Note: Descriptions are shown in the official language in which they were submitted.
CA 02755165 2011-10-12
ELECTRICAL CONTROLLER FOR ANTI-STALL TOOLS FOR DOWNHOLE
DRILLING ASSEMBLIES
BACKGROUND
The present invention provides an improvement in anti-stall tool controllers,
and more
specifically is an electrical controller to operate an anti-stall tool (AST)
for controlling
weight-on-bit during drilling operations.
Coiled tubing drilling requires the use of a downhole positive displacement
motor
(PDM) to rotate the drill bit. During drilling operations, the unloaded PDM
rotates at a
constant RPM and achieves a "freespin" motor pressure, with respect to the
fluid flow rate.
As the drill bit encounters the bottom of the hole and force is transferred to
the bit, referred to
as weight-on-bit (WOB), the motor will sense an increase in torque. This
increase in torque is
a result of increased resistance to rotating at the constant RPM (assuming a
constant flow
rate). In turn, the PDM requires additional pressure to turn the motor at the
constant RPM
while under increased resistance. If the resistance increases to a condition
which prohibits the
PDM from rotating (i.e. excessive WOB), a motor stall is encountered. During a
motor stall,
the motor stops turning, the downhole fluid path is severely restricted, and
the surface pump
pressure dramatically increases. This event can eventually cause a motor
failure, which
requires the drilling process to be stopped, and the coiled tubing to be
fatigue-cycled as the bit
is pulled off bottom and run back into the hole to start drilling again. An
anti-stall tool (AST)
is described in U.S. Patent Publication No. 2009/0173540 to Mock, et al.
A downhole tool that monitors motor pressure and sharply reduces the
occurrence of
motor stalls will increase overall drilling efficiency by:
(1) Increasing the average rate of penetration. This is achieved
by reducing the
occurrences of pulling off-bottom every time the motor stalls.
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(2) Decreasing the damage to PDMs through repeated motor stalls, thereby
decreasing occurrence of downhole failure.
(3) Decreasing the fatigue cycles on the coiled tubing. This increases the
number
of wells a coiled tubing string can service.
By achieving a more efficient drilling operation, the operators can
substantially
increase the cost savings of drilling a well.
The present invention provides an electrical controller for an anti-stall tool
that
controls WOB during drilling operations, resulting in improved overall
drilling efficiency.
SUMMARY OF THE INVENTION
Briefly, the invention comprises an electrical controller for an anti-stall
tool for use in
a downhole assembly near the bottom of the tubing adjacent a positive
displacement motor
(PDM) and the drill bit. In one embodiment, the tubing comprises a coiled
tubing, although
the invention also can be used in rotary drilling applications. The electrical
controller controls
the force applied to the drill bit during drilling to prevent the drill bit
from stalling under load.
A working pressure range of the PDM is sensed during use by a hydraulic valve
control
system and is used as an input to the controller. The controller alters weight-
on-bit (WOB) if
the downhole pressure goes beyond either end of a preset working pressure
range of the
system. The controller keeps the drill bit rotating by (1) maintaining WOB
during normal
drilling operations, (2) increasing WOB if sensed PDM working pressure
indicates that drill
bit loading is low, and (3) reducing WOB which reduces PDM back-pressure to
retract the
drill bit from the bottom if excessive working pressure is sensed due to
increased torque at the
PDM.
The anti-stall tool generally comprises one or more hydraulic cylinders for
applying an
axial force either in a forward direction or a reverse direction. The
electrical controller
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comprises electronics and a system of hydraulic valves adapted to control
piston force in
either the forward or reverse directions. An active stage of the anti-stall
tool reacts to the
PDM producing low downhole pressures (e.g. below a preset low pressure) by
actuating one
or more of the pistons in the downhole direction to increase WOB which
increases PDM
back-pressure. When the PDM is operating within its normal operating pressure
range, the
controller can lock the pistons in a passive mode, in which the pistons are
sealed and the anti-
stall tool transfers force from the tubing to the drill bit. If the controller
senses a preset high
pressure or greater due to high torque at the PDM, the valve system reverses
hydraulic flow to
the pistons, which reduces WOB to force the drill bit away from the bottom to
reduce PDM
back-pressure.
