Downhole electromagnetic telemetry apparatus

Abstract

An assembly for use in subsurface drilling includes a downhole probe having an EM telemetry signal generator and electrical contacts for carrying telemetry signals from the EM telemetry signal generator to first and second parts of a gap sub in a drill string. An outside surface of the probe and an inside surface of the gap sub are covered with layers of electrically-insulating material. Electrical conduction paths internal to the gap sub are removed, thereby increasing efficiency of EM telemetry.

What is claimed is: 
     
         1 . A probe for subsurface drilling comprising:
 an elongated metallic housing enclosing electronics including a signal generator, the elongated housing comprising first and second electrical contacts spaced apart longitudinally on an outside of the housing and an electrically-insulating gap comprising an electrically-insulating material providing electrical isolation between first and second parts of the metallic housing, the gap located between the first and second electrical contacts; and,   a first electrically-insulating layer on an outside surface of the metallic housing, the first electrically-insulating layer at least partially covering the electrically-insulating gap and extending continuously to cover a length of the outside surface of the metallic housing.   
     
     
         2 . The probe according to  claim 1  wherein the signal generator is an electromagnetic telemetry signal generator. 
     
     
         3 . The probe according to any one of  claims 1  and  2  wherein the first and second electrical contacts are located at opposed ends of the elongated metallic housing. 
     
     
         4 . The probe according to any one of  claims 1  to  3  wherein the first electrically-insulating layer continuously covers the outside surface of the metallic housing for a distance of at least 65% of a distance between the first and second electrical contacts. 
     
     
         5 . The probe according to any one of  claims 1  to  3  wherein the first electrically-insulating layer continuously covers the outside surface of the metallic housing for a distance of at least 80% of a distance between the first and second electrical contacts. 
     
     
         6 . The probe according to any one of  claims 1  to  3  wherein the first electrically-insulating layer continuously covers the outside surface of the metallic housing for a distance of at least 1 meter. 
     
     
         7 . The probe according to any one of  claims 1  to  3  wherein the first electrically-insulating layer continuously covers the outside surface of the metallic housing for a distance of at least 2 meters. 
     
     
         8 . The probe according to any one of  claims 1  to  7  wherein the first electrical contact is in electrical contact with the first part of the housing. 
     
     
         9 . The probe according to  claim 8  wherein the second electrical contact is in electrical contact with the second part of the housing. 
     
     
         10 . The probe according to any one of  claims 1  to  9  wherein the first electrically-insulating layer comprises a thermoplastic material. 
     
     
         11 . The probe according to any one of  claims 1  to  9  wherein the first electrically-insulating layer comprises a material selected from the group consisting of: thermoplastics, epoxies, ceramics, elastic polymers, and rubber. 
     
     
         12 . The probe according to any one of  claims 1  to  11  wherein the first electrically-insulating layer comprises a coating applied to an outside surface of the probe. 
     
     
         13 . The probe according to any one of  claims 1  to  11  wherein the first electrically-insulating layer comprises a pre-formed component engaged around the outside surface of the probe. 
     
     
         14 . The probe according to  claim 13  wherein the pre-formed component comprises a pre-formed tubular sleeve. 
     
     
         15 . The probe according to any one of  claims 1  to  14  wherein the first electrically-insulating layer is integrated with a centralizer. 
     
     
         16 . The probe according to  claim 15  wherein longitudinally-extending ridges or bumps are provided on an outside surface of the first electrically-insulating layer. 
     
     
         17 . A probe combination comprising the probe according to any one of  claims 1  to  16  in combination with a gap sub, the gap sub comprising an electrically-conducting uphole part comprising an uphole coupling for coupling into a drill string, an electrically-conducting downhole part comprising a downhole coupling for coupling into the drill string, a bore extending through the gap sub from the uphole coupling to the downhole coupling and an electrically-insulating gap portion electrically isolating the uphole part of the gap sub from the downhole part of the gap sub wherein the probe is located within the bore of the gap sub and the first electrical contact is in electrical contact with the uphole part of the gap sub and the second electrical contact is in electrical contact with the downhole part of the gap sub. 
     
     
         18 . The probe combination according to  claim 17  wherein the electrically-insulating gap of the probe is longitudinally spaced apart from the electrically-insulating gap portion of the gap sub. 
     
     
         19 . The probe combination according to any one of  claims 17  and  18  comprising a second electrically-insulating layer extending around the probe within the bore of the gap sub. 
     
     
         20 . The probe combination according to  claim 19  wherein the first and second electrically-insulating layers are both at least 2 meters long. 
     
     
         21 . The probe combination according to  claim 19  wherein the second electrically-insulating layer is at least 75% as long as the first electrically-insulating layer. 
     
     
         22 . The probe combination according to  claim 19  wherein the second electrically-insulating layer covers substantially the entire portion of a wall of the bore of the gap sub lying between the first and second electrical contacts. 
     
     
         23 . The probe combination of any one of  claims 19  to  22  wherein the second electrically-insulating layer lines an inner wall of the bore of the gap sub. 
     
     
         24 . The probe combination of  claim 23  wherein the second electrically-insulating layer comprises a coating applied to the inner wall of the bore of the gap sub. 
     
     
         25 . The probe combination of  claim 23  wherein the second electrically-insulating layer comprises a tubular sleeve engaged around the inner wall of the bore of the gap sub. 
     
