U.S. patent application number 17/306567 was filed with the patent office on 2021-09-02 for closed loop control of drilling toolface.
The applicant listed for this patent is Schlumberger Technology Corporation. Invention is credited to Christopher C. Bogath, Adam Bowler, Peter Hornblower, Junichi Sugiura.
Application Number | 20210270088 17/306567 |
Document ID | / |
Family ID | 1000005586734 |
Filed Date | 2021-09-02 |
United States Patent
Application |
20210270088 |
Kind Code |
A1 |
Hornblower; Peter ; et
al. |
September 2, 2021 |
CLOSED LOOP CONTROL OF DRILLING TOOLFACE
Abstract
A downhole closed loop method for controlling a drilling
toolface includes measuring first and second attitudes of the
subterranean borehole at corresponding first and second upper and
lower survey stations. The first and second attitudes are processed
downhole while drilling to compute an angle change of the
subterranean borehole between the upper and lower survey stations.
The computed angle change is compared with a predetermined
threshold. This process may be continuously repeated while the
angle change is less than the threshold. The first and second
attitudes are further processed downhole to compute a toolface
angle when the angle change of the subterranean borehole is greater
than or equal to the threshold. The toolface angle may then be
further processed to control a direction of drilling of the
subterranean borehole.
Inventors: |
Hornblower; Peter;
(Gloucester, GB) ; Bogath; Christopher C.;
(Richmond, TX) ; Bowler; Adam; (Cambridge, GB)
; Sugiura; Junichi; (Bristol, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Schlumberger Technology Corporation |
Sugar Land |
TX |
US |
|
|
Family ID: |
1000005586734 |
Appl. No.: |
17/306567 |
Filed: |
May 3, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16243125 |
Jan 9, 2019 |
10995552 |
|
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17306567 |
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14766127 |
Aug 5, 2015 |
10214964 |
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PCT/US14/31176 |
Mar 19, 2014 |
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16243125 |
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61806522 |
Mar 29, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
E21B 47/26 20200501;
E21B 7/06 20130101; E21B 45/00 20130101 |
International
Class: |
E21B 7/06 20060101
E21B007/06; E21B 45/00 20060101 E21B045/00; E21B 47/26 20060101
E21B047/26 |
Claims
1. A downhole steering tool comprising: a downhole steering tool
body; a steering mechanism for controlling a direction of drilling
a subterranean borehole; sensors for measuring an attitude of the
subterranean borehole; and a downhole controller including one or
more modules having instructions to (i) process attitude
measurements received from the sensors at a first survey station
and a second survey station to compute an angle change between the
first survey station and the second survey station and (ii) process
the angle change to compute a rate of penetration while
drilling.
2. The downhole steering tool of claim 1, wherein the rate of
penetration while drilling is computed using the following
mathematical equation: R .times. O .times. P = .beta. .DELTA.t DLS
##EQU00007## where ROP represents the rate of penetration of
drilling, DLS represents a dogleg severity of the subterranean
borehole being drilled in (a), .beta. represents the angle change
between the upper and lower survey stations, and .DELTA.t
represents a time passed between measuring the reference attitude
and the measured attitude at the upper and lower survey
stations.
3. The downhole steering tool of claim 2, wherein the one or more
modules further have instructions to compute the rate of
penetration substantially continuously while drilling.
4. The downhole steering tool of claim 1, wherein the angle change
of the subterranean borehole is computed processed using one or
more of the following mathematical equations: .beta. = ( Inc low -
Inc u .times. p ) 2 + sin 2 .function. ( Inc u .times. p ) .times.
( A .times. z .times. i low - A .times. z .times. i u .times. p ) 2
; ##EQU00008## .beta. = ( Inc low - Inc u .times. p ) 2 + sin 2
.function. ( Inc low ) .times. ( A .times. z .times. i l .times. o
.times. w - A .times. z .times. i u .times. p ) 2 ; or
##EQU00008.2## .beta. = ( Inc low - Inc u .times. p ) 2 + sin
.function. ( Inc low ) .times. sin .function. ( Inc u .times. p )
.times. ( A .times. z .times. i low - A .times. z .times. i u
.times. p ) 2 ; ##EQU00008.3## where .beta. represents the angle
change of the subterranean borehole, Inc.sub.low and Azi.sub.low
represent the measured attitude at the lower survey station, and
Inc.sub.up and Azi.sub.up represent the reference attitude at the
upper survey station.
