Method and apparatus for resistivity imaging in boreholes filled with low conductivity fluids

• US 20080068025A1
• Filed: 09/14/2006
• Published: 03/20/2008
• Est. Priority Date: 09/14/2006
• Status: Active Grant

• Current Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Original Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Inventors: Gold, Randy, Tabarovsky, Leonty A., Bespalov, Alexandre N., Khokhar, Rashid W.
• Granted Patent: US 8,203,344 B2
• Time in Patent Office: Days
• US Class Current: 324/367
• CPC Class Codes: G01V 3/24 using ac

Abstract

A mixed mode tool uses an inductive source and detects galvanic currents and/or potentials at electrodes in proximity to a borehole wall to produce a resistivity image of the earth formation.

Claims

1. An apparatus for evaluating an earth formation, the apparatus comprising:

(a) at least one antenna configured to be conveyed in a borehole in the earth formation and induce an electrical current in the earth formation upon activation of the antenna; and

(b) at least two electrodes in proximity to the formation, a difference in electrical potential between the at least two electrodes being indicative of a property of the earth formation.

2. The apparatus of claim 1 wherein the at least one antenna is mounted on a mandrel of a downhole assembly.

3. The apparatus of claim 2 wherein the at least one antenna further comprises at least three planar coils with normals distributed azimuthally about an axis of a logging tool.

4. The apparatus of claim 3 wherein at least one of the at least three coils operates at a different frequency from another one of the at least three coils.

5. The apparatus of claim 1 wherein the at least two electrodes are positioned on a first pad extendable from a mandrel of a downhole assembly to the proximity of the formation.

6. The apparatus of claim 5 further comprising a second pad disposed on an opposite side of the mandrel from the first pad, the second pad having at least two electrodes.

7. The apparatus of claim 5 wherein the at least one antenna is disposed on the first pad.

8. The apparatus of claim 5 wherein the at least two electrodes further comprise at least three pairs of electrodes, each of the pairs of electrodes being in a different azimuthal position on the first pad.

10. The apparatus of claim 1 further comprising a processor configured to determine from the difference in electrical potential a resistivity property of the earth formation.

11. The apparatus of claim 1 wherein the at least one antenna is configured to be conveyed into the borehole using one of (i) a wireline, (ii) a drilling tubular, and (iii) a slickline.

12. The apparatus of claim 1 wherein the difference is in quadrature with a current in the at least one antenna.

25. The apparatus of claim 10 wherein the processor is further configured to record the resistivity property on a suitable medium.

9. (canceled)

13. A method of evaluating an earth formation, the method comprising:

(a) passing a current through at least one antenna conveyed in a borehole in the earth formation, and inducing an electrical current in the earth formation; and

(b) measuring a difference in electrical potential between at least two electrodes in proximity the formation resulting from the induced current, and

(c) determining from the difference a resistivity property of the earth formation.

14. The method of claim 13 further comprising positioning the at least one antenna on a mandrel of a downhole assembly.

15. The method of claim 14 wherein the at least one antenna comprises at least three planar coils with normals distributed azimuthally about an axis of a logging tool.

16. The method of claim 15 further comprising operating at least one of the at least three coils at a different frequency from another one of the at least three coils.

17. The method of claim 13 further comprising positioning the at least two electrodes on a first pad extendable from a mandrel of a downhole assembly.

18. The method of claim 17 further comprising disposing a second pad having at least two electrodes on an opposite side of the mandrel from the first pad.

19. The method of claim 17 further comprising positioning the at least one antenna on the first pad.

20. The method of claim 17 wherein further comprising:

(i) using, for the at least two electrodes at least three pairs of electrodes, each of the pairs of electrodes being in a different azimuthal position on the first pad; and

(ii) the method further comprising determining an image of the property of the formation.

21. The method of claim 13 further comprising conveying the at least one antenna into the borehole using one of (i) a wireline, (ii) a drilling tubular, and (iii) a slickline.

22. The method of claim 13 wherein the measured difference is in quadrature with the current.

23. A computer readable medium for use with an apparatus for evaluating an earth formation, the apparatus comprising:

(a) at least one antenna configured to be conveyed in a borehole in the earth formation, and induce an electrical current in the earth formation upon activation of the antenna; and

(b) at least two electrodes and in proximity to a wall of the borehole;

the medium comprising instructions which enable a processor to determine from a difference in electrical potential between the at least two electrodes a property of the earth formation.

24. The medium of claim 23 further comprising at least one of (i) a RAM, (ii) a ROM, (iii) an EPROM, (iv) an EAROM, (v) a flash memory, and (vi) an optical disk.