Apparatus and method for wellbore resistivity measurements in oil-based muds using capacitive coupling

Abstract

A resistivity device for downhole use with a nonconducting mud injects a current at a frequency sufficiently high to pass capacitively through the mud and a mud cake into the formation. The frequency is further selected so that the impedance due to the dielectric constant of the formation is small compared to the formation resistivity. Dual frequency measurements may be used to further minimize the effects of the mud and mud cake resistivity. When multiple frequency measurements are used, frequency focusing may be used to get an estimate of formation resistivity that is independent of near borehole effects.

1. An apparatus for use in a borehole for obtaining a resistivity parameter of an earth formation penetrated by the borehole, the borehole having a substantially nonconducting fluid having a dielectric constant therein, the apparatus comprising:
 (a) a downhole tool including at least one measure electrode coupled to the earth formation through said nonconducting fluid, said at least one measure electrode conveying a measure current from a source thereof into the formation, said measure current having a frequency selected to make an impedance caused by said dielectric constant to be substantially less than a resistivity of said nonconductive fluid, said frequency being further selected to make an impedance caused by a dielectric constant of said formation to be substantially less greater than a resistivity of said formation; 
 (b) a device responsive to least one of (I)
 (i) the current in the at least one measure electrode, and, 
 (ii) a voltage of the at least one measure electrode for producing a measure signal representative of the resistivity parameter. 
 
 
     
     
       2. A method of obtaining a resistivity parameter of an earth formation penetrated by a borehole having a substantially nonconducting fluid therein, the method comprising:
 (a) conveying a measure current through a measure electrode into the earth formation by capacitive coupling through the nonconducting fluid, said measure current having a first frequency selected to make an impedance caused by said dielectric constant to be substantially less than a resistivity of said nonconductive fluid, and an impedance caused by a dielectric constant of said formation to be substantially less greater than a resistivity of said formation; and 
 (b) determining at least one of (I)
 (i) the current at the at least one measure electrode, and, 
 (ii) a voltage of the at least one measure electrode for producing a measure signal representative of the resistivity parameter. 
 
 
     
     
       3. A method of obtaining a resistivity parameter of an earth formation penetrated by a borehole having a substantially nonconducting fluid therein, the method comprising:
 (a) conveying a measure current through a measure electrode into the earth formation by capacitive coupling through the nonconducting fluid, said measure current having a first frequency selected to make an impedance caused by said dielectric constant to be substantially less than a resistivity of said nonconductive fluid; 
 (b) repeating (a) at a second frequency, 
 (c) determining at least one of (I)
 (i) the current at the at least one measure electrode, and, 
 (ii) a voltage of the at least one measure electrode for producing a measure signal representative of the resistivity parameter at said first frequency and said second frequency; and 
 
 (d) determining from said measure signals at said first and second frequencies said resistivity parameter. 
 
     
     
       4. The method of  claim 3  further comprising using a conducting pad for maintaining focusing of said measure current into the formation. 
     
     
       5. The method of  claim 3  wherein the at least one measure electrode comprises an array of measure electrodes, the method further comprising obtaining a resistivity image of the formation. 
     
     
       6. The method of  claim 3  further comprising selecting said first frequency for making an impedance caused by a dielectric constant of said formation to be substantially less greater than a resistivity of said formation. 
     
     
       7. The method of  claim 3  further comprising using a processor for determining from said measure signal said resistivity parameter of the formation. 
     
     
       8. The method of  claim 3  further comprising using at least one of (I) (i) a wireline, and, (ii) a drilling tubular, for conveying said at least one measure electrode into the borehole. 
     
     
       9. The method of  claim 3  wherein determining said resistivity parameter further comprises using a processor. 
     
     
       10. The method of  claim 9  wherein said processor is at a location selected from (I) a downhole location, and, (II) a surface location. 
     
     
       11. A method of obtaining a resistivity parameter of an earth formation penetrated by a borehole having a substantially nonconducting fluid having a dielectric constant therein, the method comprising:
 (a) conveying a measure current through a measure electrode into the earth formation by capacitive coupling through the nonconducting fluid at a plurality of frequencies, each of said plurality of frequencies selected to make an impedance caused by said dielectric constant to be substantially less than a resistivity of said nonconductive fluid; 
 (b) determining at least one of (I) (i) the current at the at least one measure electrode, and, (ii) a voltage of the at least one measure electrode for producing a measure signal representative of the resistivity parameter at each of said plurality of frequencies; and 
 (c) frequency focusing apparent conductivities derived from said measure signal at each of said plurality of frequencies and obtaining said resistivity parameter. 
 
     
     
       12. The method of  claim 11  further comprising selecting said frequency for making an impedance caused by a dielectric constant of the formation to be substantially less greater than a resistivity of said earth formation. 
     
     
       13. The method of  claim 11  wherein said frequency focusing further comprises representing each measured signal at said plurality of frequencies in a Taylor Series expansion. 
     
     
       14. The method of  claim 11  wherein said resistivity parameter is related to a coefficient of an ω 3/2  term in said Taylor series expansion.