There is provided an anti-stall method for controlling drilling operations in
a
downhole assembly which includes a tubing that extends downhole, a drill bit
carried on the
tubing, a drive motor for rotating the drill bit, and a spring-operated anti-
stall tool adjacent the
motor, the method comprising electrically sensing pressure in the motor,
providing a range of
operating pressures for the motor defined by high and low limits of operating
pressures, and
operating the anti-stall tool in: (1) an active stage increasing WOB forces in
a downhole
direction by applying pressure to the anti-stall tool against the bias of a
compression spring
therein, when the low limit of operating pressure is sensed, and (2) a reverse
stage for
providing a WOB force in the reverse direction via the compression spring
bias, when the
high limit of operating pressure is sensed.
An optional passive stage is contemplated in which the anti-stall tool is
locked to
transfer WOB directly from the tubing to the drill bit when the PDM is
operating within the
limits of its normal operating pressure range.
There is also provided a spring-operated anti-stall tool adapted for use in a
downhole
assembly comprising a tubing for extending downhole; a drill bit carried on
the tubing; and a
drive motor adjacent the drill bit for rotating the drill bit during drilling
operations; the spring-
operated anti-stall tool carried on the tubing and positioned adjacent the
motor for preventing
stalling of the motor due to excessive loads on the drill bit, the anti-stall
tool including at least
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one piston in a cylinder having a forward piston area and a reverse piston
area, and an
electrical controller which signals a hydraulic valve system for controlling
operation of the
piston, the forward piston area receiving hydraulic fluid to produce a force
in the downhole
direction, the reverse piston area containing a load spring adapted to apply
an upward spring
force on the piston, the electrical controller sensing operating pressure of
the drive motor and
setting a desired range of operating pressures for the motor, including an
upper limit and a
lower limit, the electrical controller adapted to: (1) supply hydraulic fluid
to the forward
piston area to increase force in a downhole direction to increase weight-on-
bit (WOB) when
operating pressure in the motor surpasses the lower limit, thereby compressing
the load spring
as the cylinder moves in the downhole direction; and (2) vent the piston
volume in the
forward piston area so the compressed spring can expand to push the tool
uphole to retract the
drill bit, to decrease WOB when operating pressure in the motor exceeds the
upper limit
Optionally, the electrical controller is adapted to lock the piston in a
passive state
when the PDM is operating within its normal operating pressure range.
Another embodiment comprises an improved anti-stall tool which produces a
controlled translational motion of the drill bit that increases drilling
efficiency. The anti-stall
tool controls the force applied to the drill bit during drilling to prevent
the drill bit from
stalling under load. The anti-stall tool comprises one or more hydraulic
cylinders for applying
an axial force in either a forward or reverse direction, and an electrical
controller adapted to
control the force applied by the one or more hydraulic cylinders to the drill
bit in response to
sensed working pressure of the drive motor during drilling operations. The
controller
comprises a system for adjusting WOB when working pressure exceeds either end
of a
working pressure range of the drive motor. The system includes (1) a passive
stage for
maintaining WOB when working pressure is within a preset normal operating
range, (2) an
active stage for applying pressure to the one or more cylinders to increase
WOB when sensed
working pressure is below a preset limit, and (3) a reverse stage for
reversing pressure to the
one or more cylinders to reduce WOB and thereby retract the drill bit from the
bottom when
sensed working pressure is above a preset limit. The tool is normally
controlled to apply
WOB at pressures within a desired wide range of pressures. When reaching a
preset anti-stall
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pressure, the tool is reversed to reduce WOB and does not resume applying WOB
over a
preset wide range of PDM back-pressure drop.
Accordingly, there is also provided a downhole assembly adapted for anti-stall
drilling
operations, the downhole assembly including a drill bit, a drive motor for
rotating the drill bit,
a tubing for supplying drilling fluid to the drive motor, and an anti-stall
tool positioned
between the tubing and the drive motor for controlling the force applied to
the drill bit during
drilling operations, to thereby prevent the drill bit from stalling under
load, the anti-stall tool
comprising: an outer housing, an internal passageway extending through the
housing for
transmitting drilling fluid from the tubing to the drive motor for rotating
the drill bit, an open
loop controller contained in the outer housing, the open loop controller
comprising: a piston
assembly slidably disposed in the outer housing, said internal passageway
extending through
the piston assembly, the piston assembly comprising one or more hydraulic
cylinders each
having a piston therein for applying axial forces in either a downhole
direction or a reverse
direction to adjust weight-on-bit (WOB) while drilling, a hydraulic control
valve system
contained in the outer housing with an inlet for receiving a supply of
hydraulic control fluid
from the drilling fluid in the internal passageway, for supplying the
hydraulic control fluid to
the piston assembly to control WOB, and an open loop electronics package
contained in the
outer housing for controlling the hydraulic control valve system, the
hydraulic control valve
system including: an adjustable first set point for indicating a desired lower
limit for hydraulic
working pressure of the drilling fluid in the tubing, and an adjustable second
set point for
indicating a desired upper limit for hydraulic working pressure of the
drilling fluid in the
tubing, the first and second set points representing a desired working
pressure range for the
drive motor, an active stage valve assembly for sensing the working pressure
in the tubing and
supplying the hydraulic control fluid to one or more of the cylinders
contained in the piston
assembly to apply an axial force in the downhole direction to increase WOB
when the sensed
working pressure is below the first set point, a passive stage valve assembly
for sensing the
working pressure in the tubing and shutting off the hydraulic control fluid
supplied to the
piston assembly to hydraulically lock the piston assembly in a passive state
for maintaining
WOB when the sensed working pressure is within the desired working pressure
range of the
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drive motor, and a reverse stage valve assembly for sensing the working
pressure in the tubing
and reversing the flow of the hydraulic control fluid supplied to one or more
of the cylinders
contained in the piston assembly to apply an axial force in the reverse
direction to retract the
drill bit to decrease WOB when the sensed working pressure reaches or exceeds
the second
set point.
