull various loads. The desired thrust capabilities of
various sizes
of the EST are summarized in the following table:
EST Diameter (in) Desired Thrust (lbs) Preferred Thrust (lbs)
2.125 1000 2000
3.375 5250 10,500
4.75 13,000 26,000
6.0 22,500 45,000
Additionally, the EST resists torsional compliance, i.e. twisting, about its
longitudinal axis. During drilling, the formation exerts a reaction torque
through the
drill bit and into the EST body. When the aft packerfoot is engaged with the
borehole
and the forward packerfoot is retracted, the portion of the body forward of
the aft
packerfoot twists slightly. Subsequently, when the forward packerfoot becomes
engaged with the borehole and the aft packerfoot is deflated, the portion of
the body to
the aft of the forward packerfoot tends to untwist. This causes the drill
string to
gradually become twisted. This also causes the body to gradually rotate about
its
longitudinal axis. The tool direction sensors must continuously account for
such
rotation. Compared to prior art tractors, the EST body is advantageously
configured to
significantly limit such twisting. Preferably, the shaft diameter is at least
1.75 inches
and the control assembly diameter is at least 3.375 inches, for this
configuration. When
such an EST is subjected to a torsional load as high as 500 ft-lbs about its
longitudinal
axis, the.shafts and control assembly twist:preferably less than 5 per,..step
pf the tractor.
Advantageously, the above-mentioned problems are substantially prevented or
-74-
CA 02542024 1999-12-17
minimized. Further, the EST design includes a non-rotational engagement of the
packerfeet and shafts, via rotation restraints 364 (Figure 49A). This prevents
torque
from being transferred to the drill string, which would cause the drill string
to rotate.
Also, the flextoe packerfeet of the EST provide improved transmission of
torque to the
borehole wall, via the flextoes.
When initiating further drilling at the bottom of a borehole, it is desirable
to "tag
bottom," before drilling. Tagging bottom involves moving at an extremely slow
speed
when approaching the end of the borehole, and reducing the speed to zero at
the
moment the drill bit reaches the end of the formation. This facilitates'smooth
starting of
the drill bit, resulting in longer bit life, fewer trips to replace the bit,
and hence lower
drilling costs. The EST has superior speed control and can reverse direction
to allow
efficient tagging of the bottom and starting the bit. Typically, the EST will
move at
near maximum speed up to the last 50 feet before the bottom of the hole. Once
within
50 feet, the EST speed is desirably reduced to about 12 feet per hour' until
within about
10 feet of the bottom. Then the speed is reduced to minimum. The tractor is
then
reversed and moved backward 1-2 feet, and then slowly moved forward.
When drilling horizontal holes, the cuttings from the bit can settle on the
bottom
of the hole. Such cuttings must be periodically be swept out by circulating
drilling fluid
close to the cutting beds. The EST has the capability of reversing direction
and walking
backward, dragging the bit whose nozzles sweep the cuttings back out.
As fluid moves through a hole, the hole wall tends to deteriorate and become
larger. The EST's packerfeet are designed to traverse holes up to 10% larger
than the
drill bit without losing traction.
Although this invention has been disclosed in the context of certain preferred
embodiments and examples, it will be understood by those skilled in the art
that the
present invention extends beyond the specifically disclosed embodiments to
other
altemative embodiments and/or uses of the invention and obvious modifications
thereof.
Thus, it is intended that the scope of the present invention herein disclosed
should not
be limited by the particular disclosed embodiments described above, but should
be
- determined only by a fair reading of the claims that follow.
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