Method and apparatus for improved current focusing in galvanic resistivity measurement tools for wireline and measurement-while-drilling applications
AssigneeBaker Hughes Incorporated
InventorsMatthias Gorek, Martin Folberth, Christian Fulda
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Abstract
A galvanic resistivity tool with one or more measurement electrodes and guard electrodes, and one or more shielding electrodes. The potential difference between the measurement electrodes and the guard electrodes is controlled to minimize current flow between the measurement electrodes and the shielding electrodes. This provides improved focusing and avoids effects of contact impedance.
Claims21
An apparatus for determining a resistivity parameter of an earth formation, the apparatus comprising:
- (a) a logging tool in a borehole in the earth formation, the logging tool having at least one measurement electrode configured to convey a measure current into the earth formation;
- (b) at least one guard electrode associated with the at least one measurement electrode configured to focus the measure current;
- (c) a shielding electrode interposed between the at least one guard electrode and the at least one measurement electrode;
- and
- (d) circuitry that minimizes a current flow between the at least one measurement electrode and the shielding electrode, the circuitry including a processor configured to alter at least one of (i) a potential of the at least one measurement electrode, (ii) an impedance between the at least one guard electrode and the at least one measurement electrode, and (iii) a potential of a least one guard electrode.
A method fo determining a resistivity parameter of an earth formation, the method comprising:
- (a) conveying a measure current into the earth formation using a measurement electrode on a logging tooling in the earth formation;
- (b) focusing the measure current using a guard electrode on the logging tool;
- (c) shielding the measurement electrode from the guard electrode by a shielding electrode;
- and
- (d) minimizing a current flow between the measurement electrode and the shielding electrode by a least one of (i) altering the potential of the measurement electrode, (ii) altering a potential of the guard electrode, (iii) altering an impedance between the guard electrode and the measurement electrode.
A computer readable medium for use with a logging tool used in a borehole in an earth formation, the logging tool comprising:
- (a) a measurement electrode which conveys a measure current into the earth formation;
- (b) a guard electrode which focuses the measure current;
- and
- (c) a shielding electrode positioned between the guard electrode and the measurement electrode,
- the medium comprising instructions enabling a processor to:
- (d) minimize a current flow between the measurement electrode and the shielding electrode by altering at least one of (i) a potential of the at least one measurement electrode, (ii) a potential of the at least one guard electrode, and (iii) an impedance between the at least one guard electrode and the at least one measurement electrode;
- and
- (e) determine an apparent resistivity from a potential of the guard or measurement electrode and the measure current.
Dependencies are inferred from the claim text.