5. The downhole steering tool of claim 1, wherein the angle change
of the subterranean borehole is processed using one or more of the
following mathematical equations: .times. .beta. = ( Inc l .times.
o .times. w - Inc u .times. p ) 2 + A .times. W .times. sin 2
.function. ( Inc u .times. p ) .times. ( A .times. z .times. i l
.times. o .times. w - A .times. z .times. i u .times. p ) 2 ;
##EQU00009## .times. .beta. = ( Inc l .times. o .times. w - Inc u
.times. p ) 2 + A .times. W .times. sin 2 .function. ( Inc l
.times. o .times. w ) .times. ( A .times. z .times. i l .times. o
.times. w - A .times. z .times. i u .times. p ) 2 ; or
##EQU00009.2## .beta. = ( Inc l .times. o .times. w - Inc u .times.
p ) 2 + AW .times. .times. sin .function. ( Inc l .times. o .times.
w ) .times. sin .function. ( Inc u .times. p ) .times. ( A .times.
z .times. i l .times. o .times. w - A .times. z .times. i u .times.
p ) 2 ; ##EQU00009.3## where .beta. represents the angle change of
the subterranean borehole, Inc.sub.low and Azi.sub.low represent
the measured attitude at the lower survey station, and Inc.sub.up
and Azi.sub.up represent the reference attitude at the upper survey
station, and AW represents a weighting factor in a range from 0 to
1.
6. The downhole steering tool of claim 5, where AW is in a range
from about 0.1 to about 0.5.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 16/243,125, filed Jan. 9, 2019, which issues as U.S. Pat.
No. 10,995,552 on May 4, 2021, which is a continuation of U.S.
patent application Ser. No. 14/766,127, now U.S. Pat. No.
10,214,964 issued on Feb. 26, 2019, which is a national stage
application of PCT Application No. PCT/US2014/031176 filed on Mar.
19, 2014, which claims priority to U.S. Provisional Patent
Application No. 61/806,522 filed on Mar. 29, 2013, the entirety of
each of which are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] Disclosed embodiments relate generally to methods for
maintaining directional control during downhole directional
drilling operations and more particularly to method for determining
a downhole toolface offset while drilling.
BACKGROUND
[0003] The use of automated drilling methods is becoming
increasingly common in drilling subterranean wellbores. Such
methods may be employed, for example, to control the direction of
drilling based on various downhole feedback measurements, such as
inclination and azimuth measurements made while drilling or logging
while drilling measurements.
[0004] One difficulty with automated drilling methods (and
directional drilling methods in general) is that directional
drilling tools exhibit tendencies to drill (or turn) in a direction
offset from the set point direction. For example, when set to drill
a horizontal well straight ahead, certain drilling tools may have a
tendency to drop inclination (turn downward) and/or to turn to the
left or right. Exacerbating this difficulty, these tendencies can
be influenced by numerous factors and may change unexpectedly
during a drilling operation. Factors influencing the directional
tendency may include, for example, properties of the subterranean
formation, the configuration of the bottom hole assembly (BHA), bit
wear, bit/stabilizer walk, an unplanned touch point (e.g. due to
compression and buckling of the BHA), stabilizer-formation
interaction, the steering mechanism utilized by the steering tool,
and various drilling parameters.
[0005] In current drilling operations, a drilling operator
generally corrects the directional tendencies by evaluating
wellbore survey data transmitted to the surface. A surface
computation of the gravity toolface of the well is generally
performed at 30 to 100 foot intervals (e.g., at the static survey
stations). While such techniques are serviceable, there is a need
for further improvement, particularly for automatically
accommodating (or correcting) such tendencies downhole while
drilling.
SUMMARY
[0006] A downhole closed loop method for controlling a drilling
toolface of a subterranean borehole is disclosed. The method
includes receiving reference and measured attitudes of the
subterranean borehole while drilling with the reference attitude
being measured at an upper survey station and the measured attitude
being measured at a lower survey station. The reference attitude
and the measured attitude are processed downhole while drilling
(using a downhole processor) to compute an angle change of the
subterranean borehole between the upper and lower survey stations.
The computed angle change is compared with a predetermined
threshold. This process may be continuously repeated while the
angle change is less than the threshold. The reference attitude and
the measured attitude are further processed downhole to compute a
toolface angle when the angle change of the subterranean borehole
is greater than or equal to the threshold. The toolface angle may
then be further processed to control a direction of drilling of the
subterranean borehole.
