MICRORESISTIVITY ANISOTROPY LOGGING TOOL EMPLOYING A MONOPOLE CURRENT INJECTION ELECTRODE

• US 20110156709A1
• Filed: 12/30/2009
• Published: 06/30/2011
• Est. Priority Date: 12/30/2009
• Status: Active Grant

• Current Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Original Assignee: Smith International Incorporated (Schlumberger NV)
• Inventors: Streinz, Christopher C., Wang, Tsili
• Granted Patent: US 8,581,594 B2
• Time in Patent Office: Days
• US Class Current: 324/366
• CPC Class Codes: G01V 3/24 using ac

Abstract

A microresistivity logging tool includes a monopole current injection electrode and at least first and second pairs of potential electrodes. The tool may further include a controller configured for making microresistivity anisotropy measurements using a single firing of the monopole current injection electrode. The controller may be configured to compute a two-dimensional tensor of the local formation resistivity from a single firing of the monopole current injection electrode. The use of a single firing tends to decrease measurement time, which in turn tends to improved azimuthal sensitivity in microresistivity anisotropy imaging while drilling applications.

Claims

1. A downhole microresistivity logging tool comprising:

a logging tool body;

a monopole current injection electrode configured to inject alternating electrical current into a formation;

a first pair of potential electrodes spaced apart from the current injection electrode in a first direction;

a second pair of potential electrodes spaced apart from the current injection electrode in a second direction; and

a controller configured to (i) apply an alternating current between the monopole current injection electrode and a return and (ii) measure first and second potential differences between the corresponding first and second pairs of potential electrodes.

2. The logging tool of claim 1, further comprising a return electrode, the return electrode providing a return path for the electrical current injected by the current injection electrode, the return electrode deployed in the tool body and spaced apart from the current injection electrode.

3. The logging tool of claim 2, wherein a ratio of an axial separation distance between the current injection electrode and the return electrode to a diameter of the logging while drilling tool body is at least ten.

4. The logging tool of claim 2, wherein a ratio of an axial separation distance between the current injection electrode and the return electrode to a separation distance between the current injection electrode and the potential electrodes is at least ten.

5. The logging tool of claim 1, wherein the monopole current injection electrode has a dipole impurity of less than about 5%.

6. The logging tool of claim 1, wherein the first direction and the second direction are orthogonal to one another.

7. The logging tool of claim 6, wherein the first direction is parallel with a longitudinal axis of the tool body.

8. The logging tool of claim 1, further comprising first and second return electrodes, the first and second return electrodes providing corresponding first and second return paths for the electrical current injected by the current injection electrode, the current injection electrode deployed axially between the first and second return electrodes.

9. The logging tool of claim 1, wherein the controller is configured to make substantially simultaneous measurements of the first potential difference and the second potential difference.

10. The logging tool of claim 1, wherein the controller is further configured to compute corresponding first and second formation resistivity values from the measured first and second potential differences.

11. A downhole microresistivity logging tool comprising:

a logging tool body;

a monopole current injection electrode configured to inject alternating electrical current into a formation;

a first array of potential electrodes spaced apart from the current injection electrode in a first direction, the first array including at least first and second pairs of potential electrodes, the first pair of potential electrodes being spaced apart from one another in the first direction and the second pair of potential electrodes being spaced apart from one another in the second direction;

a second array of potential electrodes spaced apart from the current injection electrode in the second direction, the second array including at least third and fourth pairs of potential electrodes, the third pair of potential electrodes being spaced apart from one another in the first direction and the fourth pair of potential electrodes being spaced apart from one another in the second direction; and

a controller configured to (i) apply an alternating current between the current injection electrode and a return and (ii) substantially simultaneously make first, second, third, and fourth AC voltage measurements using the corresponding first, second, third, and fourth pairs of potential electrodes.

12. The logging tool of claim 11, wherein a ratio of an axial separation distance between the current injection electrode and the return electrode to a diameter of the logging while drilling tool body is at least ten.

13. The logging tool of claim 11, wherein a ratio of an axial separation distance between the current injection electrode and the return electrode to a separation distance between the current injection electrode and the potential electrodes is at least ten.

14. The logging tool of claim 11, wherein the controller is further configured to compute a resistivity tensor of the formation from the AC voltage measurements.

15. The logging tool of claim 11, wherein the first direction and the second direction are orthogonal to one another.

16. The logging tool of claim 15, wherein the first direction is parallel with a longitudinal axis of the tool body.

17. A method for making microresistivity anisotropy logging measurements, the method comprising:

(a) deploying a microresistivity logging tool in a borehole, the logging tool including (i) a monopole current injection electrode and (ii) at least first and second pairs of potential electrodes, the first pair of potential electrodes being spaced apart from the current injection electrode in a first direction and the second pair of potential electrodes being spaced apart from the current injection electrode in a second direction;

(b) causing an alternating electrical current to be injected into a subterranean formation at the monopole current injection electrode;

(c) causing a first AC potential between the first pair of potential electrodes and a second AC potential between the second pair of potential electrodes to be measured substantially simultaneously; and

(d) computing first and second resistivity values using the corresponding first and second AC potentials measured in (c).

18. The method of claim 17, wherein the resistivity values are computed in (d) according to the following mathematical equations:

19. The method of claim 17, wherein the first direction is substantially parallel with a longitudinal axis of the logging tool and the first and second directions are orthogonal to one another.

20. The method of claim 17, wherein the microresistivity logging tool further comprises an azimuth sensor and (c) further comprises causing the azimuth sensor to measure an azimuth angle, the method further comprising:

(e) correlating the resistivity values computed in (d) with the azimuth angle measured in (c).

21. A method for making microresistivity anisotropy logging measurements, the method comprising:

(a) deploying a microresistivity logging tool in a borehole, the logging tool including (i) a monopole current injection electrode and (ii) at least first, second, third, and fourth pairs of potential electrodes, the first and second pairs of potential electrodes being spaced apart from the current injection electrode in a first direction and the third and fourth pairs of potential electrodes being spaced apart from the current injection electrode in a second direction;

(b) causing an alternating electrical current to be injected into a subterranean formation at the monopole current injection electrode;

(c) causing first, second, third, and fourth AC potentials between the corresponding first, second, third, and fourth pairs of potential electrodes to be measured substantially simultaneously;

(d) processing the first, second, third, and fourth AC potentials measured in (c) to obtain a voltage tensor; and

(e) computing a resistivity tensor from the voltage tensor obtained in (d).

22. The method of claim 21, wherein the first direction is substantially parallel with a longitudinal axis of the logging tool and the first and second directions are orthogonal to one another.

23. The method of claim 22, wherein the voltage tensor V is obtained in (d) according to the following mathematical equation:

24. The method of claim 23, wherein the resistivity tensor R, is computed in (e) according to the following mathematical equation:

25. The method of claim 21, wherein the microresistivity logging tool further comprises an azimuth sensor and (c) further comprises causing the azimuth sensor to measure an azimuth angle, the method further comprising:

(f) correlating the resistivity tensor computed in (e) with the azimuth angle measured in (c).