U.S. patent application number 12/824965 was filed with the patent office on 2010-10-21 for directional drilling control using modulated bit rotation.
This patent application is currently assigned to PRECISION ENERGY SERVICES, INC.. Invention is credited to Steven Reid Farley.
Application Number | 20100263933 12/824965 |
Document ID | / |
Family ID | 40405636 |
Filed Date | 2010-10-21 |
United States Patent
Application |
20100263933 |
Kind Code |
A1 |
Farley; Steven Reid |
October 21, 2010 |
DIRECTIONAL DRILLING CONTROL USING MODULATED BIT ROTATION
Abstract
A system for steering the direction of a borehole advanced by
cutting action of a rotary drill bit by periodically varying the
rotation speed of the drill bit. The steering system comprises a
motor disposed in a bent housing subsection and operationally
connected to a drill string and to the drill bit. The rotation
speed of the drill bit is periodically varied by periodic varying
the rotation speed of the motor or by periodic varying the rotation
speed of the drill string. Periodic bit speed rotation results in
preferential cutting of material from a predetermined arc of the
borehole wall which, in turn, resulting in borehole deviation. Both
the drill string and the drill motor are rotated simultaneously
during straight and deviated borehole drilling.
Inventors: |
Farley; Steven Reid;
(Magnolia, TX) |
Correspondence
Address: |
(Weatherford) Wong Cabello Lutsch Rutherford &Brucculeri LLP
20333 Tomball Parkway, 6th floor
Houston
TX
77070
US
|
Assignee: |
PRECISION ENERGY SERVICES,
INC.
Fort Worth
TX
|
Family ID: |
40405636 |
Appl. No.: |
12/824965 |
Filed: |
June 28, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11848328 |
Aug 31, 2007 |
7766098 |
|
|
12824965 |
|
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Current U.S.
Class: |
175/92 |
Current CPC
Class: |
E21B 33/038 20130101;
E21B 7/067 20130101; E21B 7/068 20130101 |
Class at
Publication: |
175/92 |
International
Class: |
E21B 4/00 20060101
E21B004/00 |
Claims
1. An apparatus for drilling a borehole, said apparatus comprising:
a drill string; a drill bit motor; and a drill bit; wherein said
drill string and said drill bit motor are operationally connected
to said drill bit to rotate said drill bit independently; and said
borehole is deviated by periodic varying rotation speed of said
drill bit during a bit rotation.
2. The apparatus of claim 1 wherein said periodic variation of said
drill bit speed is obtained by periodic variation of rotation speed
of said drill bit motor.
3. The apparatus of claim 1 wherein said periodic variation of said
drill bit speed is obtained by periodic variation of rotation speed
of said drill string.
4. The apparatus of claim 1 wherein said drill string and said
drill bit motor are rotated simultaneously to periodically vary
said drill bit rotation speed.
5. The apparatus of claim 1 further comprising a bent subsection
attached to a lower end of said drill string; wherein said drill
bit motor is disposed within said bent subsection; and said drill
bit is attached to a shaft of said drill bit motor.
6. A directional borehole assembly terminating a downhole end of a
drill string, said assembly comprising: a drill bit motor disposed
within a bent housing; auxiliary sensors indicating orientation and
position of said borehole assembly within said borehole; a
telemetry system for communicating between said borehole assembly
and the surface of the earth; and a downhole processor; wherein
said drill string and said drill bit motor are operationally
connected to said drill bit to rotate said drill bit independently;
said borehole is deviated by periodic varying rotation speed of
said drill bit during each bit rotation; and said periodic varying
rotation speed of said drill bit is defined by combining, within
said downhole processor, responses of said auxiliary sensors with
rotation information telemetered from said surface of the
earth.
7. The assembly of claim 6 wherein said periodic variation of said
drill bit speed is obtained by periodic variation of rotation speed
of said drill bit motor.
8. The assembly of claim 6 wherein said periodic variation of said
drill bit speed is obtained by periodic variation of rotation speed
of said drill string.
9. The assembly of claim 6 wherein said drill string and said drill
bit motor are rotated simultaneously.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation application of U.S.
patent application Ser. No. 11/848,328, entitled "Directional
Drilling Control Using Modulated Bit Rotation", filed Aug. 31, 2007
and which is incorporated by reference in its entirety, and to
which priority is claimed.