[0007] The disclosed embodiments may provide various technical
advantages. For example, the disclosed embodiments provide for
real-time closed loop control of the drilling toolface. As such,
the disclosed methods may provide for improved well placement and
reduced wellbore tortuosity. Moreover, by providing for closed loop
control, the disclosed methods tend to improve drilling efficiency
and consistency.
[0008] This summary is provided to introduce a selection of
concepts that are further described below in the detailed
description. This summary is not intended to identify key or
essential features of the claimed subject matter, nor is it
intended to be used as an aid in limiting the scope of the claimed
subject matter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] For a more complete understanding of the disclosed subject
matter, and advantages thereof, reference is now made to the
following descriptions taken in conjunction with the accompanying
drawings, in which:
[0010] FIG. 1 depicts an example drilling rig on which disclosed
embodiments may be utilized.
[0011] FIG. 2 depicts a lower BHA portion of the drill string shown
on FIG. 1.
[0012] FIG. 3 depicts a diagram of attitude and steering parameters
in a global coordinate reference frame.
[0013] FIG. 4 depicts a diagram of gravity toolface and magnetic
toolface in a global reference frame.
[0014] FIG. 5 depicts a flow chart of one disclosed closed loop
method embodiment for obtaining the drilling toolface.
[0015] FIG. 6 depicts one embodiment of a controller by which the
toolface angle obtained in the method depicted on FIG. 5 may be
processed to control the direction of drilling.
[0016] FIG. 7 depicts a cascade controller that may process the
toolface angle obtained in the method depicted on FIG. 5 to drive
the drilling tool to a target azimuth.
DETAILED DESCRIPTION
[0017] FIG. 1 depicts a drilling rig 10 suitable for using various
method and system embodiments disclosed herein. A semisubmersible
drilling platform 12 is positioned over an oil or gas formation
(not shown) disposed below the sea floor 16. A subsea conduit 18
extends from deck 20 of platform 12 to a wellhead installation 22.
The platform may include a derrick and a hoisting apparatus for
raising and lowering a drill string 30, which, as shown, extends
into borehole 40 and includes a bottom hole assembly (BHA) 50. The
BHA 50 includes a drill bit 32, a steering tool 60 (also referred
to as a directional drilling tool), and one or more downhole
navigation sensors 70 such as measurement while drilling sensors
including three axis accelerometers and/or three axis
magnetometers. The BHA 50 may further include substantially any
other suitable downhole tools such as a downhole drilling motor, a
downhole telemetry system, a reaming tool, and the like. The
disclosed embodiments are not limited in regards to such other
tools.
[0018] It will be understood that the BHA may include substantially
any suitable steering tool 60, for example, including a rotary
steerable tool. Various rotary steerable tool configurations are
known in the art including various steering mechanisms for
controlling the direction of drilling. For example, many existing
rotary steerable tools include a substantially non-rotating outer
housing employing blades that engage the borehole wall. Engagement
of the blades with the borehole wall is intended to eccenter the
tool body, thereby pointing or pushing the drill bit in a desired
direction while drilling. A rotating shaft deployed in the outer
housing transfers rotary power and axial weight-on-bit to the drill
bit during drilling. Accelerometer and magnetometer sets may be
deployed in the outer housing and therefore are non-rotating or
rotate slowly with respect to the borehole wall.
[0019] The POWERDRIVE.RTM. rotary steerable systems (available from
Schlumberger) fully rotate with the drill string (i.e., the outer
housing rotates with the drill string). The POWERDRIVE.RTM.
XCEED.TM. makes use of an internal steering mechanism that does not
require contact with the borehole wall and enables the tool body to
fully rotate with the drill string. The POWERDRIVE.RTM. X5, X6, and
POWERDRIVE ORBIT.RTM. rotary steerable systems make use of mud
actuated blades (or pads) that contact the borehole wall. The
extension of the blades (or pads) is rapidly and continually
adjusted as the system rotates in the borehole. The POWERDRIVE
ARCHER.RTM. makes use of a lower steering section joined at an
articulated swivel with an upper section. The swivel is actively
tilted via pistons so as to change the angle of the lower section
with respect to the upper section and maintain a desired drilling
direction as the bottom hole assembly rotates in the borehole.
