Oil based mud imaging tool with common mode voltage compensation

• US 8,212,568 B2
• Filed: 04/12/2010
• Issued: 07/03/2012
• Est. Priority Date: 11/04/2005
• Status: Active Grant

• Current Assignee: Halliburton Energy Services Incorporated (Halliburton Company)
• Original Assignee: Halliburton Energy Services Incorporated (Halliburton Company)
• Inventors: Morys, Marian L., Schaecher, William J., Pai, Raj, Goodman, George D.
• Primary Examiner(s): Aurora, Reena
• Application Number:
  US12/758,276
• Publication Number:
  US 20100231225A1
• Time in Patent Office: 813 Days
• Field of Search: 324/347, 324/355, 324/366
• US Class Current: 324/347
• CPC Class Codes: G01V 3/24 using ac

Abstract

An apparatus and method for minimizing the effects of a common mode voltage signal in downhole logging tools utilized to determine the resistivity of an adjacent portion of a borehole wall. Two current electrodes are energized by an excitation source to create an oscillatory electric field in a borehole wall. A voltage drop across a segment of the borehole wall is measured by two voltage electrodes, and the differential voltage is used in combination with a measured current flow to determine a resistivity value for the borehole wall. A common mode voltage in front of the two voltage electrodes is measured and minimized by controlling the excitation source, thereby reducing the resistivity measurement error.

Claims

1. A resistivity logging tool that comprises:

a biasing mechanism mounted on a tool body;

a sensor pad that is pressed against a borehole wall by the biasing mechanism;

at least two voltage electrodes positioned between at least two current electrodes on the sensorpad; and

a controller that minimizes a difference between a tool body potential and a formation potential.

2. The tool of claim 1, wherein said formation potential is a mean potential of the formation in front of the voltage electrodes.

3. The tool of claim 1, wherein the controller minimizes said difference by adjusting a current flow from each of the current electrodes.

4. The tool of claim 3, wherein the controller adaptively adjusts said current flows to balance voltages measured via said voltage electrodes.

5. The tool of claim 1, wherein the controller compensates for leakage currents from each of the current electrodes.

6. The tool of claim 1, wherein the current electrodes are insulated from a conductive body of the sensor pad, and wherein the current electrodes are capacitively coupled to said conductive body.

7. The tool of claim 1, wherein the controller further determines formation resistivity measurements and communicates them to a logging facility for storage.

8. A logging method that comprises:

passing current through a formation between a current source electrode and a current return electrode of a resistivity logging tool; and

maintaining a body potential for said tool at a formation potential, wherein said maintaining includes adaptively adjusting a current flow from each of the current source electrode and the current return electrode.

9. The method of claim 8, further comprising:

pressing a sensor pad carrying said current source and return electrodes against a borehole wall,

wherein said maintaining includes compensating for leakage current between a body of the sensor pad and said current source and return electrodes.

10. The method of claim 9, further comprising measuring formation voltages with voltage electrodes.

11. The method of claim 10, wherein said voltage electrodes are positioned between the current source electrode and the current return electrode, and wherein the voltage electrodes are further positioned on a wall-facing surface of a sensing pad.

12. The method of claim 10, wherein said formation potential is a mean potential of the formation in front of the voltage electrodes.

13. The method of claim 10, wherein said measuring includes sensing signals received via each voltage electrode with a respective differential sense amplifier that employs said body potential as a reference.

14. The method of claim 10, wherein said maintaining includes setting a current flow from each of the current source electrode and the current return electrode in response to the measured formation voltages.