Method and apparatus of using magnetic material with residual magnetization in transient electromagnetic measurement

• US 20050189945A1
• Filed: 01/18/2005
• Published: 09/01/2005
• Est. Priority Date: 02/09/2004
• Status: Active Grant

• Current Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Original Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Inventors: Reiderman, Arcady
• Granted Patent: US 8,432,167 B2
• Time in Patent Office: Days
• US Class Current: 324/333
• CPC Class Codes: G01V 3/28 using induction coils

Abstract

A transmitter antenna assembly for transient electromagnetic well logging instrument comprises an antenna coil coupled with a current source and a magnetic core having residual magnetization. Switching current in the antenna coil results in magnetization reversal in the magnetic core and change in magnetic dipole moment of the antenna. After the magnetization reversal is complete the current is removed and the new vector of magnetic dipole of the antenna maintains constant (steady-state phase of the antenna dipole) due to magnetic hysteresis of magnetic material employed for the magnetic core. No power expenditure during the steady-state phase of the magnetic dipole facilitates highly effective generation and fast switching of a large magnetic dipole. The magnetic core also serves as a shield between the antenna coil and any conductive part of the antenna assembly. Embodiments suitable for measurement-while-drilling or measurements through casing make use of residual magnetization of magnetic drill collar or magnetic casing respectively.

Claims

1. An electromagnetic transient transmitter to generate a magnetic field in materials to be analyzed, comprising:

a magnetic core made of a permeable magnetic material having magnetic hysteresis, said core magnetically coupled with the materials to be analyzed;

at least one coil to change magnetization in the magnetic core; and

a source of current driving the coil, said source of current is switched between zero and at least one non-zero steady current in the coil, said magnetic core remaining substantially magnetized when the source of current is switched to zero current.

2. The electromagnetic transient transmitter of claim 1 wherein the permeable magnetic material has magnetic properties related to dimensions of the magnetic core.

3. The transmitter of claim 2 wherein the magnetic properties include a coercive force which is substantially equal to the ratio of saturation magnetization and an effective permeability of the magnetic core in the direction of the magnetic field produced by the coil.

4. The electromagnetic transient transmitter of claim 2 wherein said permeable magnetic material having substantially rectangular hysteresis loop in the direction of magnetic field produced by the coil.

5. The electromagnetic transient transmitter of claim 2 wherein said magnetic material has the coercive force in the range 1.5-8 kA/m

6. The electromagnetic transient transmitter of claim 1 wherein the permeable magnetic material is selected from the group consisting of ferrite, electrically conductive crystalline magnetic metal ribbons or tapes and magnetic amorphous ribbons or tapes.

7. The electromagnetic transient transmitter of claim 6 wherein said magnetic field in materials to be analyzed has a switching time, and said ribbons or tapes have thickness selected in relation with said switching time.

8. The electromagnetic transient transmitter of claim 1 wherein said magnetic field in materials to be analyzed has a switching time, and the coil has inductance in relation with said switching time, said inductance depending on the number of turns in the coil and the effective permeability of the magnetic core.

9. The electromagnetic transient transmitter of claim 1 wherein said materials to be analyzed include earth formations surrounding a wellbore drilled therethrough, said wellbore having a longitudinal axis.

10. The electromagnetic transient transmitter of claim 9 wherein said at least one coil generates a magnetic dipole having longitudinal axis direction selected from the group consisting of the direction parallel to the longitudinal axis of the wellbore and a direction perpendicular to the longitudinal axis of the wellbore.

11. The electromagnetic transient transmitter of claim 9 wherein at least one coil generates a magnetic dipole having longitudinal axis direction parallel to the longitudinal axis of the wellbore, said electromagnetic transient transmitter further comprises at least one orthogonal coil generating a magnetic dipole perpendicular to the longitudinal axis of the wellbore.

12. The electromagnetic transient transmitter of claim 11 wherein said permeable magnetic material core comprises an isotropic magnetic material.

13. The electromagnetic transient transmitter of claim 11 wherein said magnetic core comprises at least two parts having different magnetic properties.

14. The electromagnetic transient transmitter of claim 9 wherein said magnetic core is disposed between the coil and a conductive support or a drill collar to substantially shield the conductive support or drill collar from a switchable magnetic field produced by the coil.

15. The electromagnetic transient transmitter of claim 14 wherein the coil has a dipole magnetic moment direction collinear with the wellbore longitudinal axis, the electromagnetic transient transmitter further comprising at least one orthogonal coil having a dipole magnetic moment direction orthogonal to the wellbore longitudinal axis, the magnetic core having an external and an internal coaxial parts, the external part having a greater coercive force.

16. The electromagnetic transient transmitter of claim 9 wherein the magnetic core comprises a longitudinal fragment of a drill collar.

17. The instrument of claim 1 wherein materials to be analyzed include earth formations between wellbores to perform a cross-well electromagnetic imaging.

18. An electromagnetic transient transmitter for logging of oil wells having a metal casing comprising:

at least one magnetic core, the magnetic core comprising an axial fragment of the casing;

at least one coil to change magnetization in the magnetic core; and

a source of current driving the coil, the source of current is switched between zero current and at least one non-zero steady current in the coil, the magnetic core remaining substantially magnetized when the source of current is switched to zero current.

19. A method of energizing of earth formations to study transient electromagnetic induction effects comprising:

using a magnetic core made of at least one type of permeable magnetic material having magnetic hysteresis, the core magnetically coupled with the earth formations;

using at least one coil to change magnetization of the magnetic core; and

using a source of current driving the coil, the source of current is switched between zero current and a non-zero steady current in the coil, the magnetic core remaining substantially magnetized when the source of current is switched to zero current.

20. The method of claim 19 wherein the step of using magnetic core comprises selecting dimensions for the magnetic core which are related to a coercive force of the at least one permeable magnetic material.

21. The method of claim 19 further comprising using at least one orthogonal coil to facilitate magnetizing of said magnetic core in at least two orthogonal directions in order to assess anisotropy of electrical properties of the earth formations

22. The method of claim 21 wherein the step of using a magnetic core comprises using of a magnetic core made of at least two different permeable magnetic materials.

23. The method of claim 19 wherein the step of using magnetic core comprises steps selected from the group consisting of using an axial segment of a drill collar when performing measurements while drilling and using an axial segment of a metal casing when performing measurements in cased wells.

24. The method of claim 19 wherein the step of using a magnetic core comprises disposing the magnetic core between a conductive support or a drill collar and the coil to substantially shielding the conductive support or drill collar from a switchable magnetic field produced by the coil, the magnetic core is selected from the group consisting of a single-material core and a bi-material core made of magnetic materials having different magnetic properties, said shielding is intended to eliminate an effect of the conductive support or drill collar on the result of measurements.