In another embodiment, the tool can apply WOB during the wide range of
operating
pressures via at least two stages, one where pressure is increasing up to a
set desired operating
pressure, and then switches the tool to a locked position at that pressure and
higher up to a
preset anti-stall limit at which flow to the pistons is reversed to lift the
drill bit. The two
stages can be operated as active/reverse stages as well.
These and other aspects of the invention, including additional embodiments,
will be
more fully understood by referring to the following detailed description and
the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view showing a downhole assembly containing an anti-
stall tool
according to principles of this invention;
FIG. 2 shows a cross-sectional view of one embodiment of a hydraulic-operated
anti-
stall tool;
FIG. 3 is an elevational view showing a further embodiment of an anti-stall
tool;
FIG. 4 is a cross-sectional view showing the anti-stall tool of FIG. 3 along
with a
schematic view of an improved controller;
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FIG. 5 shows a cross-sectional view of another alternative embodiment spring-
operated anti-stall tool;
FIG. 6 is a cross-sectional view of an electrically controlled anti-stall tool
which
includes a 2-position, 4-way pilot valve with a pressure control valve
according to principles
of this invention;
FIG. 7 shows an alternative embodiment electrically controlled anti-stall tool
which
includes a 3-position, 4-way valve; and
FIG. 8 shows a cross-sectional view of the electrically controlled AST, which
includes
an electronics package and electrical downhole motor contained in the AST.
DETAILED DESCRIPTION
FIG. 1 is a schematic diagram illustrating a coiled tubing drilling system 10
for
drilling a well bore in an underground formation 12. The coiled tubing
drilling system can
include a coiled tubing reel 14, a gooseneck tubing guide 16, a tubing
injector 18, a coiled
tubing 20, a coiled tubing connector 21, and a drill bit 22 at the bottom of
the well bore. FIG.
1 also shows a control cab 24, a power pack 26, and an alignment of other BHA
tools at 27.
A tractor (not shown), such as that described in U.S. Patent No. 7,343,982,
may be used to
move downhole equipment within the bore. During drilling, the downhole
equipment
includes a downhole motor 28, such as a positive displacement motor (PDM), for
rotating the
drill bit. An anti-stall tool (AST) 30, according to principles of this
invention, is positioned
near the bottom of the coiled tubing, upstream from the downhole motor and the
drill bit. In
one embodiment, hydraulic back pressure produced within the coiled tubing is
measured at
the surface. Torque produced at the drill bit during drilling operations is
directly related to
back-pressure. As a result, hydraulic back-pressure measurements can be sensed
and used as
inputs to a hydraulic control valve system contained in the anti-stall tool.
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The anti-stall tool 30 incorporates use of a series of hydraulic cylinders and
as few as
three pressure-actuated valves to control the applied weight-on-bit (WOB)
while drilling.
This tool will virtually create a real time, downhole motor pressure sensor
that will alter the
WOB to maintain a relatively constant drilling rate of penetration and provide
feedback to the
coiled tubing operator to adjust coiled tubing injector rates to match the PDM
pressure.
The invention uses the working pressure range of the downhole positive
displacement
motor 28 to alter the WOB if the downhole pressure surpasses either end of the
working
range. During drilling operations, the AST controls WOB through the use of
three distinct
operations: active WOB, passive WOB and reverse.
