Method and apparatus for transverse electromagnetic induction well logging

• US 5,781,436 A
• Filed: 07/26/1996
• Issued: 07/14/1998
• Est. Priority Date: 07/26/1996
• Status: Expired due to Term

• Current Assignee: Western Atlas International Incorporated (Northrop Grumman Corporation)
• Original Assignee: Western Atlas International Incorporated (Northrop Grumman Corporation)
• Inventors: Fanini, Otto N., Tabarovsky, Leonty A., Forgang, Stanislav
• Primary Examiner(s): McElheny, Jr., Donald E.
• Application Number:
  US08/686,848
• Time in Patent Office: 718 Days
• Field of Search: 364/422, 324/338, 324/339, 324/341, 324/342, 324/343
• US Class Current: 702/7
• CPC Class Codes: E21B 2200/22 Fuzzy logic, artificial intelligence, neural networks or the like
• CPC Class Codes: G01V 3/28 using induction coils

Abstract

A method of measuring the conductivity of earth formations penetrated by a wellbore. The method comprises selectively passing an alternating current through transmitter coils inserted into the wellbore. Each of the transmitter coils has a magnetic moment direction different from the magnetic moment direction of the other ones of the transmitter coils. The alternating current includes a first and a second frequency. The amplitude at the first frequency has a predetermined relationship to the amplitude at the second frequency. The relationship corresponds to the first and the second frequencies. The method includes selectively receiving voltages induced in a receiver coil having a sensitive direction substantially parallel to the axis of the corresponding transmitter coil through which the alternating current is passed. A difference in magnitudes between a component of the received voltage at the first frequency and a component of the voltage at the second frequency is measured, and conductivity is calculated from the difference in magnitudes of the components of the received voltage at the two frequencies. In a preferred embodiment, the step of measuring the difference in magnitudes between the voltage components at the two frequencies includes extracting digital signal samples of the received voltage at predetermined times with respect to a cycle of the transmitter current at the first frequency.

Claims

1. An apparatus for measuring conductivity of earth formations penetrated by a wellbore, comprising:

a plurality of transmitter coils at least one of which having a magnetic moment direction different from the magnetic moment directions of the other transmitter coils;

a plurality of receiver coils each having a sensitive direction substantially parallel to one of said magnetic moment directions of said transmitter coils;

a signal generator for conducting alternating current at at least two frequencies through selected ones of said transmitter coils, said alternating current having a first amplitude at a first one of said at least two frequencies, said alternating current having a second amplitude at a second one of said frequencies, said first amplitude related to said second amplitude by a predetermined relationship corresponding to said first and said second frequencies;

means for receiving voltages induced in said receiver coils; and

means for measuring a difference between magnitudes of said voltages induced in said receiver coils at said first frequency and induced at said second frequency.

2. The apparatus as defined in claim 1 wherein said second frequency comprises an odd multiple of said first frequency.

3. The apparatus as defined in claim 2 wherein said odd multiple is three.

4. The apparatus as defined in claim 2 wherein said first frequency comprises a range of 10 to 70 KHz.

5. The apparatus as defined in claim 2 wherein said second frequency comprises a range of 30 to 210 KHz.

6. The apparatus as defined in claim 1 further comprising three of said transmitter coils and where in said magnetic moment directions of said transmitter coils are mutually orthogonal.

7. The apparatus as defined in claim 1 further comprising three of said receiver coils and wherein said sensitive directions of said receiver coils are mutually orthogonal.

8. The apparatus as defined in claim 7 further comprising cross component receiver coils each having a sensitive direction substantially perpendicular to said magnetic moment direction of one of said transmitter coils.

9. The apparatus as defined in claim 1 wherein said signal generator comprises a digital to analog converter, a read only memory, and a digital signal processor, said digital signal processor operatively coupled to an output tap of said signal generator so that distortions of an output of said signal generator from a predetermined waveform can be corrected.

10. The apparatus as defined in claim 1 wherein said means for measuring said difference in magnitudes of said voltages includes means for digitizing said voltages, said means for digitizing further comprising means for extracting digital signal samples at predetermined times with respect to a cycle of said alternating current at said first frequency.

11. A method of measuring conductivity of earth formations penetrated by a wellbore, comprising:

passing alternating current through a selected one of a plurality of transmitter coils inserted into said wellbore, at least one of said transmitter coils having a magnetic moment direction different from the magnetic moment direction of the other ones of said transmitter coils, said alternating current comprising a first and a second frequency, said alternating current at said first frequency having a first amplitude, said alternating current at said second frequency having a second amplitude, said first amplitude and said second amplitude having a predetermined relationship to each other, said predetermined relationship corresponding to said first frequency and said second frequency;

selectively receiving voltages induced in a receiver coil having a sensitive direction substantially parallel to said magnetic moment direction of said selected one of said transmitter coils;

determining a difference in magnitude between a component of said received voltages at said first frequency and a component of said voltages at said second frequency; and

calculating said conductivity from said difference in magnitude.

12. The method as defined in claim 11 wherein said magnetic moment directions of said transmitter coils are mutually orthogonal.

13. The method as defined in claim 11 wherein said second frequency comprises an odd multiple of said first frequency.

14. The method as defined in claim 13 wherein said odd multiple is three.

15. The method as defined in claim 13 wherein said first frequency comprises a range of 10 to 70 KHz.

16. The method as defined in claim 13 wherein said second frequency comprises a range of 30 to 210 KHz.

17. The method as defined in claim 11 further comprising directly detecting a portion of said alternating current and adjusting a waveform of said alternating current to match a predetermined waveform.

18. The method as defined in claim 11 wherein said step of determining said difference in magnitude of said components of said voltage includes digitizing said voltages and extracting digital samples of said voltages at predetermined times with respect to a cycle of said alternating current at said first frequency.

19. The method as defined in claim 18 wherein said digital samples are extracted according to the formula:

##EQU4##

20. The method as defined in claim 11 further comprising selectively receiving voltages induced in receiver coils having magnetic moment directions substantially perpendicular to said magnetic moment direction of said selected one of said transmitter coils.

21. The method as defined in claim 11 wherein said predetermined relationship comprises the expression:

##EQU5##