Note: Descriptions are shown in the official language in which they were submitted.
CA 02587834 2007-05-08
TRI-STABLE ACTUATOR APPARATUS AND METHOD
BACKGROUND OF THE INVENTION
The present invention generally relates to apparatuses and methods to actuate
downhole drilling tools. More particularly, the present invention relates to
downhole
actuators to position a drill bit assembly in a desired trajectory by a rotary
steerable
assembly. More particularly still, the present invention relates to a tri-
stable actuator to
be used in a rotary steerable system to accommodate more precise positioning
of a drill
bit assembly.
Boreholes are frequently drilled into the Earth's formation to recover
deposits of
hydrocarbons and other desirable materials trapped beneath the Earth's crust.
Traditionally, a well is drilled using a drill bit attached to the lower end
of what is known
in the art as a drillstring. The drillstring is a long string of sections of
drill pipe that are
connected together end-to-end through rotary threaded pipe connections. The
drillstring is rotated by a drilling rig at the surface thereby rotating the
attached drill bit.
The weight of the drillstring typically provides all the force necessary to
drive the drill bit
deeper, but weight may be added (or taken up) at the surface, if necessary.
Drilling
fluid, or mud, is typically pumped down through the bore of the drillstring
and exits
through ports at the drill bit. The drilling fluid acts both lubricate and
cool the drill bit as
well as to carry cuttings back to the surface. Typically, drilling mud is
pumped from the
surface to the drill bit through the bore of the drillstring, and is allowed
to return with the
cuttings through the annulus formed between the drillstring and the drilled
borehole wall.
At the surface, the drilling fluid is filtered to remove the cuttings and is
often used
recycled.
CA 02587834 2007-05-08
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Patent Application
In typical drilling operations, a drilling rig and rotary table are used to
rotate a
drillstring to drill a borehole through the subterranean formations that may
contain oil
and gas deposits. At downhole end of the drillstring is a collection of
drilling tools and
measurement devices commonly known as a Bottom Hole Assembly (BHA). Typically,
the BHA includes the drill bit, any directional or formation measurement
tools, deviated
drilling mechanisms, mud motors, and weight collars that are used in the
drilling
operation. A measurement while drilling (MWD) or logging while drilling (LWD)
collar is
often positioned just above the drill bit to take measurements relating to the
properties
of the formation as borehole is being drilled. Measurements recorded from MWD
and
LWD systems may be transmitted to the surface in real-time using a variety of
methods
known to those skilled in the art. Once received, these measurements will
enable those
at the surface to make decisions concerning the drilling operation. For the
purposes of
this application, the term MWD is used to refer either to an MWD (sometimes
called a
directional) system or an LWD (sometimes called a formation evaluation)
system.
Those having ordinary skill in the art will realize that there are differences
between
these two types of systems, but the differences are not germane to the
embodiments of
the invention.
A popular form of drilling is called "directional drilling." Directional
drilling is the
intentional deviation of the wellbore from the path it would naturally take.
In other
words, directional drilling is the steering of the drill string so that it
travels in a desired
direction. Directional drilling is advantageous offshore because it enables
several wells
to be drilled from a single platform. Directional drilling also enables
horizontal drilling
through a reservoir. Horizontal drilling enables a longer length of the
wellbore to
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traverse the reservoir, which increases the production rate from the well. A
directional
drilling system may also be beneficial in situations where a vertical wellbore
is desired.
Often the drill bit will veer off of a planned drilling trajectory because of
the
unpredictable nature of the formations being penetrated or the varying forces
that the
drill bit experiences. When such a deviation occurs, a directional drilling
system may be
used to put the drill bit back on course.
A traditional method of directional drilling uses a bottom hole assembly that
includes a bent housing and a mud motor. The bent housing includes an upper
section
and a lower section that are formed on the same section of drill pipe, but are
separated
by a permanent bend in the pipe. Instead of rotating the drillstring from the
surface, the
drill bit in a bent housing drilling apparatus is pointed in the desired
drilling direction, and
the drill bit is rotated by a mud motor located in the BHA. A mud motor
converts some
of he energy of the mud flowing down through the drill pipe into a rotational
motion that
drives the drill bit. Thus, buy maintaining the bent housing at the same
azimuth relative
to the borehole, the drill bit will drill in a desired direction. When
straight drilling is
desired, the entire drill string, including the bent housing, is rotated from
the surface.
The drill bit angulates with the bent housing and drills a slightly overbore,
but straight,
borehole.
A more modern approach to directional drilling involves the use of a rotary
steerable system (RSS). In an RSS, the drill string is rotated from the
surface and
downhole devices force the drill bit to drill in the desired direction.
