Method and apparatus for determining distance for magnetic and electric field measurements

• US 5,218,301 A
• Filed: 10/04/1991
• Issued: 06/08/1993
• Est. Priority Date: 10/04/1991
• Status: Expired due to Fees

• Current Assignee: Vector Magnetics, Inc.
• Original Assignee: Vector Magnetics, Inc.
• Inventors: Kuckes, Arthur F.
• Primary Examiner(s): Strecker, Gerard R.
• Assistant Examiner(s): EDMONDS, WARREN
• Application Number:
  US07/770,890
• Time in Patent Office: 613 Days
• Field of Search: 324/338-347, 324/351-358, 324/366, 324/368, 324/369-372, 324/207.26, 324/226, 324/227, 324/207.14
• US Class Current: 324/346
• CPC Class Codes: E21B 47/02 Determining slope or direction
• CPC Class Codes: E21B 47/0228 using electromagnetic energy or detectors therefor
• CPC Class Codes: G01V 3/24 using ac

Abstract

A method and apparatus for determining the distance from a relief borehole to a target well utilizes measurements of an electric field at the target well. The electric field is produced by injecting current into the earth near the wellhead of the target well, this current producing a target current in conductive material in the target well. The electric field corresponding to the target current is measured by means of sensors in the relief borehole, and this field measurement, together with a measurement of the alternating magnetic field at the relief well due to the target current, permits determination of the distance to the target well from the relief borehole.

Claims

1. Apparatus for evaluating the distance between a bore hole in the earth and an electrically conductive target in the earth, comprising:

a target in the earth and including electrically conductive means;

first and second current supply electrodes in contact with the earth in the region of said target;

means supplying an alternating current of low frequency between said first and second current supply electrodes to produce an alternating electric potential field in the earth surrounding said target and to produce an electric field along said electrically conductive means in said target to produce an alternating target current;

a bore hole having a longitudinal, generally vertical axis;

magnetic field sensor tool means movably supported by a wireline in said bore hole and responsive to an alternating electromagnetic field produced in the earth surrounding said target by said target current to produce magnetic field sensor output signals;

electric field sensor means comprising first and second spaced sensor electrodes movably supported in said bore hole by said wireline and spaced from each other along the longitudinal axis of said bore hole and spaced from said magnetic field sensor tool means, said sensor electrodes being responsive to the electric potential field produced in the earth in the region of said bore hole by said alternating current to produce first and second electric potential field output signals;

differential amplifier means in said tool means connected to said first and second sensor electrodes and responsive to said first and second electric potential field output signals to produce an electric field sensor signal representing the electric potential field gradient in the earth surrounding said bore hole between said sensor electrodes; and

means responsive to said magnetic field sensor output signals and to said electric field sensor signal for evaluating said target current to thereby determine the horizontal distance between said electric field sensor means in said bore hole and said target.

2. The apparatus of claim 1, wherein said electrically conductive means has a known electrical resistance, whereby said target current is determinable to permit direct measurement of the horizontal distance between said target and said electric field sensor means.

3. The apparatus of claim 1, wherein said first current supply electrode comprises electrode means electrically connected to said conductive means at said target.

4. The apparatus of claim 3, wherein said target is a well having a surface wellhead and containing said electrically conductive means and wherein said second current supply electrode is a return electrode and comprises electrode means in contact with the earth'"'"'s surface and spaced from the wellhead of said target well.

5. The apparatus of claim 4, wherein said electric field sensor means is located in said bore hole at a selected distance below the earth'"'"'s surface and wherein said return electrode is spaced from said wellhead by a distance approximating said selected distance.

6. Apparatus for evaluating the distance between a bore hole in the earth and an electrically conductive target in the earth, comprising:

a target in the earth including electrically conductive means;

at least a first current supply electrode in contact with the surface of the earth in the region of said target, said first supply electrode being spaced from said target by a distance substantially equal to a selected target depth;

at least a second current supply electrode electrically contacting said target;

p1 means supplying an alternating potential of low frequency between said first and second supply electrodes to produce a generally vertical alternating electric field in the earth surrounding said target and along said conductive means to thereby produce an alternating target current therein;

a bore hole spaced from said target and having a longitudinal, generally vertical axis;

a magnetic field sensor tool movably supported in said bore hole by a wireline at about said selected depth, said tool incorporating magnetic field sensor means responsive to electromagnetic fields in the earth surrounding said bore hole and caused by said target current to produce magnetic field sensor output signals;

electric field sensor means comprising first and second sensor electrodes supported in said bore hole at about said selected depth by said wireline, at least one of said sensor electrodes being spaced from said tool, said electrodes being spaced apart vertically along the longitudinal axis of said bore hole, said sensor electrodes being responsive to the alternating potential field caused by said alternating target current to produce a corresponding electric field output signal; and

means responsive to said magnetic field sensor output signals and to said electric field output signal to produce a value representing the horizontal distance between said target and said bore hole at said selected depth.

7. A method of determining the horizontal distance between a target well containing electrically conductive material and a second well, comprising:

positioning first and second current supply electrodes at the surface of the earth in the region of the target well and of the second well, the first supply electrode being at the wellhead of the target well and the second supply electrode being spaced therefrom at a distance which is approximately equal to the selected depth in the second well at which an electric potential field gradient is to be measured;

supplying an alternating potential of low frequency across said current supply electrodes to produce ground currents in the earth formations surrounding said target well and said second well and in the electrically conductive material in the target well to produce an alternating target current I and a corresponding electric field E along the axis of the conductive material in the target well, the electric field E and the target current I having the relationship E=IR, where R is the resistance per unit length of the conductive material,

measuring the alternating electric potential field gradient due to said target current int he region of said second well by means of a pair of electric field sensors located in, and spaced apart along the axis of, the second well, and located at said selected depth thereof;

measuring by means of magnetic field sensors located in said second well at a position spaced apart from said electric field sensor'"'"'s the alternating magnetic field in the region of said second well produced by said target current;

determining, from said measured magnetic field and from said measured electric field and from said measured electric potential field gradient the horizontal distance between said target well and said second well at said selected depth of said electric field sensors.