High resolution resistivity earth imager

Abstract

An imaging tool made includes a pad whose curvature is chosen based on the expected range of borehole radius and the pad size so as to maintain the maximum standoff below a desired value. The curvature may be adjusted using fasteners.

1 . A method of estimating a resistivity property of an earth formation, the method comprising:
 determining a size of a borehole in which a resistivity imaging instrument is to be used, the resistivity imaging instrument including a pad having a face configured to have an adjustable radius of curvature;   adjusting the radius of curvature of the pad face using at least the determined borehole size;   conveying the resistivity imaging instrument into the borehole;   measuring a parameter indicative of the resistivity property using a plurality of sensors on the pad; and   imaging the resistivity property using the measurements obtained by the plurality of sensors.   
     
     
         2 . The method of  claim 1 , wherein the radius of curvature is adjusted by also using a selected value for a maximum standoff of the pad. 
     
     
         3 . The method of  claim 2  further comprising specifying the value of maximum standoff based on a simulation result. 
     
     
         4 . The method of  claim 1  wherein the plurality of sensors comprise electrodes and the measurements comprise currents. 
     
     
         5 . The method of  claim 1  wherein the plurality of sensors comprise antennas responsive to a magnetic field produced by a current in the formation. 
     
     
         6 . The method of  claim 1  further comprising conveying the resistivity imaging instrument into the borehole as part of a string of logging instruments on a wireline. 
     
     
         7 . The method of  claim 1  wherein the imaging is performed using an output of a subset of the plurality of sensors. 
     
     
         8 . An apparatus for providing an image of a resistivity property of an earth formation, the apparatus comprising:
 a resistivity imaging instrument including:
 a pad, the pad including a face having an adjustable radius of curvature; 
 a plurality of sensors responsive to a resistivity property of the earth formation; and 
   at least one processor configured to use an output of each of plurality of sensors on the pad to provide an image of the resistivity property.   
     
     
         9 . The apparatus of  claim 8 , wherein the adjustable radius of curvature of the pad reduces a stand off at one of: (i) a center of the pad, and (ii) a location away from the center of the pad. 
     
     
         10 . The apparatus of  claim 8 , wherein the radius of curvature is adjusted at one of: (i) a single location on the pad face, and (ii) a plurality of locations on the pad face. 
     
     
         11 . The apparatus of  claim 8  wherein the plurality of sensors comprise electrodes and the output of the sensors comprise currents. 
     
     
         12 . The apparatus of  claim 8  wherein the plurality of sensors comprise antennas responsive to a magnetic field produced by a current in the formation. 
     
     
         13 . The apparatus of  claim 8  further comprising a wireline configured to convey the resistivity imaging instrument into the borehole. 
     
     
         14 . The apparatus of  claim 8  further wherein the pad is extendable from a bottomhole assembly conveyed on a drilling tubular. 
     
     
         15 . A non-transitory computer-readable medium product accessible to a processor, the computer-readable medium including instructions which enable the processor to use measurements made by a resistivity imaging instrument to image a resistivity property of a formation, the resistivity imaging instrument including a pad having a face with an adjustable radius of curvature and a plurality of sensors responsive to the resistivity property of the earth formation. 
     
     
         16 . The non-transitory computer-readable medium product of  claim 15  further comprising at least one of: (i) a ROM, (ii) an EPROM, (iii) an EAROM, (iv) a flash memory, and (v) an optical disk.