Rotating the drill
string is preferable because it greatly reduces the potential for getting the
drillstring
stuck in the borehole. Generally, there are two types of RSS, "point the bit"
systems
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and "push the bit" systems. In a point system, the drill bit is pointed in the
desired
position of the borehole deviation in a similar manner to that of a bent
housing system.
In a push system, devices on the BHA push the drill bit laterally in the
direction of the
desired borehole deviation by pressing on the borehole wall.
A point the bit system works in a similar manner to a bent housing because a
point system typically includes a mechanism to provide a drill bit alignment
that is
different from the drill string axis. The primary differences are that a bent
housing has a
permanent bend at a fixed angle and a point the bit RSS typically has an
adjustable
bend angle that is controlled independent of the rotation from the surface. A
point RSS
typically has a drill collar and a drill bit shaft. The drill collar typically
includes an internal
orienting and control mechanism that counter rotates relative to the rotation
of the
drillstring. This internal mechanism controls the angular orientation of the
drill bit shaft
relative to the borehole. The angle between the drill bit shaft and the drill
collar may be
selectively controlled, but a typical angle is less than 2 degrees. The
counter rotating
mechanism rotates in the opposite direction of the drill string rotation.
Typically, the
counter rotation occurs at the same speed as the drill string rotation so that
the counter-
rotating section maintains the same angular position relative to the inside of
the
borehole. Because the counter rotating section does not rotate with respect to
the
borehole, it is often called "geo-stationary" by those skilled in the art.
Most rotary steerable systems involve the conversion of energy in the drilling
fluids into mechanical energy. Drilling fluids are typically delivered to the
drill bit through
a bore of the drillstring and return through the annulus formed between the
borehole
and the outer diameter of the drillstring. Therefore, because the cross-
sectional area of
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the bore of the drillstring is smaller than the cross-sectional area of the
annulus, the
drillstring bore fluid pressures are significantly higher than those in the
annulus. This
pressure differential is of great importance to drilling operations as it
allows various
devices to use the pressure differential to generate work and power downhole.
The
rotary steerable system is such a device, often employing the pressure
differential
between delivered and returning drilling fluids to activate thrust pads and
bend angle
actuators to achieve the rotary steerable effect of the downhole drilling
apparatus.
Former downhole actuators used with rotary steerable systems acted to divert
the high-pressure drilling fluids from the bore of the drillstring to various
devices to
produce mechanical work and push against the drilled formation in various
directions.
These actuators were typically electromagnetically activated and are
constructed as bi-
stable actuators having only "on" and "off' positions. Typically, to maintain
a bi-stable
electromagnetic actuator into one of its positions, powerful permanent magnets
were
used to retain an armature in the designated position. To overcome the
retaining force
of the permanent magnets, higher currents were necessary to break the armature
free
of the permanent magnets that would have otherwise been necessary to displace
the
armature. As such, former bi-stable actuators consumed more energy in
switching
positions than necessary. An actuator requiring less energy to switch between
two or
more stable positions would be highly desirable in the oilfield today.
SUMMARY OF THE INVENTION
The deficiencies of the prior art are addressed by an actuator to direct a
rotary
steerable tool. The actuator preferably includes a stator having an internal
cavity in
communication with a high-pressure zone, a first hydraulic port and a second
hydraulic
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port. The actuator preferably includes an armature housed within the internal
cavity,
wherein the armature includes a first valve plunger and a second valve
plunger.
Preferably, the first valve plunger is configured to block the first hydraulic
port from the
high-pressure zone when the armature is in a first position and the second
valve
plunger is configured to block the second hydraulic port from the high-
pressure zone
when the armature is in a second position. Preferably, the first and second
hydraulic
ports are in communication with the high-pressure zone when the armature is in
a third
position and the actuator is configured to be electromagnetically displaced
into the first
and second positions.
The deficiencies of the prior art are also addressed by a method to operate a
rotary steerable tool with a tri-stable actuator. The method preferable
includes installing
an armature within an internal cavity of a stator, wherein the stator has a
first hydraulic
port and a second hydraulic port. The method preferably includes hydraulically
communicating between a high-pressure zone and the internal cavity. The method
preferably includes blocking the first hydraulic port and communicating
between the
high-pressure zone and the second hydraulic port when the stator is in a first
position.
The method preferably includes blocking the second hydraulic port and
communicating
between the high-pressure zone and the first hydraulic port when the stator is
in a first
position. The method preferably includes communicating between the high-
pressure
zone and both first and second hydraulic ports when the stator is in a third
position. The
method preferably includes displacing the stator into the first and the second
positions
with electromagnetic force.
