Closed loop control of drilling toolface
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.
1. A downhole closed loop method for controlling a drilling toolface when drilling a subterranean borehole, the method comprising:
(a) drilling the subterranean borehole;
(b) measuring a first attitude of the subterranean borehole at a first upper survey station;
(c) measuring a second attitude of the subterranean borehole at a second lower survey station;
(d) processing downhole the first attitude and the second attitude to compute an angle change of the subterranean borehole between the upper and lower survey stations;
(e) repeating (b), (c), and (d) until the angle change of the subterranean borehole is greater than or equal to a predetermined threshold;
(f) processing downhole the reference attitude and the measured attitude to compute a toolface angle;
(g) processing downhole the toolface angle computed in (f) to control a direction of drilling of the subterranean borehole, processing downhole the toolface angle further comprising:
1) processing the target toolface and the toolface angle computed in (f) to obtain an error signal,
2) and processing the error signal to obtain a target toolface for a control unit; and
(h) applying the target toolface for the control unit to change the direction of drilling.
2. A downhole closed loop method for controlling a drilling toolface when drilling a subterranean borehole, the method comprising:
(a) drilling the subterranean borehole;
(b) receiving a reference attitude of the subterranean borehole, the reference attitude measured at an upper survey station;
(c) receiving a measured attitude of the subterranean borehole; the measured attitude measured at a lower survey station;
(d) processing downhole the reference attitude and the measured attitude to compute an angle change of the subterranean borehole between the upper and lower survey stations;
(e) comparing the angle change of the subterranean borehole with a predetermined threshold;
(f) repeating (c), (d), and (e) when the angle change of the subterranean borehole is less than the predetermined threshold;
(g) processing downhole the reference attitude and the measured attitude to compute a toolface angle when the angle change of the subterranean borehole is greater than or equal to the predetermined threshold;
(h) processing downhole the toolface angle computed in (g) to control a direction of drilling of the subterranean borehole, processing downhole the toolface angle further comprising:
1) processing the target toolface and the toolface angle computed in (g) to obtain an error signal, and
2) processing the error signal to obtain a target toolface for a control unit; and
(i) applying the target toolface for the control unit to change the direction of drilling.
3. The method of claim 2 , wherein (h) further comprises:
(i) processing a target azimuth and the measured attitude received in (c) to compute a target toolface;
(ii) processing the target toolface and the toolface angle computed in (g) to obtain an error signal;
(iii) processing the proportional and integral gains of the error signal to obtain a target toolface for a control unit; and
(iv) applying the target toolface for a control unit to change the direction of drilling.
4. The method of claim 2 , wherein the angle change of the subterranean borehole is computed in (d) using one of the following mathematical equations:
β
=
(
Inc
low
-
Inc
up
)
2
+
sin
2
(
Inc
up
)
(
Azi
low
-
Azi
up
)
2
;
β
=
(
Inc
low
-
Inc
up
)
2
+
sin
2
(
Inc
low
)
(
Azi
low
-
Azi
up
)
2
;
β
=
(
Inc
low
-
Inc
up
)
2
+
sin
(
Inc
low
)
sin
(
Inc
up
)
(
Azi
low
-
Azi
up
)
2
;
wherein β represents the angle change of the subterranean borehole, Inc low and Azi low represent the measured attitude at the lower survey station, and Inc up and Azi up represent the reference attitude at the upper survey station.
5. The method of claim 2 , wherein (h) further comprises:
(i) processing the toolface angle computed in (g) and a set point toolface to obtain a toolface error;
(ii) processing the toolface error to obtain a toolface correction; and
(iii) applying the toolface correction to change the direction of drilling.