Accelerometer and magnetometer sets may rotate with the drill
string or may alternatively be deployed in an internal
roll-stabilized housing such that they remain substantially
stationary (in a bias phase) or rotate slowly with respect to the
borehole (in a neutral phase). To drill a desired curvature, the
bias phase and neutral phase are alternated during drilling at a
predetermined ratio (referred to as the steering ratio). Again, the
disclosed embodiments are not limited to use with any particular
steering tool configuration.
[0020] The downhole sensors 70 may include substantially any
suitable sensor arrangement used making downhole navigation
measurements (borehole inclination, borehole azimuth, and/or tool
face measurements). Such sensors may include, for example,
accelerometers, magnetometers, gyroscopes, and the like. Such
sensor arrangements are well known in the art and are therefore not
described in further detail. The disclosed embodiments are not
limited to the use of any particular sensor embodiments or
configurations. Methods for making real-time while drilling
measurements of the borehole inclination and borehole azimuth are
disclosed, for example, in commonly assigned U.S. Patent
Publications 2013/0151157 and 2013/0151158. In the depicted
embodiment, the sensors 70 are shown to be deployed in the steering
tool 60. Such a depiction is merely for convenience as the sensors
70 may be deployed elsewhere in the BHA.
[0021] It will be understood by those of ordinary skill in the art
that the deployment illustrated on FIG. 1 is merely an example. It
will be further understood that disclosed embodiments are not
limited to use with a semisubmersible platform 12 as illustrated on
FIG. 1. The disclosed embodiments are equally well suited for use
with any kind of subterranean drilling operation, either offshore
or onshore.
[0022] FIG. 2 depicts the lower BHA portion of drill string 30
including drill bit 32 and steering tool 60. As described above
with respect to FIG. 1, the steering tool may include navigation
sensors 70 including tri-axial (three axis) accelerometer and
magnetometer navigation sensors. Suitable accelerometers and
magnetometers may be chosen from among substantially any suitable
commercially available devices known in the art. FIG. 2 further
includes a diagrammatic representation of the tri-axial
accelerometer and magnetometer sensor sets. By tri-axial it is
meant that each sensor set includes three mutually perpendicular
sensors, the accelerometers being designated as A.sub.x, A.sub.y,
and A.sub.z and the magnetometers being designated as B.sub.x,
B.sub.y, and B.sub.z. By convention, a right handed system is
designated in which the z-axis accelerometer and magnetometer
(A.sub.z and B.sub.z) are oriented substantially parallel with the
borehole as indicated (although disclosed embodiments are not
limited by such conventions). Each of the accelerometer and
magnetometer sets may therefore be considered as determining a
plane (the x and y-axes) and a pole (the z-axis along the axis of
the BHA).
[0023] FIG. 3 depicts a diagram of attitude in a global coordinate
reference frame at first and second upper and lower survey stations
82 and 84. The attitude of a BHA defines the orientation of the BHA
axis (axis 86 at the upper survey station 82 and axis 88 at the
lower survey station 84) in three-dimensional space. In wellbore
surveying applications, the wellbore attitude represents the
direction of the BHA axis in the global coordinate reference frame
(and is commonly understood to be approximately equal to the
direction of propagation of the drill bit). Attitude may be
represented by a unit vector the direction of which is often
defined by the borehole inclination and the borehole azimuth. In
FIG. 2 the borehole inclination at the upper and lower survey
stations 82 and 84 is represented by Inc.sub.up and Inc.sub.low
while the borehole azimuth is represented by Azi.sub.up and
Azi.sub.low. The angle .beta. represents the overall angle change
of the borehole between the first and second survey stations 82 and
84.
[0024] FIG. 4 depicts a further diagram of attitude and toolface in
a global coordinate reference frame at the second lower survey
station 84. The Earth's magnetic field and gravitational field are
depicted at 91 and 92. The borehole inclination Mom, represents the
deviation of axis 88 from vertical while the borehole azimuth
Azi.sub.low, represents the deviation of a projection of the axis
88 on the horizontal plane from magnetic north. Gravity toolface
(GTF) is the angular deviation about the circumference of the
downhole tool of some tool component with respect to the highside
(HS) of the tool collar (or borehole). In this disclosure gravity
tool face (GTF) represents the angular deviation between the
direction towards which the drill bit is being turned and the
highside direction (e.g., in a slide drilling operation, the
gravity tool face represents the angular deviation between a bent
sub scribe line and the highside direction). Magnetic toolface
(MTF) is similar to GTF but uses magnetic north as a reference
direction. In particular, MTF is the angular deviation in the
horizontal plane between the direction towards which the drill bit
is being turned and magnetic north.