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Patent Application
The deficiencies of the prior art are also addressed by a tri-stable actuator.
The
tri-stable actuator preferably includes a stator having an internal cavity in
communication with a high-pressure zone, a first hydraulic port and a second
hydraulic
port. The tri-stable actuator preferably includes an armature housed within
the internal
cavity, wherein the armature includes a first valve plunger and a second valve
plunger.
Preferably, the first valve plunger is configured to block the first hydraulic
port from the
high-pressure zone when the armature is in a first stable position.
Preferably, the
second valve plunger is configured to block the second hydraulic port from the
high-
pressure zone when the armature is in a second stable position. Preferably,
the first
and second hydraulic ports are in communication with the high-pressure zone
when the
armature is in a third stable position and the actuator is configured to be
electromagnetically displaced into the first and second stable positions.
Preferably, the
armature is configured to be held into the first stable position by the force
of fluids
flowing from the high-pressure zone through the second port and held into the
second
stable position by the force of fluids flowing from the high-pressure zone
through the
first port.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic representation of a tri-stable actuator in accordance
with
an embodiment of the present invention.
Figure 2 is a schematic representation of the tri-stable actuator of Figure 1
engaged in a first position.
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Figure 3 is a schematic representation of the tri-stable actuator of Figure 1
engaged in a second position.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring initially to Figure 1, a tri-stable actuator assembly 100 in
accordance
with an embodiment of the present invention is shown schematically. Tri-stable
actuator
assembly 100 is shown having a stator 102 and an armature 104. Stator 102
preferably
is in communication with high-pressure fluids and has two ports 106, 108 in
communication with lower pressure zones. Actuator 100 selectively diverts high-
pressure flow (e.g. drilling fluids) from internal cavity 110 to low-pressure
ports 106,
108. Biasing springs 112, 114 between stator 102 and armature 104 centralize
armature 104 along its axis 116 within stator 102 when no other forces are
present.
Armature 102 is shown in Figure 1 in a neutral position, one where both port
106 and
108 are unobstructed and in communication with high-pressure fluids inside
cavity 110.
Because ports 106 and 108 are of substantially the same size and geometry and
because armature 104 is symmetrical, high-pressure fluid flowing from cavity
110
through ports 106,108 will not displace armature 104 with respect to stator
102 along
axis 116. As such, with armature 102 in position shown in Figure 1, high-
pressure fluid
flows through both ports 106 and 108 at substantially the same rate to drive
any
equipment attached thereto with equal amounts of energy.
Furthermore, stator 102 and armature 104 are preferably components of an
electromagnetic system whereby armature is thrust along its axis 116 to close
ports 106
and 108. Typically, an electromagnetic system comprises a magnetic field and
an
electric coil. The field is preferably a permanent magnet but can be
constructed as an
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electromagnet, one that requires electric current to be magnetized, if
desired. The
electric coil is preferably constructed as a coil of electrically conductive
wire. The
number of coils, the gauge of the wire, and the amount and potential of
current
applied thereto designate the amount of electromagnetic force. Depending on
the
polarity of the electrical charge applied to the coil, the coil will create a
magnetic force
that interacts with the magnetic properties of the field. If the poles of the
field and the
coil are reversed, they are attracted to one another. If the poles are
aligned, then
they are repelled.
In Figure 1, stator 102 is shown constructed as field and armature 104
is constructed as a coil. While this configuration is preferred, it should be
understood
that alternate configurations could be accomplished by one of ordinary skill
without
departing from the scope of the present invention. When constructed as a coil,
Armature 104 is able to conserve mass. The operation of armature 104 is more
efficient and exhibits greater responsiveness when the mass thereof is
minimized.
Stator 102 is shown constructed from a ferromagnetic material having a
positive
pole (+) and a negative pole (-). Alternatively, stator 102 can be constructed
as an
electromagnetic device also having a coil so that the magnitude and polarity
of the
magnetic field generated thereby can be varied or reversed. With no current
applied
to coil of armature 104, springs 112 and 114 center armature upon axis 116 and
allow hydraulic communication between both ports 106 and 108 and high-pressure
fluids within cavity 110. When either port 106 or 108 is intended to be
closed, an
electrical current is applied to the coil of armature 104 and one of two
plungers
118, 120 is driven into port 106 or 108 depending on the polarity of armature
104 with
respect to the polarity of stator 102.