     
         26 . The probe combination of any one of  claims 19  to  22  further comprising a drill collar coupled to a downhole end of the gap sub wherein the second electrically-insulating layer lines an inner wall of a bore of the drill collar. 
     
     
         27 . The probe combination of  claim 26  wherein the second electrically-insulating layer comprises a coating applied to the inner wall of the bore of the drill collar. 
     
     
         28 . The probe combination of  claim 26  wherein the second electrically-insulating layer comprises a pre-formed component engaged around the inner wall of the bore of the drill collar. 
     
     
         29 . The probe combination of any one of  claims 19  to  23  and  26  wherein the second electrically-insulating layer comprises a tubular sleeve formed with longitudinally-extending lobes that contact the first electrically-insulating layer on the outside surface of the metallic housing. 
     
     
         30 . The probe combination of  claim 29  wherein at least one of the first electrically-insulating layer and the second electrically-insulating layer comprises a material that provides mechanical damping. 
     
     
         31 . The probe combination of any one of  claims 17  to  30  wherein the gap sub comprises inwardly-extending parts projecting inwardly on an inside of the bore. 
     
     
         32 . The probe combination of  claim 31  wherein the inwardly-extending parts comprise longitudinally-extending ridges. 
     
     
         33 . The probe combination of  claim 32  wherein the longitudinally-extending ridges comprise rounded lobes. 
     
     
         34 . The probe combination of any one of  claims 32  and  33  wherein the longitudinally-extending ridges comprise metal ridges integrally-formed with one or both of the uphole and downhole parts of the gap sub. 
     
     
         35 . The probe combination of any one of  claims 31  to  34  wherein the inwardly extending parts extend to support the probe from a plurality of different circumferential directions. 
     
     
         36 . The probe combination of any one of  claims 17  to  25  comprising a drill collar coupled to a downhole end of the gap sub wherein the drill collar comprises a bore and inwardly-extending parts projecting inwardly on an inside of the bore of the drill collar wherein the probe extends into the drill collar. 
     
     
         37 . A subsurface drilling assembly comprising:
 a gap sub, the gap sub comprising an electrically-conducting uphole part comprising an uphole coupling for coupling into a drill string, an electrically-conducting downhole part comprising a downhole coupling for coupling into the drill string, a bore extending through the gap sub from the uphole coupling to the downhole coupling and an electrically-insulating gap portion electrically isolating the uphole part of the gap sub from the downhole part of the gap sub;   a probe extending within the bore, the probe comprising an elongated housing enclosing electronics including a signal generator, the elongated housing comprising first and second electrical contacts spaced apart longitudinally on an outside of the housing; and   a fluid-carrying channel bypassing the probe, wherein walls of the fluid-carrying channel are electrically-insulating at least in a section of the channel extending longitudinally from a location above the electrically-insulating gap portion to a location below the electrically-insulating gap portion.   
     
     
         38 . A subsurface drilling assembly according to  claim 37  wherein the fluid-carrying channel is configured to carry all fluid flowing in the bore above the probe. 
     
     
         39 . A subsurface drilling assembly according to  claim 38  wherein the fluid-carrying channel is annular in cross-section and extends around the probe. 
     
     
         40 . A subsurface drilling method performed using a drill string comprising a gap sub and an electronics package located in a bore of the gap sub wherein the electronics package comprises first and second electrical contacts that are in electrical contact with electrically-conductive parts of the gap sub, the method comprising:
 passing a drilling fluid down a bore of the drill string; and,   at the location of the electronics package, channeling the drilling fluid into a channel that is electrically insulated from both the electrically conductive parts of the gap sub and electrically conductive parts of the housing of the electronics package.   
     
     
         41 . The method of  claim 40  wherein the channel is annular in cross section. 
     
     
         42 . The method according to any one of  claims 40  and  41  wherein the channel surrounds at least that portion of the electronics package between the electrical contacts. 
     
     
         43 . The method according to any one of  claims 40  to  42  comprising carrying the drilling fluid in the channel for a distance of at least 1 meter. 
     
     
         44 . The method according to any one of  claims 40  to  42  comprising carrying the drilling fluid in the channel for a distance of at least 1½ meters. 
     
     
         45 . The method according to any one of  claims 40  to  42  comprising carrying the drilling fluid in the channel for a distance of at least 65% of a distance between the first and second electrical contacts. 
     
     
         46 . A subsurface drilling assembly comprising:
 a gap sub, the gap sub comprising an electrically-conducting uphole part comprising an uphole coupling for coupling into a drill string, an electrically-conducting downhole part comprising a downhole coupling for coupling into the drill string, a bore extending through the gap sub from the uphole coupling to the downhole coupling and an electrically-insulating gap portion electrically isolating the uphole part of the gap sub from the downhole part of the gap sub;   an EM telemetry signal generator housed within a wall of the gap sub, the EM telemetry signal generator having output leads electrically coupled to the uphole and downhole parts of the gap sub; and,   an electrically-insulating sleeve lining the bore of the gap sub, the electrically insulating sleeve covering at least one interface between the electrically insulating gap portion of the gap sub and one of the uphole and downhole parts of the gap sub and extending continuously along the one of the uphole and downhole parts of the gap sub.   
     
     
         47 . Apparatus having any new and inventive feature, combination of features, or sub-combination of features as described herein. 
     
     
         48 . Methods having any new and inventive steps, acts, combination of steps and/or acts or sub-combination of steps and/or acts as described herein.