[0025] It will be understood that the disclosed embodiments are not
limited to the above described conventions for defining borehole
coordinates depicted in FIGS. 2, 3, and 4. It will be further
understood that these conventions can affect the form of certain of
the mathematical equations that follow in this disclosure. Those of
ordinary skill in the art will be readily able to utilize other
conventions and derive equivalent mathematical equations.
[0026] FIG. 5 depicts a flow chart of one disclosed closed loop
method embodiment 100 for obtaining the drilling toolface. A
subterranean borehole is drilled at 102, for example, via rotating
a drill string, pumping drilling fluid through a downhole mud
motor, or the like. A directional drilling tool (steering tool) may
also be actuated so as to control the direction of drilling (the
drilling attitude) and thereby steer the drill bit. A reference
attitude is received at 104. The reference attitude may include,
for example, a previously measured attitude. A measured attitude is
received 106. The reference and measured attitudes may include
inclination and azimuth values measured using substantially any
suitable downhole sensor arrangements, for example, including the
aforementioned accelerometers, magnetometers, and gyroscopic
sensors. The reference attitude may include a previously measured
attitude obtained from an upper survey station while the measured
attitude may include a currently measured attitude obtained from a
lower survey station.
[0027] At 108 the reference and measured attitudes are processed to
compute an overall angle change .beta. of the borehole between
first and second survey stations (see FIG. 3). The angle .beta. is
then compared with a predetermined threshold value at 110. When
.beta. is less than the threshold, the method returns to 106 and
receives a subsequent measured attitude (an attitude measured later
in time as compared to the previously measured attitude) and then
re-computes .beta. at 108. When .beta. is greater than or equal to
the threshold value at 110, the reference and measured attitudes
are further processed at 112 to compute the toolface angle (e.g.,
the GTF and/or the MTF) of the drill bit (i.e., the tool face angle
towards which the drill bit is turning). The computed toolface
angle is then further processed at 200 as described in more detail
below with respect to FIGS. 6 and 7 to control the direction of
drilling. At 114 the reference attitude (originally received at
104) is reset such that it equals the most recently measured
attitude received at 106. The method then cycles back to 106 and
receives another measured attitude and then re-computes .beta. at
108.
[0028] The attitude received at 106 may be measured, for example,
using static and/or continuous inclination and azimuth measurement
techniques. Static measurements may be obtained, for example, when
drilling is temporarily suspended to add a new pipe stand to the
drill string. Continuous measurements may be obtained, for example,
from corresponding continuous measurements of the axial component
of the gravitational and magnetic fields (A.sub.z and B.sub.z in
FIG. 2) using techniques known to those of ordinary skill in the
art (e.g., as disclosed in U.S. Patent Publication 2013/0151157
which is fully incorporated by reference herein). The continuous
inclination and azimuth measurements may further be filtered to
reduce the effects of noise. For example, a suitable digital filter
may include a first-order infinite impulse response (IIR) filter.
Such filtering techniques are also known to those of ordinary skill
in the art and need not be discussed further herein.
[0029] The reference and measured attitudes may be processed at 108
to compute the angle .beta. between the upper and lower survey
stations, for example, as follows:
.beta.=arccos{cos(Inc.sub.low-Inc.sub.up)-sin(Inc.sub.low)sin(Inc.sub.up-
)[1-cos(Azi.sub.low-Azi.sub.up)]} (1)
[0030] where Inc.sub.low and Azi.sub.low represent the measured
attitude (inclination and azimuth) and Inc.sub.up and Azi.sub.up
represent the reference attitude (inclination and azimuth). Given
that the overall angle change of the well is often small in a
continuous drilling operation, one or more of the following
approximations may be used when .beta. is small (e.g., less than
about 5 degrees):
.beta. = ( Inc low - Inc u .times. p ) 2 + sin .function. ( Inc low
) .times. sin .function. ( Inc u .times. p ) .times. ( A .times. z
.times. i low - A .times. z .times. i u .times. p ) 2 ( 2 ) .times.
.beta. = ( Inc low - Inc u .times. p ) 2 + sin 2 .function. ( Inc
low ) .times. ( A .times. z .times. i .times. o .times. w - A
.times. z .times. i u .times. p ) 2 ( 3 ) .times. .beta. = ( Inc
low - Inc u .times. p ) 2 + sin 2 .function. ( Inc u .times. p )
.times. ( A .times. z .times. i low - A .times. z .times. i u
.times. p ) 2 ( 4 ) ##EQU00001##
[0031] When making continuous (while drilling) attitude
measurements, the continuous azimuth measurements are commonly
noisier than the continuous inclination measurements. As such,
Equations 2-4 may be modified to include a weighting factor AW to
desensitize the effect of the noisier azimuth on the overall angle
change .beta..