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Patent Application
If the forces from springs 112, 114 are not strong enough to maintain armature
104 in a centralized position between ports 106 and 108, an electromagnetic
device 122
of stator 102 can be used in conjunction with a corresponding electromagnetic
device
124 of armature 104 to retain armature 104. Using electromagnetic devices 122,
124,
armature 104 can be kept clear of ports 106, 108 in circumstances where high
turbulent
flow from high-pressure cavity 110 through ports 106, 108 might otherwise
cause
movement of armature 104 along axis 116. Furthermore, since it can be
difficult to
extend electrical leads to armature 104 from inside cavity 110 of stator 102,
springs
112, 114 can optionally be constructed as electrical conductors to activate
coils within
armature 104. Alternatively, armature 104 can be constructed as a permanent
magnetic field and stator 102 can contain the electrical coil.
Referring briefly now to Figure 2, actuator assembly 100 is shown with
armature
102 displaced such that plunger 118 is in engaged within port 106 allowing
high-
pressure fluids to flow from cavity 110 only through port 108. The
particularities of the
seal between plunger 118 and port 106 (or plunger 120 and port 108) are not
shown,
but as long as engagement of plunger 118 into or against port 106 acts to seal
off port
106 with sufficient integrity, the actuator assembly 100 functions as desired.
Therefore,
port 106 can include a socket configured to seal with plunger 118 or the face
of plunger
118 can alternatively be configured to seal access to port 106 without
engagement
therein.
Furthermore, once plunger 118 of actuator assembly 100 engages port 106,
electromagnetic force is no longer required to retain armature 104 in its
displaced
position so long as pressure of fluids flowing from cavity 110 through port
108 is
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maintained. High-pressure fluids flowing from cavity 110 through port 108 act
upon face
126 and thrust plunger 118 further into engagement with port 106. As such,
considerable electrical energy is conserved in that actuator assembly 100 is
considered
"stable" in the position shown in Figure 2. To disengage plunger 118 from port
106,
only enough current is needed to activate to coil within armature 104 to
overcome the
pressure forces thrusting against face 126. Former systems either required
continuous
current to flow through coil of armature 104 or permanent magnets to retain a
plunger
within a port. The former systems necessitated that current to be consumed
throughout
the engagement period and the latter system required elevated current to
disengage the
armature from the permanent magnet holding the plunger in place. The actuator
assembly 100 of the present invention only consumes electrical energy when the
position of armature 104 is changed and requires less current than prior art
systems in
changing that position.
Referring briefly to Figure 3, actuator assembly 100 is shown with armature
104
displaced such that plunger 120 is engaged with port 108. Like the position of
armature
104 in Figure 2, the position of armature 104 shown in Figure 3 is also
"stable" when no
current flows through armature 104 or stator 102 as high-pressure fluids
flowing from
cavity 110 through port 106 act upon face 128 to keep plunger 120 engaged.
Similarly,
as there is no magnetic force holding plunger 120 in engagement with port 108,
electrical current in coil of armature 102 is only required to overcome the
force of the
flow against face 128.
Actuator assembly 100 has three "stable" positions, shown in Figures 1, 2, and
3
respectively, enabling various modes of communication between high-pressure
fluids in
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Patent Application
cavity 110 and ports 106 and 108. Particularly, Actuator 100 has three
selectable
modes: 1) port 106 in communication with cavity 110; 2) port 108 in
communication
with cavity 110; and 3) both ports 106 and 108 in simultaneous communication
with
cavity 110. With these three modes, a rotary steerable system (or any other
downhole
tool) using tri-stable actuator 100 in accordance with the present invention
can divert
drilling fluids in one of three ways to enable the tool to be more precisely
controlled and
much more efficiently.
Former systems consumed more electrical energy than the tri-stable actuator
100 of the present invention and only provided for two positions, on and off.
Therefore,
former systems required the installation and control of several actuators to
completely
control a downhole tool. Using actuators in accordance with the present
invention, the
tool designer has the option of either using fewer actuators (thereby
conserving
valuable space) or using the same number of actuators, but with much more
precise
control than previously possible. Most downhole systems operate on limited
power
supplies that are either generated downhole or delivered and stored downhole
in lithium
battery packs. Because the power output of these devices is always finite, a
reduction
in electrical power consumption for downhole equipment is highly desirable.
Numerous embodiments and alternatives thereof have been disclosed. While
the above disclosure includes the best mode belief in carrying out the
invention as
contemplated by the inventors, not all possible alternatives have been
disclosed. For
that reason, the scope and limitation of the present invention is not to be
restricted to
the above disclosure, but is instead to be defined and construed by the
appended
claims.
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