FIG. 2 illustrates one embodiment of the anti-stall tool 30 which includes a
series of
axially aligned hydraulic cylinders with separate pistons that define piston
areas Al and A2,
A3A and A3B, and A3C and A3D. The torque section of the tool is shown at 35.
FIG. 2 also
schematically shows a hydraulic controller 34 contained in the anti-stall
tool. The controller
includes a pressure reducing valve 36, a reverser valve 38, and a vent valve
40. Hydraulic
control fluid passes through a filter 42.
In the description to follow, specific operating pressure set points or values
are related
to operative ranges for coiled tubing equipment. Use of the anti-stall tool in
rotary drilling
operations, for example, would involve use of different operating pressure
ranges or control
valve set points.
The first stage of the hydraulic anti-stall tool is activated when the
unloaded PDM
produces low downhole pressures. For example, if the PDM creates a back
pressure of 200
psi (adjustable to specific motor requirements), the anti-stall tool will be
in the active WOB
stage. This causes pressure to be supplied to all pistons that will produce a
force in the
downhole direction (Al, A3A and possibly A3C). As the WOB is applied, the
normal
reaction is for the PDM to generate more pressure. As the anti-stall tool
senses the increase in
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pressure to 250 psi (adjustable to specific motor requirements), the pressure
reducing valve 36
will shut off additional flow to the pistons and hydraulically lock the
pistons in the passive
WOB stage.
In the passive WOB stage, the anti-stall tool transfers the force from the
tubing to the
bit. The tool is acting as a rigid member and is monitoring the PDM back-
pressure. The
pressure reducing valve 36 is closed and is sealing the fluid in the pistons
(A3A and possibly
A3C) that produce a force in the downhole direction. All of the resultant
pressure from the
WOB will be contained in the sealed piston volumes.
During the final stage of the anti-stall tool, the back pressure due to high
torque in the
PDM triggers the reverser valve 38 and vent valve 40 to reduce WOB. Once the
back
pressure reaches 1,000 psi (adjustable to specific motor requirements), the
reverser valve 38
switches the flow of fluid to the pistons that produce force in the uphole
direction (A2, A3B,
A3D). At the same time, the vent valve 40 vents the opposite side of those
pistons. This
allows the tool to travel uphole, reducing WOB and thereby reducing the PDM
back pressure.
As the PDM back pressure falls below the reverser valve setting (including
hysteresis) the
reverser valve 38 will switch back to its original position.
The anti-stall tool is designed to be in the fully expanded position at low
pressures.
This bias allows the tool to have the full length of stroke available to
retract as much as
needed until the PDM back-pressure reduces below the lower limit of the vent
valve. The
anti-stall tool will then try to fully expand, but the pressure may rise to
the pressure control
valve setting or higher and limit the expansion. Therefore, the long stroke
length will allow
several retraction steps before the stroke length is used up. The coiled
tubing operator can
adjust the input speed of the coiled tubing into the hole to prevent the anti-
stall tool from fully
retracting. The operator will see a change in pump pressure with each
retraction to signal the
need to reduce the coiled tubing input speed.
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The anti-stall tool operates as an open loop system. Drilling fluid from the
surface is
pumped down the bore in the tubing through the tool, to the motor for rotating
the drill bit.
Most of the fluid flow in the system is used for driving the drill bit. A
small amount of the
fluid is used for the controller and is jetted out to the sides and into the
annulus during use.
The anti-stall tool includes splines in a torque section 44 which contains an
outer
spline housing and splines contained internally on the piston housing. The
splines allow the
BHA to maintain its orientation relative to the motor and drill bit, without
undesired twisting.
The splines allow the tool to be used with a steerable BHA. Steerable BHAs can
be
controlled to drill the hole to a desired location, while changing the
direction of the hole while
drilling to achieve this goal. The splines allow the PDM and bit to maintain
alignment with
the orienting tools that would be uphole of the anti-stall tool. The torque
load is transferred
from the PDM across the outermost housings and across the spline of the anti-
stall tool to the
tools uphole of the anti-stall tool. The inner shafts do not see direct
loading due to torque.
The spline section functions in both the expansion and retraction of the anti-
stall tool.
FIGS. 3 and 4 show an improved anti-stall tool 30' which produces a three-
stage
controlled translational motion to the drill bit that increases drilling
efficiency.
This illustrated embodiment includes a series of axially aligned hydraulic
cylinders
with pistons that cooperate to form piston areas Sl, Al and A2, and A3A and
A3B. The
torque section of the tool is shown at 44 along with a hydraulic controller
contained in the
anti-stall tool and shown schematically at 46. The controller includes a
pressure control valve
48, a pilot valve 50, a sequence valve 52, and a vent valve 54. A filter for
the hydraulic
controller is shown at 56.