FIELD OF THE INVENTION
[0002] This invention is related to the directional drilling of a
well borehole. More particularly, the invention is related to
steering the direction of a borehole advanced by a rotary drill bit
by periodically varying rotational speed of the drill bit during a
revolution of the drill string to which the drill bit is
operationally connected.
BACKGROUND
[0003] The complex trajectories and multi-target oil wells require
precision placement of well borehole path and the flexibility to
continually maintain path control. It is preferred to control or
"steer" the direction or path of the borehole during the drilling
operation. It is further preferred to control the path rapidly
during the drilling operation at any depth and target as the
borehole is advanced by the drilling operation.
[0004] Directional drilling is complicated by the necessity to
operate a drill bit steering device within harsh borehole
conditions. The steering device is typically disposed near the
drill bit, which terminates a lower or "down hole" end of a drill
string. In order to obtain the desired real time directional
control, it is preferred to operate the steering device remotely
from the surface of the earth. Furthermore, the steering device
must be operated to maintain the desired path and direction while
being deployed at possibly a great depth within the borehole and
while maintaining practical drilling speeds. Finally, the steering
device must reliably operate under exceptional heat, pressure, and
vibration conditions that can be encountered during the drilling
operation.
[0005] Many types of directional steering devices, comprising a
motor disposed in a housing with an axis displaced from the axis of
the drill string, are known in the prior art. The motor can be a
variety of types including electric, or hydraulic. Hydraulic
turbine motors operated by circulating drilling fluid are commonly
known as a "mud" motors. A rotary bit is attached to a shaft of the
motor, and is rotated by the action of the motor. The axially
offset motor housing, commonly referred to as a bent subsection or
"bent sub", provides axial displacement that can be used to change
the trajectory of the borehole. By rotating the drill bit with the
motor and simultaneously rotating the drill bit with the drill
string, the trajectory or path of the advancing borehole is
parallel to the axis of the drill string. By rotating the drill bit
with the motor only, the trajectory of the borehole is deviated
from the axis of the drill string. By alternating these two
methodologies of drill bit rotation, the path of the borehole can
be controlled. A more detailed description of directional drilling
using the bent sub concept is presented in U.S. Pat. Nos.
3,713,500, 3,841,420 and 4, 492,276, which are herein entered into
this disclosure by reference.
[0006] The prior art contains methods and apparatus for adjusting
the angle of "bend" of a bent sub housing thereby directing the
angle of borehole deviation as a function of this angle. The prior
art also contains apparatus and methods for dealing with unwanted
torques that result from steering operations including clutches
that control relative bit rotation in order to position the bit
azimuthally as needed within the walls of the borehole. Prior art
steering systems using variations of the bent sub concept typically
rely upon complex pushing or pointing forces and the associated
equipment which directs the hole path by exerting large pressures
on the bit perpendicular to the borehole path while rotating the
drill string. These forces are often obtained using hydraulic
systems that are typically expensive and present additional
operational risks in the previously mentioned harsh drilling
environment. Furthermore, these perpendicular forces typically
require the steering device to be fabricated with mechanically
strong components thereby further increasing the initial and
operating cost of the steering device.
SUMMARY OF THE INVENTION
[0007] This invention comprises apparatus and methods for steering
the direction of a borehole advanced by cutting action of a rotary
drill bit terminating a lower or "down hole" end of a drill string.
The rotation speed of the bit is periodically varied during a
rotation of the drill string thereby cutting a disproportionately
larger amount of material from an azimuthal arc of wall of the
borehole, which will results in an azimuthal deviation in borehole
direction.
[0008] The steering device, which is disposed at the downhole end
of a drill string, comprises a motor disposed in a bent housing
subsection or "bent sub". A rotary drill bit is attached to a shaft
of the motor. The drill bit is rotated by both the motor and by the
rotary action of the drill string.
[0009] As stated above, the steering system is designed so that the
rotating drill bit disproportionally cuts material along the wall
of the borehole in a predetermined azimuthal arc to direct the
advancement of the borehole in a desired trajectory. In the
disclosed examples of the invention, the rotation rate of the bit
is periodically slowed in this predetermined arc cutting a
disproportionally small amount of material from the borehole wall.
As a result, the bit moves to the opposite side of the borehole and
cuts disproportionately larger amount of material from the borehole
wall. The borehole then tends to deviate and advance in the
azimuthal direction in which the disproportional large amount of
borehole wall material has been removed.
