Electromagnetic telemetry gap sub

Abstract

A downhole tool and method of assembly. A sleeve is moved along a longitudinal axis of a first sub to radially surround an inner mandrel of the first sub that extends along the longitudinal axis. A gap sub is moved along the longitudinal axis to be disposed around an extended section of the inner mandrel, wherein the gap sub includes nonconductive material. The gap sub is configured to electrically separate the first sub from a second sub of the downhole tool. A lock nut is fixedly connected to the inner mandrel to secure the gap sub against the sleeve.

What is claimed is: 
     
         1 . A downhole tool of a bottomhole assembly for use in a wellbore, comprising:
 a voltage source comprising a first pole electrically connected to a first portion of the bottomhole assembly and a second pole electrically connected to a second portion of the bottomhole assembly, wherein the first portion and the second portion are electrically separated by electrically nonconductive material between the first portion and the second portion;   wherein the first portion includes a housing defining an outer housing surface of the first portion and wherein the second portion includes an inner mandrel and a lock nut fixedly connected to the inner mandrel, wherein the lock nut secures the electrically nonconductive material to the housing.   
     
     
         2 . The downhole tool of  claim 1 , wherein the lock nut has an outer lock nut diameter and the outer housing surface has an outer housing surface diameter that is larger than the outer lock nut diameter. 
     
     
         3 . The downhole tool of  claim 1 , further including at least one friction shim disposed axially between the housing and the second portion. 
     
     
         4 . The downhole tool of  claim 1 , further including a torque sleeve disposed axially between the lock nut and the electrically nonconductive material, the torque sleeve including an anti-rotation element that prevents rotation of the torque sleeve relative to the inner mandrel. 
     
     
         5 . The downhole tool of  claim 1 , wherein the lock nut is fixedly connected to the inner mandrel by a thread. 
     
     
         6 . The downhole tool of  claim 1 , wherein the lock nut secures the electrically nonconductive material at least partially by clamping. 
     
     
         7 . The downhole tool of  claim 3 , wherein the friction shim comprises one or more microparticles at one or more surfaces of the friction shim. 
     
     
         8 . The downhole tool of  claim 3 , wherein the friction shim comprises an anti-rotation element that prevents rotation of the friction shim relative to the inner mandrel. 
     
     
         9 . The downhole tool of  claim 5 , wherein the thread comprises two mating surfaces and wherein the two mating surfaces are in electrical contact. 
     
     
         10 . The downhole tool of  claim 1 , wherein the housing and/or the second portion are in direct contact with a drilling fluid in an annulus between a wall of the wellbore and the downhole tool. 
     
     
         11 . The downhole tool of  claim 1 , further comprising a sleeve, wherein the inner mandrel is disposed within the sleeve and wherein the sleeve is electrically isolated from the inner mandrel. 
     
     
         12 . The downhole tool of  claim 1 , wherein the electrically nonconductive material comprises one or more insulating rings. 
     
     
         13 . A method of assembling a downhole tool suitable for use in a wellbore, the method comprising:
 moving a sleeve along a longitudinal axis of a first sub to radially surround an inner mandrel of the first sub that extends along the longitudinal axis;   moving a gap sub along the longitudinal axis to be disposed around an extended section of the inner mandrel wherein the gap sub comprises electrically nonconductive material, the gap sub configured to electrically separate the first sub from a second sub of the downhole tool; and   fixedly connecting a lock nut to the inner mandrel to secure the gap sub against the sleeve.   
     
     
         14 . The method of  claim 13 , further comprising
 compressing the sleeve and the gap sub relative to the inner mandrel and/or elongating the inner mandrel relative to the gap sub;   connecting the lock nut to the inner mandrel while the sleeve and the gap sub is compressed relative to the inner mandrel and/or while the inner mandrel is elongated relative to the gap sub, respectively; and   releasing, after connecting the lock nut, the sleeve and the gap sub from compression relative to the inner mandrel and/or the inner mandrel from elongation relative to the gap sub, respectively.   
     
     
         15 . The method of  claim 13 , wherein the lock nut has an outer lock nut diameter and the gap sub has an outer gap sub diameter that is larger than the outer lock nut diameter. 
     
     
         16 . The method of  claim 13 , wherein the gap sub comprises one or more friction shims. 
     
     
         17 . The method of  claim 13 , wherein the gap sub comprises one or more torque sleeves, the one or more torque sleeves including an anti-rotation element that prevents rotation of the one or more torque sleeves relative to the inner mandrel. 
     
     
         18 . The method of  claim 13 , wherein the lock nut is fixedly connected to the inner mandrel by a thread that comprises two mating surfaces, and wherein the two mating surfaces are in electrical contact. 
     
     
         19 . The method of  claim 16 , wherein the one or more friction shims comprise an anti-rotation element that prevents rotation of the one or more friction shims relative to the inner mandrel. 
     
     
         20 . The method of  claim 13 , wherein during assembling the downhole tool no electrically nonconductive material in the downhole tool is subject to a friction force that is one percent or higher of the highest friction force that occurs between any of the conductive components during assembling the downhole tool.