In one embodiment, the controller has the three stages of operation: (1)
active, (2)
passive, and (3) retraction. The control valves contained in the controller
area of the tool are
shown schematically in FIG. 4: pressure lines are shown as solid lines, pilot
lines are shown
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as dashed lines, and exhaust lines are shown in dotted lines. In the following
description, the
pressure ranges are used as examples only; they are adjustable to specific
motor requirements.
The active stage applies downward force to the drill bit based on motor back-
pressure
from the positive displacement motor. If pressure is less than 400 psi, for
example, the
hydraulic pistons apply a downward force which generates more PDM back-
pressure. The
vent valve 54 of the controller is open and supplies a pilot signal to the
pilot valve 50. If
pressure reaches 400 psi, the vent valve 54 closes and vents the pilot line
for the pilot valve
50. But the detented pilot valve stays in position, and the PDM back-pressure
is sensed by the
pressure control valve 48. The pistons apply the downward force until sensed
downhole
pressure reaches 650 psi, for example, which represents a desired working
pressure.
The pressure control valve then switches the anti-stall tool to the passive
mode when
sensed pressure reaches the desired drilling pressure of 650 psi, for example.
Here the
pressure control valve 48 shuts off flow to the pistons and hydraulically
locks the pistons in
the passive WOB mode. The pressure control valve 48 is closed and no pressure
is sent to the
pistons. The pistons are sealed, and existing force is transferred to the
drill bit. Motor
pressure is not increased. Downhole pressure continues to be monitored in the
passive mode
via the vent valve 54 and sequence valve 52, which monitor pressure change in
the coiled
tubing. The passive state continues until sensed back-pressure reaches 800
psi, for example.
Once downhole pressure reaches the 800 psi level, the anti-stall tool switches
to the
reverse mode. That is, if torque in the PDM increases, it causes an increase
in back-pressure.
Motor stall is prevented by sensing and reacting to back pressure at a level
below motor stall,
e.g., 800 psi, or other pressure below that at which stall can occur.
When sensed pressure reaches 800 psi, the normally-closed sequence valve 52 is
opened, sending a pilot signal to the pilot valve 50 which reverses flow of
hydraulic fluid to
the pistons to produce a force in the uphole direction, to reduce WOB.
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As back pressure falls below 800 psi, the pilot signal from the sequence valve
52 to
the pilot valve 50 is closed. The sequence valve 52 vents the pilot signal,
and this continues
until sensed PDM pressure falls to 400 psi, where the vent valve 54 opens and
sends a pilot
signal to the pilot valve 50 to shift back to the active mode, by supplying
fluid pressure to the
pistons to expand and to apply downward force to increase WOB.
Thus, in this embodiment, the tool is normally controlled to apply WOB when
drilling
at pressures within a desired wide range of pressures. These can be from 400
to 800 psi, for
example. When reaching a preset anti-stall pressure, such as 800 psi, which
would be a safe
level below the pressure at which stall actually occurs, the tool is reversed
and does not
resume applying WOB over a preset wide range of pressure drop, before resuming
active
WOB operations. This wide range of pressure drop can be from about 200 to
about 2,000 psi.
In the illustrated embodiment, the range of pressure drop is 400 psi (from 800
to 400 psi),
before WOB is resumed.
The tool applies WOB during the desired wide range of operating pressures via
two
stages, one stage where pressure is increasing up to a set desired operating
pressure, for
example 650 psi, and then switches to a second-stage locked position at that
pressure and
higher up until an anti-stall limit, of say 800 psi is reached, before
reversing flow to the
pistons and lifting the drill bit.
A key feature of the anti-stall tool is the single input necessary for the
tool to operate.
The tool need only sense and respond to the back-pressure created by the PDM.
Stated
another way, the anti-stall tool operates on constant (although adjustable)
working pressure
set points. The fixed set points can be fine-tuned to control the thresholds
at which the
control valves open and close, and as a result, drill bit penetration rate is
more uniform.
An alternate embodiment of the invention comprises a two-phase anti-stall
method for
controlling drilling operations in a downhole assembly, which includes the
tubing that extends
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downhole, the drill bit carried on the tubing, the positive displacement motor
(PDM) for
rotating the drill bit, and the anti-stall tool adjacent the PDM. This method
comprises sensing
pressure in the PDM, providing a range of operating pressures for the PDM
defined by high
and low limits of operating pressures, and operating the anti-stall tool in:
(1) an active stage
increasing WOB forces in the downhole direction when the low limit of
operating pressure is
sensed, and (2) a reverse stage providing a force in the reverse direction,
reducing the WOB,
when the high limit of operating pressure is sensed.