[0010] The removal of material from the wall of the borehole, thus
the steering of the borehole trajectory, is accomplished by
periodically varying the rotational speed of the drill bit during a
rotation of the drill string. The steering system uses two elements
for rotating the drill bit. The first element used to rotate the
drill bit is the rotating drill string. The second element used to
rotate the drill bit is the motor disposed within the bent sub and
operationally connected to the drill bit. The final drill bit
rotational speed is the sum of the rotational speeds provided by
the drill string and the motor.
[0011] It is preferred that both the drill string and the motor
rotate simultaneously. If a constant borehole trajectory is
desired, both the drill string and motor rotation speeds are held
constant throughout a drill string revolution. The procession of
the bit rotation around the borehole removes essentially the same
amount of material azimuthally around the borehole wall. If a
deviated borehole trajectory is desired, the rotation speed of the
drill bit is varied as it passes through a predetermined azimuthal
sector of the borehole wall. This periodic variation in bit speed
can be accomplished by periodically varying the rotational speed of
the motor, or by periodically varying the rotational speed of the
drill string. Both methodologies remove disproportionately small
amounts from one side of the borehole and remove disproportionately
larger amounts of material the opposite side of the borehole. The
borehole is deviated in the direction of disproportionately large
amount of material removal. Both methodologies will be discussed in
detail in subsequent sections of this disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The manner in which the above recited features and
advantages, briefly summarized above, are obtained can be
understood in detail by reference to the embodiments illustrated in
the appended drawings.
[0013] FIG. 1 illustrates borehole assembly comprising a bent sub
and motor disposed in a well borehole by a drill string
operationally attached to a rotary drilling rig;
[0014] FIG. 2 is a cross section of a cylindrical borehole and is
used to define certain parameters used in the steering methodology
of the invention;
[0015] FIG. 3 is a cross section of a borehole in which the
rotation speed of the borehole has been varied thereby removing a
disproportionately small amount of material from one side of the
borehole and a disproportionately large amount of material from the
opposite side of the borehole;
[0016] FIG. 4a is a plot of a constant rate of rotation of the
drill string as a function of a plurality of rotational cycles;
[0017] FIG. 4b is a plot of a periodic decreasing rotation rate of
the motor as a function of a plurality of drill string
rotations;
[0018] FIG. 4c is a plot of a periodic decreasing and periodic
increasing rotation rate of the motor as a function of a plurality
of drill string rotation cycles;
[0019] FIG. 5a is a plot of a periodic decreasing rotation rate of
the drill string as a function of a plurality of drill string
rotations; and
[0020] FIG. 5b is a plot of a constant rate of rotation of the
motor as a function of a plurality of rotational cycles.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] This invention comprises apparatus and methods for steering
the direction of a borehole advanced by cutting action of a rotary
drill bit. The invention will be disclosed in sections. The first
section is directed toward hardware. The second section details
basic operating principles of the invention. The third section
details two embodiments of the invention that will produce the
desired borehole steering results.
[0022] Directional drilling is obtained by periodically varying the
rotation rate of the drill bit. For purposes of this disclosure
"periodic variation" is defined as varying the drill bit rotation
speed in a plurality of 360 degree drill string rotations or
"cycles" at the same azimuthal arc in the plurality of
rotations.
Hardware
[0023] Attention is directed to FIG. 1, which illustrates a
borehole assembly (BHA) 10 suspended in a borehole 30 defined by a
wall 50 and penetrating earth formation 36. The upper end of the
BHA 10 is operationally connected to a lower end of a drill pipe 35
by means of a suitable connector 20. The upper end of the drill
pipe 35 is operationally connected to a rotary drilling rig, which
is well known in the art and represented conceptually at 38.
Surface casing 32 extends from the borehole 30 to the surface 44 of
the earth. Elements of the steering apparatus are disposed within
the BHA 10. Motor 14 is disposed within a bent sub 16. The motor 14
can be electrical or a Monyo or turbine type motor. A rotary drill
bit 18 is operationally connected to the motor 14 by a motor shaft
17, and is rotated as illustrated conceptually by the arrow
R.sub.B.