.beta. w .times. e .times. i .times. g .times. h .times. t .times.
e .times. d = ( Inc low - Inc u .times. p ) 2 + AW .times. .times.
sin .function. ( Inc low ) .times. sin .function. ( Inc u .times. p
) .times. ( A .times. z .times. i .times. o .times. w - A .times. z
.times. i u .times. p ) 2 ( 5 ) .beta. w .times. e .times. i
.times. g .times. h .times. t .times. e .times. d = ( Inc low - Inc
u .times. p ) 2 + A .times. W .times. sin 2 .function. ( Inc low )
.times. ( A .times. z .times. i low - A .times. z .times. i u
.times. p ) 2 ( 6 ) .beta. w .times. e .times. i .times. g .times.
h .times. t .times. e .times. d = ( Inc low - Inc u .times. p ) 2 +
A .times. W .times. sin 2 .function. ( Inc u .times. p ) .times. (
A .times. z .times. i l .times. o .times. w - A .times. z .times. i
u .times. p ) 2 ( 7 ) ##EQU00002##
[0032] wherein the weighting factor AW is in a range from 0 to 1
and may be selected based on the noise levels in the inclination
and azimuth values. In certain embodiments, the weighting factor AW
may be in a range from about 0.1 to about 0.5 (although the
disclosed embodiments are by no means limited in this regard).
Equations 2-7 may be advantageously utilized on a downhole
computer/processor as they reduce the number of trig functions
(which tend to use substantial computational resources).
[0033] Substantially any suitable threshold may be used at 110, for
example, in a range from about 0.25 to about 2.5 degrees. In
general increasing the value of the threshold reduces the error in
the toolface value computed at 112. In one embodiment, a toolface
error in a range from about 5-10 degrees may be achieved using a
threshold value of 0.5 degrees. Using a threshold value of 1.0
degree may advantageously further reduce the toolface error. It
will be understood that the threshold is related to the curvature
of the wellbore section being drilled and the distance drilled. For
example, at a curvature of 5 degrees per 100 feet of wellbore, a
threshold of 0.5 degrees corresponds to a distance drilled of 10
feet. As such the control loop depicted in FIG. 5 may be thought of
as being a substantially depth-domain controller.
[0034] It will be further understood that the measured value of
.beta. may be processed downhole to obtain an approximate rate of
penetration ROP of drilling, for example, as follows:
R .times. O .times. P = .beta. .DELTA. .times. .times. t DLS ( 8 )
##EQU00003##
[0035] where DLS represents the dogleg severity (curvature) of the
borehole section being drilled and .DELTA.t represents the time
passed between making measurements at the first and second upper
and lower survey stations. This estimated ROP may be advantageously
used, for example, to project the continuous survey sensor
measurements to the bit (or other locations in the string). It will
be understood that "static" and/or substantially continuous ROP
values may be computed. For example, a static ROP may be computed
at 112 when .beta. exceeds the threshold. A substantially
continuous ROP may be computed, for example, at 108 when computing
.beta. thereby giving a near instantaneous rate of penetration.
Such a near instantaneous rate of penetration may optionally be
filtered, for example, using a rolling average window or other
filtering technique.
[0036] The reference and measured attitudes may be further
processed at 112 to compute the GTF or MTF angles, for example, as
follows:
.times. GTF = arctan [ sin .function. ( In .times. c l .times. o
.times. w ) .times. sin .function. ( A .times. z .times. i l
.times. o .times. w - A .times. z .times. i u .times. p ) cos
.function. ( Inc u .times. p ) .times. sin .function. ( In .times.
c l .times. o .times. w ) .times. cos .function. ( Az .times. i l
.times. o .times. w - A .times. z .times. i u .times. p ) - sin
.function. ( In .times. c u .times. p ) .times. cos .function. ( In
.times. c l .times. o .times. w ) ] ( 9 ) MTF = arctan .function. [
cos 2 .function. ( Inc u .times. p ) .times. sin .function. ( Inc l
.times. o .times. w ) .times. sin .function. ( A .times. z .times.