This two-phase anti-stall method can be accomplished by adjusting the setting
of the
sequence valve 52 equal to or lower than the pressure control valve 48, but
still above the
setting of the vent valve 54.
The anti-stall tool also can be operated by the two-phase method, combined
with a
passive range that operates (as described above) between a small range of
pressure settings.
Different orifice adjustments can be used to control the speed at which the
tool
responds. In FIG. 2, the orifice is not shown. The orifice can be on the
exhaust of the
reverser valve 38.
Although the schematic in FIG. 4 depicts a single orifice 55, those skilled in
the art
would understand that the two-position/four-way valve contains two exhaust
ports. Each of
the ports vents a different piston area, either the piston area to produce
downhole force
(expand) or uphole force (retract). Using the high and low limits of the
operating pressures,
the orifice sizes can be calculated to restrict the volumetric flow rate of
fluid exhausted
through the valve and thereby control the speed at which the tool expands or
retracts. The
expansion and retraction of the tool can be controlled individually by
different orifice sizes.
As an alternative, WOB can be controlled by a combination of control valve
settings
and adjustments to orifice sizes.
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EXAMPLE
The following specifications illustrate one embodiment of the anti-stall tool:
Description Characteristic
Tool OD 3.00 in
Tool ID .75 in
Length - Expanded 8.1 ft
Length - Collapsed 7.4 ft
Stroke 9 in
Max Temp 300 F
Tensile Strength 50,000 lbs
Max Motor Torque 2,000 ft-lbs
Max Dog Leg 25 /100 ft
Tool Joint 2 3/8 PAC
The design is flexible in that the pressure settings and orifice size may be
changed to fine-tune
the tool. If a much larger WOB change is needed, then the shaft can be
replaced to allow
installation of additional pistons.
# of Pistons Total Downhole Area Pressure Control Max WOB
from
(sq. in.) Valve Setting (psi) AST (lbs)
1 4.8 650 3,055
2 7.9 650 5,135
3 11.0 650 7,150
The anti-stall tool cylinders and valves may be manufactured from various
corrosion-
resistant materials including tungsten carbide, Inconel, high strength nickel
alloyed steel such
as MP35, beryllium-copper, and the like.
Examples of improvements provided by the anti-stall tool are:
(1) Active WOB: The tool will attempt reset into the fully extended
position when the
pressure falls below 650 psi. If a motor stall has occurred and the AST has
pulled the
bit off bottom, the Active WOB stage will produce a minimum WOB and thrust the
bit
downhole until the PDM pressure exceeds 650 psi.
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(2) Passive WOB: Shuts off the Active WOB stage and allows the coiled
tubing to
transfer WOB to the bit. Prevents excessive WOB that can be developed as PDM
pressure rises and acts on the pistons producing force downhole.
(3) Reverse: Reduces WOB to prevent motor stalls.
(4) Torque section will transfer torque through the AST into the coiled
tubing.
A downhole tool that monitors motor pressure and sharply reduces the
occurrence of
motor stalls will increase the overall drilling efficiency by:
(1) Increasing the average rate of penetration. This is achieved
reducing the occurrences
of pulling off bottom for motor stalls.
(2) Decreasing the damage to PDMs through repeated motor stalls, thereby
decreasing
occurrence of downhole failure.
(3) Decreasing the fatigue cycles on the coiled tubing. The increases
the number of wells
a coiled tubing string can service.
FIG. 5 illustrates a spring-operated anti-stall tool 130 according to this
invention. In
the description to follow, motor pressure values are examples only; they are
dependent upon
and adjustable to specific motor requirements.
The FIG. 5 embodiment includes a series of axially aligned hydraulic cylinders
with
separate pistons that define piston areas Al and A2, A3A and A3B, and A3C and
A3D. The
torque section of the tool is shown at 35. The piston area A3B contains a
compression spring
that applies a spring force Fl and a piston area A3D which contains a
compression spring that
applies a spring force F2. FIG. 2 also schematically shows a controller 34
contained in the
anti-stall tool. The controller includes a pressure reducing valve 136 and a
vent valve 138.
Hydraulic fluid passes through a filter 140.
In the description to follow, specific operating pressure set points or values
are related
to operative ranges for coiled tubing equipment. Use of the anti-stall tool in
rotary drilling
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CA 02755165 2011-10-12
operations, for example, would involve use of different operating pressure
ranges or control
valve set points.