[0024] Again referring to FIG. 1, the BHA 10 also comprises an
auxiliary sensor section 22, a power supply section 24, an
electronics section 26, and a downhole telemetry section 28. The
auxiliary sensor section 22 comprises directional sensors such as
magnetometers and inclinometers that can be used to indicate the
orientation of the BHA 10 within the borehole 30. This information,
in turn, is used in defining the borehole trajectory path for the
steering methodology. The auxiliary sensor section 22 can also
comprise other sensors used in Measurement-While-Drilling (MWD) and
Logging-While-Drilling (LWD) operations including, but not limited
to, sensors responsive to gamma radiation, neutron radiation and
electromagnetic fields. The electronics section 26 comprises
electronic circuitry to operate and control other elements within
the BHA 10. The electronics section 26 preferably comprise downhole
memory (not shown) for storing directional drilling parameters,
measurements made by the sensor section, and directional drilling
operating systems. The electronic section 26 also preferably
comprises a downhole processor to process various measurement and
telemetry data. Elements within the BHA 10 are in communication
with the surface 44 of the earth via a downhole telemetry section
28. The downhole telemetry section 28 receives and transmits data
to an uphole telemetry section (not shown) preferably disposed
within surface equipment 42. Various types of borehole telemetry
systems are applicable including mud pulse systems, mud siren
systems, electromagnetic systems and acoustic systems. A power
supply section 24 supplies electrical power necessary to operate
the other elements within the BHA 10. The power is typically
supplied by batteries.
[0025] Once again referring to FIG. 1, drilling fluid or drilling
"mud" is circulated from the surface 44 downward through the drill
string comprising the drill pipe and BHA 10, exits through the
drill bit 18, and returns to the surface via the borehole-drill
string annulus. Circulation is illustrated conceptually by the
arrows 12. The drilling fluid system is well known in the art and
is represented conceptually at 40. If the motor 14 is a turbine or
"mud" motor, the downward flow of drilling fluid imparts rotation
to the drill bit 18 through the shaft 17, as indicated by the arrow
R.sub.M. For purposes of illustration in FIG. 1, it is assumed that
the motor 14 is a mud motor. The steering system utilizes a
periodic variation in the rotational speed of the drill bit 18 in
defining trajectory of the advancing borehole 30. In one embodiment
of the invention, the rotational speed of the drill bit 18 is
periodically varied by periodically varying the rotation of the
motor 14. Since in FIG. 1 it is assumed that the motor 14 is a mud
motor, rotational speed is varied by varying drilling fluid flow
through the mud motor. This is accomplished with a fluid flow
restriction or fluid release element which can be disposed within
the drill string (as shown conceptually at 39) or at the surface 44
within (not shown) the mud pump system 40. The fluid flow
restriction or fluid release element is illustrated with broken
lines since it is not needed if the motor 14 is electric. Although
a mud motor is assumed from purposes of discussion, an electrical
motor can also be used eliminating the need for the fluid flow
restriction or fluid flow release element 39. Electric motor speed
is controlled electrically by the cooperating electronics section
26 and power supply sections 24. The connection between the power
supply section 24 and the motor 14 is shown as a broken line since
the connection is not needed if the motor is of the turbine
type.
[0026] Still referring to FIG. 1, the rotary rig 38 imparts an
additional rotation component, indicated conceptually by the arrow
R.sub.D, to the rotary drill bit 18 by rotating the drill pipe 35
and BHA 10. Drill string rotation speed is typically controlled
from the surface, using the surface equipment 42, based upon
predetermined trajectory information or from BHA orientation
information telemetered from sensors in the auxiliary sensor
section 22. Motor rotation speed (indicated conceptually by the
arrow R.sub.M) is typically controlled by signals telemetered from
the surface using BHA 10 position and orientation information
measured by the auxiliary section 22 and telemetered to the
surface. Alternately, motor rotational speed R.sub.M can be
controlled using orientation information measured by the auxiliary
sensor section cooperating with predetermined control information
stored in a downhole processor within the electronics section
26.
Basic Operating Principles
[0027] The BHA 10 shown in FIG. 1, when rotated at a constant
rotation speed within the borehole 30, sweeps a circular path
drilling a borehole slightly larger than the diameter of the drill
bit 18. This larger diameter, defined by the borehole wall 50, is
due to the angle defined by the axis of the drill pipe 35 and the
axis of the bent sub housing 16.
