Downhole motor assembly and method for torque regulation

1. A downhole motor assembly for driving a drill bit to form a borehole, the downhole motor assembly including:

a hydraulic drive section operatively connected to the drill bit and a drill string, the hydraulic drive section including a stator and a rotor located inside the stator, the stator and rotor forming a flow path between the stator and the rotor such that fluid flowing through the flow path in response to a pressure differential across the hydraulic drive section creates an operative force to rotate the drill bit; and

a regulation mechanism including a valve and a fluid flow diversion bore for diverting at least some fluid from the flow path when the pressure differential across the hydraulic drive section is greater than or equal to a transition pressure differential, wherein the fluid flow diversion bore extends radially outward through the stator to an annulus between the drill string and a sidewall of the borehole.

2. The downhole motor assembly of claim 1 , where the valve includes a biased closed, self-regulating valve that operates based on the pressure differential across the hydraulic drive section to maximize the operative force on the drill bit for a given resistive torque on the drill bit.

3. The downhole motor assembly of claim 1 , where the fluid flow diversion bore includes a flow path through a portion of the length of the stator.

4. The downhole motor assembly of claim 1 , where the pressure differential across the hydraulic drive section remains less than a stall pressure differential for the hydraulic drive section.

5. The downhole motor assembly of claim 1 , where the transition pressure differential is less than a stall pressure differential for the hydraulic drive section.

6. The system of claim 1 , where the transition pressure differential is less than a stall pressure differential for the hydraulic drive section.

7. A method of drilling a subterranean wellbore using a drill bit including:

applying an operative force to the drill bit using a downhole motor assembly, the downhole motor assembly coupled to a drill string and including:

a hydraulic drive section including a stator and a rotor located inside the stator, the stator and rotor forming a flow path between the stator and the rotor; and

a regulation mechanism including a valve and a fluid flow diversion bore extending radially outward through the stator to an annulus between the drillstring and a sidewall of the wellbore;

where applying the operative force includes flowing fluid through the flow path to create the operative force;

decreasing the amount operative force on the drill bit when the pressure differential across the hydraulic drive section is greater than or equal to a transition pressure differential by operating the regulation mechanism to divert at least some fluid from the flow path and into the fluid flow diversion bore; and

maintaining at least some operative force on the drill bit while diverting fluid flow.

8. The method of claim 7 , where:

the valve includes a biased closed, self-regulating valve;

decreasing the operative force on the drill bit includes diverting at least some fluid flow by operating the valve based on the pressure differential across the hydraulic drive section; and

operating the valve includes maximizing the operative force on the drill bit for a given resistive force.

9. The method of claim 7 , where decreasing the operative force on the drill bit includes diverting at least some fluid flow by operating the valve using an actuator to divert fluid into the diversion bore.

10. The method of claim 7 , where the fluid flow diversion bore includes a flow path through a portion of the length of the stator.

11. The method of claim 7 , further including maintaining the pressure differential across the hydraulic drive section less than a stall pressure differential for the hydraulic drive section.

12. The method of claim 7 , where maintaining at least some operative force on the drill bit while diverting fluid flow includes at least one of opening the valve, lowering the RPM of the rotor, and lowering the amount of available torque to counteract the resistive torque.

13. The method of claim 12 , where lowering the amount of available torque on the drill bit comprises flowing fluid under a lower pressure through the flow path.

14. A subterranean wellbore drilling system including:

a drill bit connected with a drill string; and

a downhole motor assembly within the drill string for driving the drill bit, the downhole motor assembly including:

a hydraulic drive section operatively connected to the drill bit, the hydraulic drive section including a stator and a rotor located inside the stator, the stator and rotor forming a flow path between the stator and the rotor such that fluid flowing through the flow path in response to a pressure differential across the hydraulic drive section creates an operative force to operate the drill bit; and

a regulation mechanism including a valve and a fluid flow diversion bore for diverting at least some fluid from the flow path when the pressure differential across the hydraulic drive section is greater than or equal to a transition pressure differential, wherein the fluid flow diversion bore extends radially outward through the stator to an annulus between the drill string and a sidewall of the wellbore.

15. The system of claim 14 , where the valve includes a biased closed, self-regulating valve that operates based on the pressure differential across the hydraulic drive section to maximize the operative force on the drill bit for a given resistive torque.

16. The system of claim 14 , where the fluid flow diversion bore includes a flow path through a portion of the length of the stator.

17. The system of claim 14 , where the pressure differential across the hydraulic drive section remains less than a stall pressure differential for the hydraulic drive section.