The first stage of the spring operated anti-stall tool 130 is activated when
the unloaded
PDM produces low downhole pressures. For example, if the PDM 20 creates a back
pressure
of 200 psi, the spring-operated tool will be in the active WOB stage. This
causes pressure to
be supplied to all pistons that will produce a force in the downhole direction
(Al, A3A and
possibly A3C). This will compress and load the springs with a spring force F1
and F2. As
the WOB is applied, the normal reaction is for the PDM to generate more
pressure. As the
tool senses the increase in pressure to 250 psi (adjustable to specific motor
requirements), the
pressure reducing valve 136 will shut off additional flow to the pistons and
hydraulically lock
the pistons in the passive WOB stage.
In the passive WOB stage, the spring-operated tool transfers the force from
the coil to
the bit. The tool is acting as a rigid member and is monitoring the PDM back-
pressure. The
pressure reducing valve 136 is closed and is sealing the fluid in the pistons
(A3A and possibly
A3C) that produce a force in the downhole direction. All of the resultant
pressure from the
WOB is contained in the sealed piston volumes.
During the final stage of the spring-operated tool, the back pressure due to
high torque
in the PDM triggers the vent valve 138 to pull the bit off-bottom. Once the
back pressure
reaches 1,000 psi (adjustable to specific motor requirements), the vent valve
138 vents piston
volumes A3A and A3C. The resultant force Fl and F2 of the compressed springs
will push
the tool uphole, reducing WOB and thereby reducing the PDM back-pressure. As
the PDM
back-pressure falls below the vent valve setting (including hysteresis), the
tool will switch
back to one of its other stages of operation.
FIGS. 1-5 illustrate two AST designs having hydraulic controllers, however
FIG. 6
shows an alternative embodiment electrically controlled AST 58 having a 2-
position, 4-way
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pilot valve 60 with pressure control valve 62. An electrical controller could
be incorporated
into the AST embodiments shown in FIGS. 1-5. The embodiment, as shown in FIG.
6, is
based on using the working pressure range of a downhole positive displacement
motor (PDM)
28 to alter the weight-on-bit (WOB) if the downhole pressure surpasses either
end of the
PDM's working range. Alternatively, this embodiment can use the working power
range of an
electric downhole motor 64 to alter the WOB if the power consumption surpasses
either end
of the motor's working range. During drilling operations, the anti-stall tool
(AST) 58 will
control the WOB through the use of three distinct operations; active WOB,
passive WOB,
and Off Bottom.
The first stage of the AST is activated when the unloaded PDM produces low
downhole pressures. For example, if the PDM and drill bit jets create a back
pressure of 300
psi, the AST will be in the active WOB stage. This means that pressure will be
supplied to all
pistons in the AST that will produce a force in the downhole direction (Al,
A3A). As the
WOB is applied, the normal reaction is for the PDM to generate more pressure.
When the
pressure reaches 400 psi, for example, the electronics package 66, which
receives a pressure-
related input from a pressure transducer (P) 68, will signal the motor 64 to
shift the pilot valve
(PV) 60. The PV is a motor-driven, two position, four way spool valve. The PV
controls
whether the AST will expand or contract. As the AST senses the increase in
pressure to 650
psi (adjustable to specific motor requirements), the pressure control valve
(PCV) 62 will shut
off additional flow to the pistons and hydraulically lock the pistons in the
AST in the passive
WOB stage.
In the passive WOB stage, the AST simply transfers the force from the coil
tubing to
the drill bit. The AST is acting as a rigid member and is simply monitoring
the PDM back
pressure. The PCV is closed and is sealing the fluid in the pistons (Al, A3A)
that produce a
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CA 02755165 2011-10-12
force in the downhole direction. All of the resultant pressure from the WOB
will be contained
in the sealed piston volumes.
During the final stage of the AST, the back pressure due to high torque in the
PDM
signals the electronics package 66 via the pressure transducer 68 to shift the
pilot valve 60 and
pull the bit off bottom. Once the back pressure reaches 800 psi (off bottom
setting, adjustable
to specific motor requirements), the pressure transducer/electronics package
will signal the
electric downhole motor 64 to shift the PV's position. This switches the flow
of fluid to the
AST pistons that produce force in the uphole direction (A2, A3B). This allows
the tool to
travel uphole, reducing WOB and thereby reducing the PDM back pressure. When
the
pressure falls below the active WOB setting (400 psi), the pressure
transducer/electronics
package will signal the electric downhole motor to shift the PV's positions.
