Directional drilling control using periodic perturbation of the drill bit

Abstract

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 comprises 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 is varied by periodic varying the rotation speed 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, resulting in borehole deviation. Action of the drill bit can be varied independent 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) a bent sub; and   (b) a bit perturbation device; 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 comprises a mud motor and a cooperating brake/clutch assembly. 
   
   
       4 . The apparatus of  claim 1  wherein said bit perturbation device comprises a mud motor and a drilling fluid variable bypass orifice. 
   
   
       5 . The apparatus of  claim 1  wherein said bit perturbation device comprises an element that imparts axial and azimuthal force components at said drill bit. 
   
   
       6 . The apparatus of  claim 1  wherein said drill string and said drill bit are rotated simultaneously. 
   
   
       7 . A method for deviating a borehole advanced by a rotating drill bit operationally attached to a drill string, the method comprising periodic perturbing action of said drill bit with a bit perturbation device to preferentially remove a disproportional amount of material in an azimuthal arc of a wall of said borehole while maintaining continuous rotation of said drill string. 
   
   
       8 . The method of  claim 7  wherein said bit perturbation device comprising a mud motor and a cooperating brake/clutch assembly. 
   
   
       9 . The method of  claim 7  wherein said bit perturbation device comprising a mud motor and a drilling fluid variable bypass orifice. 
   
   
       10 . The method of  claim 7  wherein said bit perturbation device comprising an element that imparts axial and azimuthal force components at said drill bit. 
   
   
       11 . The method of  claim 7  wherein:
 (a) said bit perturbation device is operationally attached to said drill bit and to said drill string; and   (b) said drill bit and said drill string are simultaneously rotated while rotation speed of said drill bit is periodically varied.   
   
   
       12 . The method of  claim 7  wherein:
 (a) said bit perturbation device is operationally attached to said drill bit and to a drill string; and   (b) rate of penetration of said drill bit is periodically varied while said drill string is simultaneously rotated.   
   
   
       13 . The method of  claim 7  further comprising:
 (a) periodically varying, at a variation angle, said action of said drill bit from a first action rate to a second action rate;   (b) maintaining said second action rate through a dwell angle; and   (c) subsequently resuming said first action rate.   
   
   
       14 . The method of  claim 13  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 periodic varying said bit action. 
   
   
       15 . The method of  claim 13  further comprising:
 (a) storing said variation angle and said dwell angle in a downhole memory; and   (b) transferring said variation angle and said dwell angle to a downhole processor cooperating with said bit perturbation device thereby periodic varying said action of said drill bit.   
   
   
       16 . A directional borehole assembly terminating a downhole end of a drill string, said assembly comprising:
 (a) a bit perturbation device cooperating with a bent sub;   (b) auxiliary sensors indicating orientation and position of said borehole assembly within said borehole;   (c) a telemetry system for communicating between said borehole assembly and the surface of the earth; and   (d) a downhole processor; wherein   (e) 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;   (f) said borehole is deviated by periodic varying action of said drill bit while continuously rotating said drill string; and   (g) 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.   
   
   
       17 . The assembly of  claim 16  wherein said bit perturbation device comprises a mud motor and a cooperating brake/clutch assembly or a mud motor and a drilling fluid variable bypass orifice or an element that imparts axial and azimuthal force components at said drill bit. 
   
   
       18 . The assembly of  claim 16  wherein said action of said drill bit speed is varied by periodic variation of rotation speed of said drill bit. 
   
   
       19 . The assembly of  claim 16  wherein said periodic variation of said drill bit is obtained by periodic variation of rate of penetration of said drill bit. 
   
   
       20 . The assembly of  claim 16  wherein said drill string and said drill bit are rotated simultaneously.