[0028] As discussed previously, two components of drill bit
rotation are present. The first component results from the action
of the drilling rig 38 that rotates the entire drill string at a
rotation rate of R.sub.D. The second component of rotation results
from the action of the motor 10 that rotates the bit at a rate
R.sub.M. The rotation speed of the drill bit, R.sub.B, is the sum
of these two components. Stated mathematically, the rotation speed
R.sub.B
R.sub.B=R.sub.D+R.sub.M (1)
[0029] As shown above, the two components R.sub.D and R.sub.M
comprising the final drill bit rotation speed R.sub.B are generally
considered separable where directional control is required. As a
prior art example, if R.sub.D is set to zero, then the motor 14
will continue to turn the drill bit 18 at a rotation speed R.sub.M.
The drill bit will increase borehole deviation angle at a constant
azimuthal angle defined by the position of the non rotating bent
sub 16, with the drill string sliding down the borehole behind the
advancing drill bit. Alternately, if a constant trajectory hole is
require to be drilled, then the drill string rotation R.sub.D is
initiated along with motor rotation R.sub.M, the azimuthal angle of
the bent sub 16 is no longer constant due to the rotation of the
BHA 10, and the drill bit rotating at R.sub.B=R.sub.M+R.sub.D cuts
equally into all sides of hole.
[0030] In the periodic procession of the drill bit around the wall
of the borehole described above, where R.sub.D and R.sub.M are not
equal to zero, the drill bit 18 cuts a different azimuthal section
of the hole as a function of procession time. It is during this
periodic drill bit procession that R.sub.B can be instantaneously
and periodically changed during each revolution of the BHA 10 to
preferentially cut one side of the hole at a different rate than it
cuts the opposite side of the hole. This also results in increasing
borehole deviation angle, while still rotating the drill string.
There are operational advantages to continue to rotate the drill
string, as will be discussed in a subsequent section of this
disclosure. The periodic change in R.sub.B per revolution of the
drill string can be implemented by varying either R.sub.D or
R.sub.M, as will be discussed in detail in subsequent sections of
this disclosure.
[0031] FIG. 2 is a cross section of a cylindrical borehole 30 and
is used to define certain parameters used in the steering
methodology. The center of the borehole is indicated at 52, and a
borehole or "zero" azimuthal reference angle is indicated at 51.
For purposes of discussion, assume that R.sub.D and R.sub.M are non
zero, and during the procession of the drill bit within the
borehole, the drill bit rotation speed R.sub.B=R.sub.D+R.sub.M is
decreased to a value R.sub.Bd beginning essentially at speed
variation angle .alpha. indicated at 54 and continued through a
"dwell" angle of magnitude .sigma. indicated at 60. The azimuthal
position of the variation angle .alpha. angle is preferably defined
with respect to the reference angle 51. The bit rotation speed then
resumes essentially to R.sub.B for the remainder of the 360 degree
rotation cycle. The instantaneous and periodic change from R.sub.B
to R.sub.Bd can be obtained by decreasing either R.sub.D or R.sub.M
(or both), as will be discussed in subsequent sections of this
disclosure. This decrease in cutting power during the dwell angle
.sigma. (shown at 60) will leave a surplus of borehole wall
material essentially at the azimuthal dwell angle .sigma.. This
surplus of material naturally causes the drill bit to move radially
to the opposite side of the hole to an azimuthal arc section
.sigma./2 is indicated at 57 that terminates at an angle .beta.,
where:
.beta.=.alpha.-180.degree.+.sigma./2 (2)
and .beta. is indicated at 56. Drill bit rotation speed through the
arc .sigma./2 to the angle .beta. is R.sub.B or greater which is,
of course, greater than R.sub.Bd. This results in the removal of a
disproportionally large amount of borehole wall material
essentially in the azimuthal arc 57 thereby deviating the borehole
in this azimuthal direction.
