Formation resistivity imaging in conductive boreholes

Abstract

An apparatus for providing an image of a resistivity property of an earth formation surrounding a borehole is provided. The apparatus, in one aspect, may include a plurality of measure electrodes that are configured to convey an electrical current into the earth formation. In one aspect, each measure electrode may be substantially surrounded by an associated insulator that extends away from a tool body in a manner that blocks at least a portion of a vertical current flowing in a conductive fluid when the tool is operated to log the borehole. A processor provides an image of the resistivity property of the earth formation using the current in plurality of measure electrodes.

What is claimed is: 
     
         1 . An apparatus configured to provide an image of a resistivity property of an earth formation, the apparatus comprising:
 a logging tool configured to be conveyed in a borehole having a conductive fluid therein;   at least one measure electrode on a pad of the logging tool configured to sense an electrical current in the earth formation;   at least one return electrode;   an associated insulator surrounding at least one of the at least one measure electrode and the at least one return electrode and limiting a current path between the at least one measure electrode and the at least one return electrode; and   at least one processor configured to provide an image of the resistivity property using the current sensed by the at least one measure electrode.   
     
     
         2 . The apparatus of  claim 1  wherein:
 each associated insulator extends away from the pad of the logging tool, such that a gap exists between an outer face of the pad and a wall of the borehole, the gap extending vertically along the borehole to axial ends of the pad. 
 
     
     
         3 . The apparatus of  claim 1  wherein:
 each associated insulator limits a current path in the conductive fluid along the borehole between the at least one measure electrode and the at least one return electrode. 
 
     
     
         4 . The apparatus of  claim 1 , further comprising at least one guard electrode encircling the at least one measure electrode. 
     
     
         5 . The apparatus of  claim 4  wherein:
 each associated insulator further comprises a spacer that maintains a separation between a body of the corresponding associated insulator and a wall of the borehole, such that the at least one measure electrode is in direct contact with a continuous body of the conductive fluid in direct contact with the guard electrode where and when the pad is urged against the wall of the borehole. 
 
     
     
         6 . The apparatus of  claim 4  wherein:
 the at least one guard electrode is at a potential substantially equal to a potential of the at least one measure electrode. 
 
     
     
         7 . The apparatus of  claim 1  wherein the return electrode is at a location that is selected from: (i) on the logging tool, and (ii) at a remote location. 
     
     
         8 . A method of imaging a resistivity property of an earth formation, the method comprising:
 conveying a logging tool into a borehole having a conductive fluid therein;   using at least one measure electrode on a pad of the logging tool to sense an electrical current in the earth formation;   reducing a flow of current in the fluid in the borehole by surrounding at least one of the at least one measure electrode and the at least one return electrode with an associated insulator that limits a current path between the at least one measure electrode and the at least one return electrode; and   providing an image of the resistivity property using the current sensed by the at least one measure electrode.   
     
     
         9 . The method of  claim 8  wherein:
 each associated insulator extends away from the pad of the logging tool, such that a gap exists between an outer face of the pad and a wall of the borehole, the gap extending vertically along the borehole to axial ends of the pad. 
 
     
     
         10 . The method of  claim 8  further comprising:
 using each associated insulator to limit a current path in the conductive fluid along the borehole between the at least one measure electrode and the at least one return electrode. 
 
     
     
         11 . The method of  claim 8 , further comprising encircling the at least one measure electrode with at least one guard electrode. 
     
     
         12 . The method of  claim 11  further comprising:
 using, for each associated insulator, an insulator that further comprises a spacer that maintains a separation between a body of the corresponding associated insulator and a wall of the borehole, such that the at least one measure electrode is in direct contact with a continuous body of the conductive fluid in direct contact with the guard electrode where and when the pad is urged against the wall of the borehole. 
 
     
     
         13 . The method of  claim 11  further comprising:
 further reducing the axial flow of current in the borehole by maintaining the at least one guard electrode at a potential substantially equal to a potential of the at least one measure electrode. 
 
     
     
         14 . The method of  claim 8  further comprising providing the return electrode at a location that is selected from: (i) on the logging tool, and (ii) at a remote location. 
     
     
         15 . The method of  claim 8  further comprising:
 positioning the logging tool as part of a string of logging instruments; and 
 using a wireline configured to convey the logging string into the borehole. 
 
     
     
         16 . A non-transitory computer-readable medium product having stored thereon instructions that when read by at least one processor cause the at least one processor to perform a method, the method comprising:
 providing an image of a resistivity property of an earth formation using a current sensed by at least one measure electrode on a logging tool of an apparatus, the apparatus further including at least one return electrode, the logging tool conveyed in a borehole with a conductive fluid therein, an associated insulator surrounding at least one of the at least one measure electrode and the at least one return electrode.   
     
     
         17 . The non-transitory computer-readable medium product of  claim 16  further comprising at least one of: (i) a ROM, (ii) an EPROM, (iii) an EAROM, (iv) a flash memory, and (v) an optical disk.