Directional drilling control using periodic perturbation of the drill bit

Abstract

Disclosed herein is a system for steering the direction of a borehole advanced by cutting action of a rotary drill bit by periodically varying action of a drill bit while continuously rotating a drill string to which the drill bit is operationally attached. The steering system can include a bit perturbation device cooperating with a bent housing subsection and operationally connected to the drill string and to the drill bit. Drill bit action can be varied by periodically varying the rotation speed and/or rate of penetration of the drill bit. Periodic drill bit action results in preferential cutting of material from a predetermined arc of the borehole wall which, in turn, results in borehole deviation. Action of the drill bit can be varied independently of the rotation rate of the drill string.

1 . An apparatus for drilling a borehole with a drill string and a cooperating drill bit, said apparatus comprising:
 a bent sub; and   a bit perturbation device comprising at least one element that imparts axial and azimuthal force components at said drill bit; wherein
 said drill string and said perturbation device are operationally connected to said drill bit to operate said drill bit independent of rotation of said drill string, 
 said bit perturbation device periodically varies action of said drill bit, and 
 said borehole is deviated by said periodic variation of action of said drill bit. 
   
     
     
         2 . The apparatus of  claim 1  wherein said drill string rotates continuously during said borehole deviation. 
     
     
         3 . The apparatus of  claim 1  wherein said bit perturbation device further comprises a mud motor and a cooperating brake/clutch assembly. 
     
     
         4 . The apparatus of  claim 1  wherein said bit perturbation device further comprises a mud motor and a drilling fluid variable bypass orifice. 
     
     
         5 . The apparatus of  claim 1  wherein said drill string and said drill bit are rotated simultaneously. 
     
     
         6 . A method for deviating a borehole advanced by a rotating drill bit operationally attached to a drill string, the method comprising periodically perturbing action of said drill bit with a bit perturbation device comprising an element that imparts axial and azimuthal force components at said drill bit to vary the rate of penetration of said drill bit, thereby preferentially removing a relatively larger amount of material in an azimuthal arc of a wall of said borehole while maintaining continuous rotation of said drill string. 
     
     
         7 . The method of  claim 6  wherein said bit perturbation device further comprises a mud motor and a cooperating brake/clutch assembly. 
     
     
         8 . The method of  claim 6  wherein said bit perturbation device further comprises a mud motor and a drilling fluid variable bypass orifice. 
     
     
         9 . The method of  claim 6  further comprising:
 periodically varying, at a variation angle, said action of said drill bit from a first action rate to a second action rate; 
 maintaining said second action rate through a dwell angle; and 
 subsequently resuming said first action rate. 
 
     
     
         10 . The method of  claim 9  further comprising telemetering, from the surface of the earth, said variation angle and said dwell angle to a downhole processor cooperating with said bit perturbation device thereby periodically varying said bit action. 
     
     
         11 . The method of  claim 9  further comprising:
 storing said variation angle and said dwell angle in a downhole memory; and 
 transferring said variation angle and said dwell angle to a downhole processor cooperating with said bit perturbation device thereby periodically varying said action of said drill bit. 
 
     
     
         12 . A directional borehole assembly terminating a downhole end of a drill string, said assembly comprising:
 a bit perturbation device cooperating with a bent sub, the bit perturbation device comprising an element that imparts axial and azimuthal force components at said drill bit;   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 bit perturbation device are operationally connected to said drill bit to operate said drill bit independent of rotation of said drill string;   said borehole is deviated by periodic varying action of said drill bit while continuously rotating said drill string wherein said periodic variation of said drill bit is obtained at least in part by periodic variation of a rate of penetration of said drill bit; and   said periodic varying action of said drill bit is defined by combining, within said downhole processor, responses of said auxiliary sensors with information telemetered from said surface of the earth.   
     
     
         13 . The assembly of  claim 12  wherein said bit perturbation device further comprises a mud motor and at least one of a cooperating brake/clutch assembly or a drilling fluid variable bypass orifice. 
     
     
         14 . The assembly of  claim 12  wherein said action of said drill bit speed is also varied by periodic variation of rotation speed of said drill bit. 
     
     
         15 . The assembly of  claim 12  wherein said drill string and said drill bit are rotated simultaneously. 
     
     
         16 . A borehole assembly for drilling a deviated borehole, the borehole assembly comprising:
 a bent sub configured to be operatively connected to a drill bit and including a bit perturbation device having an assembly configured to periodically vary a rate of penetration of the drill bit within a single revolution of the drill bit; and   an electronics section configured to receive signals from the surface and operate the bit perturbation device so as to preferentially remove a relatively larger amount of material throughout a predetermined azimuthal arc of a wall of the borehole.   
     
     
         17 . The borehole assembly of  claim 16  wherein the assembly configured to periodically vary a rate of penetration of the drill bit comprises at least one of a vibrator or hammer configured to rise and fall under control of the electronics section. 
     
     
         18 . The borehole assembly of  claim 16  wherein the assembly configured to periodically vary a rate of penetration of the drill bit comprises a rotary mass that may be actuated by the electronics section to periodically vibrate the borehole assembly, thereby improving weight transfer to the face of the drill bit. 
     
     
         19 . A method for deviating a borehole, the method comprising periodically varying the rate of penetration of a drill bit throughout a predetermined sector of a single drill bit rotation, thereby preferentially removing a relatively larger amount of material in a predetermined azimuthal arc of a wall of the borehole. 
     
     
         20 . The method of  claim 20  wherein periodically varying the rate of penetration of the drill bit comprises imparting varying axial and azimuthal forces at the drill bit face.