Once the PV
switches back to its original position, the AST will return to the active WOB
stage. The
electronics package 74 receives a pressure input from pressure transducer (P)
76 and signals
motor 72 to shift pilot valve 70.
FIG. 7 shows a 3-position, 4-way pilot valve 70. The alternative embodiment,
shown
in FIG. 7, is based on using a single valve to control the AST. The pilot
valve 70 is a motor
72 driven, three-position, four-way spool valve. The center position of the
pilot valve
generates the hydraulic lock necessary for the passive WOB stage.
FIG. 8 illustrates a cross sectional view of the electrically controlled AST
58 which
includes a torque transducer 78 (or alternative sensing method), electronics
package 66 and
the electric motor 64 driven pilot valve 60 within the housing.
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EXAMPLE
Specifications for one embodiment AST illustrated in FIGS. 6-8:
Description Characteristic
Tool OD 3.00 in
Tool ID .75 in
Length - Expanded 8.1 ft
Length ¨ Collapsed 7.4 ft
Stroke 9 in
Max Temp 300 F
Tensile Strength 50,000 lbs
Max Motor Torque 2,000 ft-lbs
Max Dog Leg 25 0/100 ft
Tool Joint 2 3/8 PAC
The design of an electrically controlled AST is flexible in that the pressure
settings
may be changed to fine tune the AST. Programmable pressure settings may be
changed on
surface or while in operation. Current available communication techniques
include mud pulse
telemetry, fiber optic and wireline.
If a large increase in WOB is needed, then the shaft of the AST can be
replaced to
allow the installation of additional pistons.
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CA 02755165 2011-10-12
# of Piston Total Downhole Pressure Control Max
WOB from
Area (sq. in) Valve Setting (psi) AST (lbs)
1 4.8 650 3,055
2 7.9 650 5,135
3 11.0 650 7,150
Features and Benefits:
The following illustrates features of an electrically controlled AST:
= Active WOB: The tool will attempt reset into the fully extended position
when the
pressure falls below 650 psi. If a motor stall has occurred and the AST has
pulled the bit off
bottom, the Active WOB stage will produce a minimum WOB and thrust the bit
downhole
until the PDM pressure exceeds 650 psi.
= Passive WOB: Shuts off the Active WOB stage and allows the coiled tubing
to
transfer WOB to the bit. Prevents excessive WOB that can be developed as PDM
pressure
rises and acts on the pistons producing force downhole.
= Off Bottom: Pulls the bit off bottom to prevent motor stalls.
= Torque section will transfer torque through the AST into the coiled
tubing.
The downhole tool monitors motor pressure and sharply reduces the occurrence
of
motor stalls to thereby increase the overall drilling efficiency by:
* Increasing the average Rate of Penetration (ROP). This is achieved
reducing the
occurrences of pulling off bottom for motor stalls.
= Decreasing the damage to PDMs through repeated motor stalls, thereby
decreasing
occurrence of downhole failure.
= Decreasing the fatigue cycles on the coiled tubing. This increases the
number of wells
a coiled tubing string can service.
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CA 02755165 2011-10-12
By achieving a more efficient drilling operation, the operators can
substantially
increase the cost savings of drilling a well.
Alternative Methods of Operations:
Instead of an absolute pressure transducer, the AST may sense:
* Differential Pressure between annulus and bore using a Differential
Pressure
Transducer
= Load transferred through the tool or Weight on Bit (WOB) using a Load
Cell
= Torque transferred through the tool using a Torque Transducer
= Rotational deceleration using a Torsional Accelerometer
If the system is connected to an electric motor instead of a hydraulic motor,
the AST
may sense:
= Change in voltage
= Change in electric current
The AST system itself may be hydraulically powered, electrically powered,
battery
powered, mechanically assisted, or any combination.
Although the invention has been described in connection with oil well drilling
and use
with a coiled tubing, the invention has other applications, including: jointed
pipe, or rotary
drilling; in operations besides drilling where it is useful to retract a tool
at high pressures; or
where adjustments to the drill bit are made to keep contact with the formation
or to pick up
the bit completely off the formation. Although the invention has been
described with
reference to a drill bit used in drilling oil wells in underground formations,
the invention also
may be used with other pressure-inducing tools such as high pressure jetting
tools.
The anti-stall tool cylinders and valves may be manufactured from various
corrosion-
resistant materials including tungsten carbide, Inconel, high strength nickel
alloyed steel such
as MP35, beryllium-copper, and the like.
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