i l .times. o .times. w ) - sin .function. ( Inc u .times. p )
.times. cos .function. ( Inc u .times. p ) .times. sin .function. (
A .times. z .times. i u .times. p ) .times. cos .function. ( Inc l
.times. o .times. w ) + sin 2 .function. ( Inc u .times. p )
.times. sin .function. ( Inc l .times. o .times. w ) .times. cos
.function. ( A .times. z .times. i u .times. p ) .times. sin
.function. ( A .times. z .times. i l .times. o .times. w - A
.times. z .times. i u .times. p ) cos 2 .function. ( Inc u .times.
p ) .times. sin .function. ( Inc l .times. o .times. w ) .times.
cos .function. ( A .times. z .times. i l .times. o .times. w ) -
sin .function. ( Inc u .times. p ) .times. cos .function. ( Inc u
.times. p ) .times. cos .function. ( A .times. z .times. i u
.times. p ) .times. cos .function. ( Inc l .times. o .times. w ) -
sin 2 .function. ( Inc u .times. p ) .times. sin .function. ( Inc l
.times. o .times. w ) .times. sin .function. ( A .times. z .times.
i u .times. p ) .times. sin .function. ( A .times. z .times. i l
.times. o .times. w - A .times. z .times. i u .times. p ) ] .times.
( 10 ) ##EQU00004##
[0037] An approximate GTF may be computed based on the assumption
that .beta. is small (e.g., less than about 5 degrees), for
example, as follows:
G .times. T .times. F = arctan .function. ( ( A .times. z .times. i
l .times. o .times. w - A .times. z .times. i u .times. p ) .times.
sin .function. ( Inc u .times. p ) Inc l .times. o .times. w - Inc
u .times. p ) ( 11 ) ##EQU00005##
[0038] Likewise, an approximate MTF may be computed when the
borehole inclination is small (e.g., less than about 5 degrees) at
the upper and lower survey stations, for example, as follows:
M .times. T .times. F = arctan .function. ( sin .function. ( Inc l
.times. o .times. w ) .times. sin .function. ( A .times. z .times.
i l .times. o .times. w ) - sin .function. ( Inc u .times. p )
.times. sin .function. ( A .times. z .times. i u .times. p ) sin
.function. ( Inc l .times. o .times. w ) .times. cos .function. ( A
.times. z .times. i l .times. o .times. w ) - sin .function. ( Inc
u .times. p ) .times. cos .function. ( A .times. z .times. i u
.times. p ) ) ( 12 ) ##EQU00006##
[0039] Equations 11 and 12 require less intensive computation and
may therefore be advantageous when implementing the disclosed
method on a downhole controller. It will be understood that the MTF
and/or the GTF may alternatively (and/or additionally) be computed
using other known mathematical relations, for example, utilizing
inclination and magnetic dip angle or inclination, azimuth, and
magnetic dip angle. Such mathematical relations are disclosed, for
example, in U.S. Pat. No. 7,243,719 and U.S. Patent Publication
2013/0126239, each of which is incorporated by reference in its
entirety herein.
[0040] The computed toolface values may be compared with a toolface
set point value to compute toolface offset values (the error or
offset between the set point value and the actual measured value)
in substantially real time while drilling. The toolface offset
values may be further processed to obtain a transfer function of
the directional drilling system. This transfer function may be
further evaluated in combination with various drilling and BHA
parameters (e.g., formation type, rate of penetration, BHA
configuration, etc) to evaluate the performance of the drilling
system.
[0041] FIG. 6 depicts one embodiment of a controller 200 by which
the toolface angle may be processed to control the direction of
drilling. The toolface angle obtained from method 100 may be
combined at 202 with the toolface set point value (e.g., the
desired toolface angle set by the drilling operator) to obtain a
toolface error. The toolface error may be in turn be combined at
204 with a previous toolface correction to obtain a current
toolface correction which may be further combined at 206 with the
toolface set point value to obtain a toolface reference. It will be
understood that the control structure depicted on FIG. 6 functions
like a proportional integral (P+I) controller (with a P gain of 1)
for changes in the toolface set point value and like an integral
only controller when responding to toolface disturbances. The
disclosed embodiments are of course not limited to any particular
type of controller. For example, other controllers such as a
proportional controller, a proportional differential controller, or
a proportional integral differential controller may be used. Non
classic controllers, such as a model predictive controller, a fuzzy
controller, and the like may also be used.
