Closed-Loop Geosteering Device and Method

Abstract

A closed-loop method for geosteering a subterranean borehole includes rotating a roll-stabilized control unit in the borehole, obtaining formation evaluation sensor measurements via sensors located in the control unit, and processing downhole the sensor measurements to obtain a corrective steering tool setting that may be applied to a steering tool to control a direction of drilling. A logging while drilling method includes obtaining sensor measurements via sensors located on a rotating roll-stabilized control unit and processing the sensor measurements to obtain an LWD image. A downhole tool includes a downhole tool body and a roll-stabilized control unit. The roll-stabilized control unit is deployed in a through bore of the tool body such that it is free to rotate with respect to the tool body. A tool face sensor and a partially shielded gamma ray sensor are deployed in the roll-stabilized control unit.

I claim: 
     
         1 . A closed-loop method for geosteering a subterranean borehole, the method comprising:
 (a) causing a bottom hole assembly to drill a subterranean borehole, the bottom hole assembly including a steering tool having a roll-stabilized control unit, the roll stabilized control unit including at least one formation evaluation sensor and a tool face sensor deployed therein;   (b) causing the roll-stabilized unit to rotate with respect to the borehole while drilling in (a);   (c) causing the formation evaluation sensor and the tool face sensor to obtain corresponding sensor measurements while rotating in (b);   (d) processing the sensor measurements obtained in (c) to compute a borehole image parameter;   (e) processing the borehole image parameter to obtain a corrective steering tool setting; and   (f) applying the corrective steering tool setting to the steering tool to change a direction of drilling.   
     
     
         2 . The method of  claim 1 , further comprising:
 (g) continuously repeating (c), (d), (e), and (f) while drilling in (a).   
     
     
         3 . The method of  claim 1 , wherein the formation evaluation sensor comprises a gamma ray sensor deployed in the roll-stabilized control unit. 
     
     
         4 . The method of  claim 3 , wherein the gamma ray sensor comprises a substantially cylindrical scintillator deployed co-axially in a semi-cylindrical shield. 
     
     
         5 . The method of  claim 3 , wherein the gamma ray sensor comprises a substantially cylindrical scintillator deployed in a cylindrical shield, the scintillator being axially offset from the cylindrical shield. 
     
     
         6 . The method of  claim 3 , wherein the gamma ray sensor comprises a substantially cylindrical scintillator deployed in the roll-stabilized control unit and a partially cylindrical shield is deployed about the roll-stabilized control unit on an inner surface of a rotary steerable tool body. 
     
     
         7 . The method of  claim 1 , wherein the borehole image parameter is at least one of a difference or a ratio between high side gamma ray counts and low side gamma ray counts, a relative dip angle between the borehole and a formation boundary of interest, and an azimuthal width and intensity of a gamma ray peak or trough. 
     
     
         8 . The method of  claim 1 , wherein (d) further comprises processing the sensor measurements obtained in (c) and a rate of penetration in (a) to compute the borehole image parameter. 
     
     
         9 . The method of  claim 1 , wherein (e) further comprises processing the borehole image parameter and a borehole azimuth measurement to obtain the corrective steering tool setting. 
     
     
         10 . The method of  claim 1 , further comprising:
 (g) receiving a borehole azimuth measurement;   (h) processing the borehole azimuth measurement to obtain a second corrective steering tool setting, the second steering tool setting operative to change a borehole azimuth; and   (i) applying second corrective steering tool setting to the steering tool to change a direction of drilling.   
     
     
         11 . The method of  claim 1 , wherein (e) comprises processing the borehole image parameter in combination with a target image parameter to obtain a corrective steering tool setting. 
     
     
         12 . The method of  claim 11 , further comprising:
 (g) causing a surface system to execute an outer control loop to obtain the target image parameter; and   (h) downlinking the target image parameter from the surface system to a downhole processor for processing in (e).   
     
     
         13 . A downhole tool comprising:
 a downhole tool body;   a roll-stabilized control unit deployed in a through bore of the downhole tool body, the roll-stabilized control unit being free to rotate with respect to the downhole tool body;   a tool face sensor deployed in the roll-stabilized control unit; and   a partially shielded gamma ray sensor deployed in the roll-stabilized control unit, the gamma ray sensor including a scintillator crystal and a substantially cylindrical shield that subtends an angle less than 360 degrees.   
     
     
         14 . The downhole tool of  claim 13 , wherein the tool is a rotary steerable tool and the downhole tool body is a rotary steerable tool body; 
     
     
         15 . The downhole tool of  claim 13 , wherein the tool is a logging while drilling tool and the downhole tool body is a logging while drilling tool body; 
     
     
         16 . The downhole tool of  claim 13 , wherein the downhole tool body is configured to be connected with a drill string such that the tool body is rotationally coupled with the drill string. 
     
     
         17 . The downhole tool of  claim 13 , wherein the scintillator is substantially cylindrical and deployed co-axially in a semi-cylindrical shield with both the scintillator and the shield being deployed in the roll-stabilized control unit. 
     
     
         18 . The downhole tool of  claim 13 , wherein the scintillator is substantially cylindrical and deployed in a cylindrical shield, the scintillator being axially offset from the cylindrical shield with both the scintillator and the shield being deployed in the roll-stabilized control unit. 
     
     
         19 . The downhole tool of  claim 13 , wherein the scintillator is substantially cylindrical and deployed in the roll-stabilized control unit and the shield is partially cylindrical and deployed about the roll-stabilized control unit on an inner surface of a rotary steerable tool body. 
     
     
         20 . A logging while drilling method comprising:
 (a) deploying a logging while drilling tool in a subterranean borehole, the logging while drilling tool including a roll-stabilized control unit deployed in a tool body, the roll stabilized control unit including at least one formation evaluation sensor and a tool face sensor deployed therein;   (b) causing the logging while drilling tool body to be rotationally stationary with respect to the borehole;   (c) causing the roll-stabilized control unit to rotate with respect to the borehole;   (d) causing the formation evaluation sensor and the tool face sensor to obtain corresponding sensor measurements while rotating in (c);   (e) processing the corresponding formation evaluation sensor measurements and the tool face measurements obtained in (d) to obtain an image.