Patent ID: 7854275

Claim:
A spring-operated anti-stall tool adapted for use in a downhole assembly comprising a tubing for extending downhole; a drill bit carried on the tubing; and a drive motor adjacent the drill bit for rotating the drill bit during drilling operations; the spring-operated anti-stall tool carried on the tubing and positioned adjacent the motor for preventing stalling of the motor due to excessive loads on the drill bit, the anti-stall tool including at least one piston in a cylinder having a forward piston area and a reverse piston area, and a controller comprising a hydraulic valve system for controlling operation of the piston, the forward piston area receiving hydraulic fluid to produce a force in the downhole direction, the reverse piston area containing a load spring adapted to apply an upward spring force on the piston, the controller sensing operating pressure of the drive motor and setting a desired range of operating pressures for the motor, including an upper limit and a lower limit, the controller adapted to: (1) supply hydraulic fluid to the forward piston area to increase force in the downhole direction to increase weight-on-bit (WOB) when operating pressure in the motor surpasses the lower limit, thereby compressing the load spring as the piston moves in the downhole direction; (2) vent the piston volume in the forward piston area so the compressed spring can expand to push the tool uphole to retract the drill bit, to decrease WOB when operating pressure in the motor exceeds the upper limit; and (3) optionally lock the piston in a passive state when the motor is operating within its normal operating pressure range under the bias of the spring.