[0032] The previously discussed effects of varying the drill bit
rotation speed are illustrated conceptually in the borehole cross
sectional view of FIG. 3. Drill bit rotation speed is reduced from
R.sub.B to R.sub.Bd when the bit reaches angle .alpha. denoted at
54. The drill bit in this azimuthal position is depicted as 18a.
Because of the reduction in bit rotation speed, there is an excess
of material along the borehole wall at 50a, which corresponds to
the dwell angle .sigma. shown in FIG. 2. Drill bit rotational speed
is subsequently increased to R.sub.B, and the bit moves to the
opposite side of the borehole 30 to the azimuthal arc 57
terminating at angle .beta.. The drill bit in this position is as
depicted conceptually at 18b. With the drill bit rotating at
R.sub.B or faster (due to lack of resistance in moving across the
borehole), a disproportionally large amount of borehole wall is
removed at 50b. By periodically reducing the rotation speed of the
bit at the speed variation angle .alpha. as the BHA rotates within
the borehole 30, the angle of borehole deviation continues to build
in the azimuthal region defined by the arc 57 and the angle
.beta..
[0033] It should be understood that borehole deviation can also be
obtained by periodically increasing R.sub.B thereby removing a
disproportional amount of borehole wall at the angle of periodic
rotation increase.
Techniques For Periodically Varying Bit Rotation Speed
[0034] Equation (1) illustrates mathematically that drill bit
rotation speed R.sub.B can be varied by varying either the motor
rotation speed R.sub.M or the drill string rotation speed
R.sub.D.
[0035] FIGS. 4a, 4b and 4c illustrate graphically methodology for
periodically varying R.sub.B by periodically varying R.sub.M and
holding R.sub.D at a constant.
[0036] Curve 70 in FIG. 4a represents R.sub.D as a function of
angle through which the BHA 10 is rotated. Expanding on the
examples discussed above and illustrated in FIGS. 2 and 3, the
reference or "zero" angle is again denoted at 51. A complete 360
degree BHA rotation cycle is represented at 59, with three such
cycles being illustrated. The drill string is, therefore, rotating
at a constant speed R.sub.D shown at 53.
[0037] With the drill string rotating at a constant value of 53,
curve 72 in FIG. 4b represents drill bit rotation speed R.sub.M as
a function of angle through which the BHA 10 is rotated. As in FIG.
4a, the reference angle for a drill string rotation cycle is
denoted at 51, with three cycles 59 again being depicted. Further
expanding on the examples discussed above and illustrated in FIGS.
2 and 3, R.sub.M is periodically decreased, as indicated by
excursions 76, to a value at 74 beginning at an angle 54 (which
corresponds to the speed variation angle .alpha.) for a dwell angle
of 60 (which corresponds to the dwell angle of magnitude .sigma.).
This variation in R.sub.M is repeated periodically during rotation
cycles of the drill string.
[0038] As discussed previously, a decrease in bit rotation on one
side of the borehole causes the drill bit to move to the opposite
side of the borehole where bit rotation speed returns to normal or
even increases. FIG. 4c is an illustration similar to FIG. 4b, but
illustrates a periodic decrease and increase in R.sub.M. The
excursions 76 again illustrate a decrease in R.sub.M to a value 74
at azimuthal angle 54 (corresponding to the angle .alpha.). In
addition, the excursions 78 illustrate an increase in the value of
R.sub.M to 80 at azimuthal arc 57 terminating at angle 56
(corresponding to the angle .beta.).
[0039] Considering illustrations shown In FIGS. 4a, 4b and 4c, it
can be seen that when R.sub.D is held constant and R.sub.M is
varied periodically, the rotation speed or the drill bit
R.sub.B=R.sub.D+R.sub.M is varied periodically thereby resulting in
the desired borehole deviation.
[0040] The periodic variation in R.sub.M can be controlled in real
time while drilling using various techniques. Attention is again
directed to FIG. 1 as well as FIGS. 4a, 4b and 4c. These real time
steering methods typically utilize BHA 10 orientation and position
measured with sensors within the auxiliary sensor section 22. A
first method comprises the storing of a plurality of drill bit
rotation speed variation responses (as a function of .alpha. and
.sigma.) within downhole memory in the electronics section 26. An
appropriate sequence is then selected by a signal telemetered from
the surface based upon BHA orientation telemetered to the surface
along with the known borehole target. The appropriate sequence is
typically determined using a surface processor within the surface
equipment 42. This method is similar to the "look-up table" concept
used in numerous electronics systems. A second method comprises
telemetering values of .alpha. and .sigma. from the surface
equipment 42 to the BHA 10 to direct the drilling to the target.
