Calibration method for a microresistivity logging tool
First Claim
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1. A method for making calibrated microresistivity logging measurements, the method comprising:
- (a) deploying a microresistivity logging tool in a nonconductive medium, the tool including (i) a current injection electrode configured to inject electrical current into a formation, (ii) a return electrode spaced apart from the current injection electrode, the return electrode providing a return path for the electrical current, and (iii) at least first and second spaced potential electrodes;
(b) applying a first alternating current between the current injection electrode and the return electrode;
(c) measuring a first AC potential difference between the first and second potential electrodes;
(d) redeploying the logging tool in a subterranean borehole including a nonconductive drilling fluid;
(e) applying a second alternating current between the current injection electrode and the return electrode;
(f) measuring a second AC potential difference between the first and second potential electrodes;
(g) causing a processor to subtract a first parameter derived from the first AC potential difference measured in (c) from a second parameter derived from the second AC potential difference measured in (f) to obtain a calibrated impedance; and
(h) causing a processor to compute a formation resistivity from the calibrated impedance obtained in (g).
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Abstract
Methods for making calibrated microresistivity logging measurements in nonconductive drilling fluid are disclosed. A calibrated measurement parameter (e.g., a calibrated electrical impedance or a calibrated potential difference) is obtained and further utilized to compute a formation resistivity. The methods tend to be particularly well suited for drilling applications in which the borehole environment is highly resistive (i.e., boreholes in which both the drilling fluid and the formation have a high resistivity).
37 Citations
16 Claims
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1. A method for making calibrated microresistivity logging measurements, the method comprising:
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(a) deploying a microresistivity logging tool in a nonconductive medium, the tool including (i) a current injection electrode configured to inject electrical current into a formation, (ii) a return electrode spaced apart from the current injection electrode, the return electrode providing a return path for the electrical current, and (iii) at least first and second spaced potential electrodes; (b) applying a first alternating current between the current injection electrode and the return electrode; (c) measuring a first AC potential difference between the first and second potential electrodes; (d) redeploying the logging tool in a subterranean borehole including a nonconductive drilling fluid; (e) applying a second alternating current between the current injection electrode and the return electrode; (f) measuring a second AC potential difference between the first and second potential electrodes; (g) causing a processor to subtract a first parameter derived from the first AC potential difference measured in (c) from a second parameter derived from the second AC potential difference measured in (f) to obtain a calibrated impedance; and (h) causing a processor to compute a formation resistivity from the calibrated impedance obtained in (g). - View Dependent Claims (2, 3, 4, 5, 6)
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7. A method for making calibrated microresistivity logging measurements, the method comprising:
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(a) deploying a microresistivity logging tool in a nonconductive medium, the tool including (i) a current injection electrode configured to inject electrical current into a formation, (ii) a return electrode spaced apart from the current injection electrode, the return electrode providing a return path for the electrical current, and (iii) at least first, second, and third spaced potential electrodes; (b) applying an alternating current between the current injection electrode and the return electrode; (c) measuring a first AC potential difference between the first and second potential electrodes and a second AC potential difference between the second and third potential electrodes; (d) causing a processor to compute a ratio of the first and second AC potential differences measured in (c); (e) redeploying the logging tool in a subterranean borehole including a nonconductive drilling fluid; (f) repeating (b) and (c); (g) causing a processor to apply the ratio computed in (d) to the first and second potential differences measured in (f) to obtain a calibrated potential difference; and (h) causing a processor to compute a formation resistivity from the calibrated potential difference obtained in (g) wherein the ratio is computed in (d) according to the following mathematical equation; - View Dependent Claims (8, 9, 10, 11, 12, 13)
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14. A logging while drilling tool comprising:
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a logging while drilling tool body; a current injection electrode deployed in the tool body and configured to inject electrical current into a formation; a return electrode spaced apart from the current injection electrode, the return electrode providing a return path for the electrical current; at least first, second, and third potential electrodes deployed between the current injection electrode and the return electrode; and a controller configured to (i) apply an alternating current between the current injection electrode and the return electrode when the logging while drilling tool is deployed in a nonconductive medium;
(ii) measure a first AC potential difference between the first and second potential electrodes and a second AC potential difference between the second and third potential electrodes;
(iii) compute a ratio of the first and second AC potential differences measures in (ii);
(iv) repeat (i) and (ii) when the logging while drilling tool is deployed in a subterranean borehole;
(v) apply the ratio computed in (iii) to the first and second potential differences measured in (iv) to obtain a calibrated potential difference; and
(vi) compute a formation resistivity from the calibrated potential difference obtained in (v). - View Dependent Claims (15, 16)
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Specification