Controlling release of torsional energy from a drill string

1. A system comprising:

a rotation sensor operable to facilitate rotational speed measurements indicative of rotational speed of a top drive for rotating a drill string;

a torque sensor operable to facilitate torque measurements indicative of torque output by the top drive; and

a processing device comprising a processor and a memory storing computer program code, wherein the processing device is operable to:

monitor the rotational speed measurements and the torque measurements during drilling operations;

determine that a lower portion of the drill string has become stuck downhole when:

the rotational speed measurements indicate that the rotational speed of the top drive is decreasing; and

the torque measurements indicate that the torque output by the top drive has increased to a predetermined maximum torque level; and

decrease a torque set-point of the top drive in response to determining that the lower portion of the drill string has become stuck downhole.

2. The system of claim 1 wherein the processing device is operable to determine that the lower portion of the drill string has become stuck downhole when the rotational speed measurements indicate that the rotational speed of the top drive has decreased to zero.

3. The system of claim 1 wherein the processing device is operable to determine that the lower portion of the drill string has become stuck downhole when the torque measurements indicate that the torque output by the top drive has increased to the predetermined maximum torque level for at least a predetermined period of time.

4. The system of claim 1 further comprising a pressure sensor operable to facilitate pressure measurements indicative of pressure of drilling mud being pumped through the drill string by mud pumps, wherein the processing device is further operable to:

monitor the pressure measurements during the drilling operations; and

determine that the lower portion of the drill string has become stuck downhole when, also, the pressure measurements indicate that the pressure of the drilling mud being pumped by the mud pumps is increasing.

5. The system of claim 1 wherein, after determining that the lower portion of the drill string has become stuck downhole, the processing device is further operable to:

decrease a rotational speed set-point of the top drive and decrease the torque set-point of the top drive; and then

cause a drawworks to lift the drill string to free the drill string.

6. The system of claim 1 wherein, after determining that the lower portion of the drill string has become stuck downhole, the processing device is further operable to output information indicative that the lower portion of the drill string has become stuck downhole to a video output device for viewing by rig personnel.

7. The system of claim 1 wherein determining that the lower portion of the drill string has become stuck downhole comprises determining that at least one of a drill bit and a bottom-hole assembly has become stuck downhole.

8. A method comprising:

controlling release of torsional energy from a drill string having a lower portion that is stuck against a subterranean formation and a top end rotated by a top drive by:

decreasing a rotational speed set-point of the top drive and decreasing a torque set-point of the top drive; and then

lifting the drill string to free the drill string.

9. The method of claim 8 wherein decreasing the rotational speed set-point of the top drive comprises decreasing the rotational speed set-point of the top drive to zero.

10. The method of claim 8 wherein decreasing the torque set-point of the top drive comprises decreasing the torque set-point of the top drive to a minimum torque level that the top drive can output.

11. The method of claim 8 wherein decreasing the rotational speed set-point of the top drive and decreasing the torque set-point of the top drive causes the top drive to permit being rotated by the torsional energy in the drill string, thereby dissipating the torsional energy in the drill string.

12. The method of claim 8 further comprising, while lifting the drill string:

monitoring torque measurements indicative of torque output by the top drive; and

determining that the drill string has been freed from the formation when the torque measurements indicate that the torque output by the top drive is decreasing.

13. The method of claim 8 wherein controlling the release of torsional energy from the drill string further comprises, before lifting the drill string, decreasing flow rate of drilling mud being pumped downhole via the drill string, thereby decreasing torque output by a mud motor rotating a drill bit.

14. The method of claim 13 further comprising, while lifting the drill string:

monitoring pressure measurements indicative of pressure of the drilling mud being pumped downhole via the drill string; and

determining that the drill string has been freed from the formation when the pressure measurements indicate that the pressure of the drilling mud is decreasing.

15. A method comprising:

monitoring rotational speed measurements indicative of rotational speed of a top drive rotating a drill string;

monitoring torque measurements indicative of torque output by the top drive to the drill string; and

determining that a lower portion of the drill string has become stuck downhole in response to:

the rotational speed measurements indicating that the rotational speed of the top drive is decreasing; and

the torque measurements indicating that the torque output by the top drive has increased to a predetermined maximum torque level; and

decreasing a torque set-point of the top drive in response to determining that the lower portion of the drill string has become stuck downhole.

16. The method of claim 15 comprising determining that the lower portion of the drill string is stuck downhole in response to the rotational speed measurements indicating that the rotational speed of the top drive has decreased to zero.

17. The method of claim 15 comprising determining that the lower portion of the drill string is stuck downhole in response to the torque measurements indicating that the torque output by the top drive has increased to the predetermined maximum torque level for at least a predetermined period of time.

18. The method of claim 15 further comprising:

monitoring pressure measurements indicative of pressure of drilling mud being pumped downhole through the drill string by mud pumps; and

determining that the lower portion of the drill string has become stuck downhole in response to, also, the pressure measurements indicating that the pressure of the drilling mud being pumped is increasing.

19. The method of claim 15 further comprising, after determining that the lower portion of the drill string has become stuck downhole, controlling release of torsional energy stored in the drill string by:

decreasing a rotational speed set-point of the top drive to zero and decreasing the torque set-point of the top drive; and then

lifting the drill string to free the drill string.

20. The method of claim 19 wherein controlling the release of torsional energy stored in the drill string further comprises, before lifting the drill string, decreasing flow rate of drilling mud being pumped downhole via the drill string, thereby decreasing torque output by a mud motor rotating a drill bit.