[0042] FIG. 7 depicts a cascade controller 200' that may process
the toolface angle obtained from method 100 to drive the drilling
tool to a target azimuth. The depicted controller includes a P+I
outer closed loop 220 to drive the drill cycle survey azimuth to a
target azimuth downlinked by a drilling operator and a P+I inner
closed loop 240 to drive the measured toolface (MTF or GTF) to the
target toolface. At the start of the implantation (e.g., at the
beginning of an automated drilling operation) it may be desirable
to disable (switch off) the outer loop 220 to enable the tuning of
the inner loop 240 via setting gains kpAzi and kpAzi equal to
zero.
[0043] In the outer loop 220, the target azimuth targetAzi is
combined at 222 with the measured azimuth cAzi from method 100 to
obtain an azimuth error signal: e.sub.1 [n]=targetAzi-cAzi. The
azimuth error signal is further combined at 224 with a weighted
value of the measured inclination k sin(cInc) to obtain a weighted
azimuthal error signal: e'.sub.1 [n]=e.sub.1 [n]ksin (cInc).
Proportional and integral gains of the weighted azimuthal error
signal are computed at 226 and 228 and combined at 230 to obtain a
target toolface of the well:
targetTF=kpAzie'.sub.1[n]+kiAzi.SIGMA..sub.1.sup.ne'.sub.1[n]. The
target toolface may be either a GTF or a MTF and may be
automatically (or manually) selected at 235, for example, based on
the inclination of the wellbore.
[0044] In the inner loop 240 a target GTF or a target MTF are
computed and input into control unit 260 that controls the
direction of drilling. When the MTF/GTF switch 235 is set to select
GTF, the target toolface of the well targetTF is combined at 242
with a GTF obtained from method 100 to obtain a GTF error signal:
e.sub.3 [n]=targetTF-GTF. Proportional and integral gains of the
GTF error signal are computed at 244 and 246 and combined at 248 to
obtain the target GTF of the control unit: targetGTF=kpGTFe.sub.3
[n]+kiGTF.SIGMA..sub.1.sup.ne.sub.3 [n]. When the MTF/GTF switch
235 is set to select MTF, the target toolface of the well targetTF
is combined at 252 with an MTF obtained from method 100 to obtain
an MTF error signal: e.sub.2 [n]=targetTF-MTF. Proportional and
integral gains of the MTF error signal are computed at 254 and 256
and combined at 258 to obtain the target MTF of the control unit:
targetMTF=kpMTFe.sub.2 [n]+kiMTFE.sub.1.sup.ne.sub.2 [n].
[0045] The methods described herein are configured for downhole
implementation via one or more controllers deployed downhole (e.g.,
in a steering/directional drilling tool). A suitable controller may
include, for example, a programmable processor, such as a
microprocessor or a microcontroller and processor-readable or
computer-readable program code embodying logic. A suitable
processor may be utilized, for example, to execute the method
embodiments described above with respect to FIGS. 5, 6, and 7 as
well as the corresponding disclosed mathematical equations. A
suitable controller may also optionally include other controllable
components, such as sensors (e.g., a depth sensor), data storage
devices, power supplies, timers, and the like. The controller may
also be disposed to be in electronic communication with the
attitude sensors (e.g., to receive the continuous inclination and
azimuth measurements). A suitable controller may also optionally
communicate with other instruments in the drill string, such as,
for example, telemetry systems that communicate with the surface. A
suitable controller may further optionally include volatile or
non-volatile memory or a data storage device.
[0046] With continued reference to FIG. 7, disclosed embodiments
may further include a downhole steering tool having a downhole
steering tool body, a steering mechanism for controlling a
direction of drilling a subterranean borehole and sensors for
measuring an attitude of the subterranean borehole. The steering
tool may further include a downhole controller including (i) a
toolface module having instructions (as in method 100 on FIG. 5) to
process attitude measurements received from the sensors to obtain a
drilling toolface, (ii) an outer control loop having instructions
to process the attitude measurements received from the sensors and
a target azimuth to obtain a target toolface, (iii) an inner loop
having instructions to process the drilling toolface and the target
toolface to obtain an error signal, and (iv) a control unit target
including instructions to process the error signal to obtain
instructions for the steering mechanism for controlling the
direction of drilling.
[0047] Although closed loop control of drilling toolface and
certain advantages thereof have been described in detail, it should
be understood that various changes, substitutions and alterations
may be made herein without departing from the spirit and scope of
the disclosure as defined by the appended claims.
* * * * *