6. The method of claim 2 , wherein the toolface angle is computed in (g) using one of the following equations:
GTF
=
arctan
[
sin
(
Inc
low
)
sin
(
Azi
low
-
Azi
up
)
cos
(
Inc
up
)
sin
(
Inc
low
)
cos
(
Azi
low
-
Azi
up
)
-
sin
(
Inc
up
)
cos
(
Inc
low
)
]
MTF
=
arctan
[
cos
2
(
Inc
up
)
sin
(
Inc
low
)
sin
(
Azi
low
)
-
sin
(
Inc
up
)
cos
(
Inc
up
)
sin
(
Azi
up
)
cos
(
Inc
low
)
+
sin
2
(
Inc
up
)
sin
(
Inc
low
)
cos
(
Azi
up
)
sin
(
Azi
low
-
Azi
up
)
cos
2
(
Inc
up
)
sin
(
Inc
low
)
cos
(
Azi
low
)
-
sin
(
Inc
up
)
cos
(
Inc
up
)
cos
(
Azi
up
)
cos
(
Inc
low
)
-
sin
2
(
Inc
up
)
sin
(
Inc
low
)
sin
(
Azi
up
)
sin
(
Azi
low
-
Azi
up
)
]
wherein GTF represents gravity toolface, MTF represents magnetic toolface, Inc low and Azi low represent the measured attitude at the lower survey station, and Inc up and Azi up represent the reference attitude at the upper survey station.
7. The method of claim 2 , wherein (h) further comprises:
(i) processing the measured attitude received in (c) in an outer control loop to obtain a target toolface; and
(ii) processing the target toolface and the toolface angle computed in (g) in an inner control loop to control the direction of drilling of the subterranean borehole.
8. The method of claim 2 , wherein an inclination of the subterranean borehole is less than about 5 degrees and the toolface angle is computed in (g) using the following equation:
MTF
=
arctan
(
sin
(
Inc
low
)
sin
(
Azi
low
)
-
sin
(
Inc
up
)
sin
(
Azi
up
)
sin
(
Inc
low
)
cos
(
Azi
low
)
-
sin
(
Inc
up
)
cos
(
Azi
up
)
)
wherein MTF represents magnetic toolface, Inc low and Azi low represent the measured attitude at the lower survey station, and Inc up and Azi up represent the reference attitude at the upper survey station.
9. The method of claim 2 , wherein the angle change of the subterranean borehole is less than about 5 degrees and the toolface angle is computed in (g) using the following equation:
GTF
=
arctan
(
(
Azi
low
-
Azi
up
)
sin
(
Inc
up
)
Inc
low
-
Inc
up
)
wherein GTF represents gravity toolface, Inc low and Azi low represent the measured attitude at the lower survey station, and Inc up and Azi up represent the reference attitude at the upper survey station.
10. The method of claim 2 , further comprising:
(i) setting the reference attitude equal to the measured attitude when the angle change of the subterranean borehole is greater than or equal to the predetermined threshold; and
(j) repeating (c), (d), (e), (f), (g), (h), (h)(1), (h)(2), and (i).
11. The method of claim 2 , wherein the predetermined threshold is in a range from about 0.25 to about 2.5 degrees.
12. The method of claim 2 , wherein the angle change of the subterranean borehole is computed in (d) using one of the following mathematical equations:
β
=
(
Inc
low
-
Inc
up
)
2
+
AW
sin
2
(
Inc
up
)
(
Azi
low
-
Azi
up
)
2
;
β
=
(
Inc
low
-
Inc
up
)
2
+
AW
sin
2
(
Inc
low
)
(
Azi
low
-
Azi
up
)
2
;
β
=
(
Inc
low
-
Inc
up
)
2
+
AW
sin
(
Inc
low
)
sin
(
Inc
up
)
(
Azi
low
-
Azi
up
)
2
;
wherein β represents the angle change of the subterranean borehole, Inc low and Azi low represent the measured attitude at the lower survey station, and Inc up and Azi up represent the reference attitude at the upper survey station, and AW represents a weighting factor in a range from 0 to 1.
13. The method of claim 12 , wherein AW is in a range from about 0.1 to about 0.5.
14. The method of claim 2 , further comprising:
(i) processing the angle change of the subterranean borehole to compute a rate of penetration of the drilling in (a).
15. The method of claim 14 , wherein the rate of penetration of drilling in (a) is computed using the following mathematical equation:
ROP
=
β
Δ
t
·
DLS
wherein ROP represents the rate of penetration of drilling, DLS represents a dogleg severity of the subterranean borehole being drilled in (a), and Δt represents a time passed between measuring the reference attitude and the measured attitude at the upper and lower survey stations.
16. The method of claim 14 , wherein the rate of penetration of drilling is computed in (d) substantially continuously while drilling in (a).