The values of .alpha. and .sigma. are again selected by considering
both BHA orientation data (measured with sensors disposed in the
auxiliary sensor section 22) telemetered to the surface and the
directional drilling target. Telemetered values of speed variation
and dwell angles .alpha. and .sigma., respectively, are input into
an operating program preferably resident in a downhole processor
within the electronics section 26. Output supplied by the downhole
processor is then used to control and periodically vary the
rotation speed of the motor 14 to direct the borehole 30 to a
desired formation target. Stated summarily, periodic varying
rotation speed of said drill bit is defined by combining, within
said downhole processor, responses of the auxiliary sensors with
rotation information telemetered from said surface of the
earth.
[0041] It should be understood that other techniques can be used to
obtain periodic variations in R.sub.M including, but not limited
to, the use of preprogrammed variation instructions stored in
downhole memory of the electronics section 26 and combined with
measured BHA orientation data using sensors in the auxiliary sensor
section 22. This method requires no real time telemetry
communication with the surface equipment 42.
[0042] The rotation speed of the bit R.sub.B can also be varied by
varying R.sub.D, the rotation speed of the drill string. Attention
is directed to FIGS. 5a and 5b. Curve 95 of FIG. 5b shows the motor
14 rotating at a constant speed R.sub.M 97 as a function of angle
through which the BHA 10 is rotated. As in FIGS. 4a, 4b and 4c, the
reference angle for a drill string rotation cycle is denoted at 51,
with three drill string rotation cycles 59 again being depicted.
FIG. 5a shows the rotation speed R.sub.D of the drill string being
periodically varied. Using again the previously discussed example,
the first rotation R.sub.D is periodically decreased, as indicated
by the excursions 92, to a second rotation speed at 93 beginning at
a speed variation angle 54 (which corresponds to the angle .alpha.)
for a dwell angle of 60 (which corresponds to the angle .sigma.).
This variation in R.sub.D between the first and second rotation
speeds is repeated periodically during rotation cycles of the drill
string.
[0043] Considering illustrations shown In FIGS. 5a and 5b, it can
bee seen that when R.sub.M is held constant and R.sub.D is varied
periodically, the rotation speed or the drill bit
R.sub.B=R.sub.D+R.sub.M is varied periodically thereby resulting in
the desired borehole deviation.
[0044] The periodic variation in R.sub.B is typically controlled at
the surface of the earth using the surface equipment 42 (into which
values of .alpha. and .sigma. are input) cooperating with the
rotary table (not shown) of the drilling rig 38.
[0045] It should be understood that the rate at which a borehole
deviation angle is built depends upon a number of factors including
the magnitude of increase or decrease of the periodic variation of
the rotation speed of the drill bit. For a given variation of drill
bit rotation speed, the value of R.sub.B can be varied at
periodically staggered drill string rotation cycles, such as every
other rotation, every third rotation, every fourth rotation, and
the like. It should also be understood that R.sub.B can be varied
by periodically and synchronously varying both R.sub.D and R.sub.M
using techniques disclosed above.
[0046] In an alternate embodiment of the invention, two telemetry
systems are used. A first system is dedicated controlling the
periodic variation of the drill bit rotation speed R.sub.B. A
second telemetry system is dedicated to telemetering measurements
made by sensors disposed within the auxiliary sensor section 22 of
the BHA 10.
SUMMARY
[0047] This invention comprises apparatus and methods for steering
the direction of a borehole advanced by cutting action of a rotary
drill bit. Steering is accomplished by periodically varying, during
a 360 degree rotation cycle of the drill string, the rotation speed
of the drill bit thereby preferentially cutting differing amounts
of material from the wall of the borehole within predetermined
azimuthal arcs. The borehole deviates in an azimuthal direction in
which a proportionally large amount of borehole wall has been cut.
The drill bit is rotated by simultaneously rotating both the drill
bit motor and the drill string. The invention requires little if
any forces perpendicular to the axis of the borehole. Deviation is
instead achieved by relying only on variation in rotation speed of
the bit to preferentially remove material from the borehole wall
while simultaneously maintaining drills string rotation. This
allows the borehole path objectives to be achieved using lower
strength, less expensive materials that are required in other such
methods and associated devices. Furthermore, the invention does not
require the use of hydraulics to push drill string members into the
desired direction of deviation. Continuous rotation of the drill
string, while drilling both straight and deviated borehole,
provides superior heat dissipation and more torque at the drill
bit.
[0048] The above disclosure is to be regarded as illustrative and
not restrictive, and the invention is limited only by the claims
that follow.
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