Directional drilling control using modulated bit rotation

Abstract

A system for steering the direction of a borehole advanced by cutting action of a rotary drill bit by periodically varying the rotation speed of the drill bit. The steering system comprises a motor disposed in a bent housing subsection and operationally connected to a drill string and to the drill bit. The rotation speed of the drill bit is periodically varied by periodic varying the rotation speed of the motor or by periodic varying the rotation speed of the drill string. Periodic bit speed rotation results in preferential cutting of material from a predetermined arc of the borehole wall which, in turn, resulting in borehole deviation. Both the drill string and the drill motor are rotated simultaneously during straight and deviated borehole drilling.

1 . An apparatus for drilling a borehole, said apparatus comprising:
 a drill string;   a drill bit motor; and   a drill bit; wherein
 said drill string and said drill bit motor are operationally connected to said drill bit to rotate said drill bit independently; and 
 said borehole is deviated by periodic varying rotation speed of said drill bit during a bit rotation. 
   
     
     
         2 . The apparatus of  claim 1  wherein said periodic variation of said drill bit speed is obtained by periodic variation of rotation speed of said drill bit motor. 
     
     
         3 . The apparatus of  claim 1  wherein said periodic variation of said drill bit speed is obtained by periodic variation of rotation speed of said drill string. 
     
     
         4 . The apparatus of  claim 1  wherein said drill string and said drill bit motor are rotated simultaneously to periodically vary said drill bit rotation speed. 
     
     
         5 . The apparatus of  claim 1  further comprising a bent subsection attached to a lower end of said drill string; wherein
 said drill bit motor is disposed within said bent subsection; and   said drill bit is attached to a shaft of said drill bit motor.   
     
     
         6 . A directional borehole assembly terminating a downhole end of a drill string, said assembly comprising:
 a drill bit motor disposed within a bent housing;   auxiliary sensors indicating orientation and position of said borehole assembly within said borehole;   a telemetry system for communicating between said borehole assembly and the surface of the earth; and   a downhole processor; wherein   said drill string and said drill bit motor are operationally connected to said drill bit to rotate said drill bit independently;   said borehole is deviated by periodic varying rotation speed of said drill bit during each bit rotation; and   said periodic varying rotation speed of said drill bit is defined by combining, within said downhole processor, responses of said auxiliary sensors with rotation information telemetered from said surface of the earth.   
     
     
         7 . The assembly of  claim 6  wherein said periodic variation of said drill bit speed is obtained by periodic variation of rotation speed of said drill bit motor. 
     
     
         8 . The assembly of  claim 6  wherein said periodic variation of said drill bit speed is obtained by periodic variation of rotation speed of said drill string. 
     
     
         9 . The assembly of  claim 6  wherein said drill string and said drill bit motor are rotated simultaneously.