High frequency induction log for determining resistivity and dielectric constant of the earth

• US 4,278,941 A
• Filed: 10/30/1978
• Issued: 07/14/1981
• Est. Priority Date: 10/30/1978
• Status: Expired due to Term

• Current Assignee: Shell Oil Company (Shell Plc)
• Original Assignee: Shell Oil Company (Shell Plc)
• Inventors: Freedman, Robert
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US05/955,673
• Time in Patent Office: 988 Days
• Field of Search: 324/338, 324/339, 324/341
• US Class Current: 324/341
• CPC Class Codes: G01V 3/28 using induction coils

Abstract

A method and apparatus for logging rock formations penetrated by a borehole to determine both the resistivity and dielectric constant of the formations. The apparatus comprises an induction logging tool having at least a transmitting coil and two receiver coils and operating in the radio frequency range from 20 to 60 megahertz. The method utilizes both the in-phase and out-of-phase, with respect to the transmitter current, voltage components which are measured at each receiver. The in-phase and out-of-phase voltage components are used to compute the phase shift between the voltage signals and the relative attenuation of the voltage amplitudes between the receivers. The phase shifts and relative attenuations are relatively insensitive to the borehole (e.g., borehole radius and drilling mud resistivity) and can be used as inputs into theoretically constructed nomograms to determine both the resistivity and dielectric constant of the formation. If the invasion of the drilling mud into the formation is not negligible then a tool with four spatially separated receiver coils is employed and the phase shifts and relative attenuations between the three adjacent pairs of receiver coils are computed from the in-phase and out-of-phase voltage components measured at each receiver. These data are used to determine the resistivity and dielectric constant of the invaded zone, the radius of the invaded zone, and the resistivity and dielectric constant of the non-invaded formation.

Claims

1. A method of induction logging a borehole to obtain the resistivity and dielectric constant of formations surrounding the borehole, said method comprising:

producing an alternating electric current in a transmitter coil at a first location in the borehole, said electric current having a frequency of between 20 and 60 megahertz;

measuring at second and third locations both the voltage signals V_I,1 and V_I,2 that are in-phase with respect to said current and the voltage signals V_Q,1 and V_Q,2 which are out-of-phase with respect to said current, said second and third locations being spaced different distances from said first location;

determining the phase shift Δ

φ

between the voltage signals at the second and third locations by solving ##EQU4## determining the relative attenuation between the voltage amplitudes at the second and third locations by taking the logarithm of the voltages at the second and third locations, where the voltage amplitudes are the square roots of the sums of the squares of the in-phase and out-of-phase voltage components at each receiver; and

,

utilizing said phase shifts and relative attenuations to derive the dielectric constant and resistivity of the formations from a previously prepared nomogram

2. The method of claim 1 wherein the in-phase and out-of-phase voltages are measured at a fourth location spaced from said second and third locations.

3. The method of claim 2 wherein said additional in-phase and out-of-phase voltages are used to compensate for the invaded zone.

4. The method of claim 1 wherein the voltage amplitudes are obtained from the square root of the sum of the squares of the in-phase and out-of-phase voltage components.

5. An apparatus for induction logging a borehole to obtain the resistivity and dielectric constant of the formation surrounding the borehole, said apparatus comprising:

a logging tool adapted for lowering into the borehole and having three spaced coils disposed thereon;

one coil being a transmitter coil and the remaining two coils being receiver coils;

a power source having a frequency between 20 and 60 megahertz, said power source being coupled to said transmitter coil;

a phase sensitive detector and amplifier network coupled to the said two receiver coils, said phase sensitive detector and amplifier network measuring at both receiver coils the voltage that is in-phase with respect to the power source and the voltage that is out-of-phase with respect to the power source;

computer means disposed to utilize the in-phase and out-of-phase signals to compute the phase shift in the voltages between the receiver coils and the relative attenuation of the voltage amplitudes;

a recording mechanism to record as a function of wireline depth the phase shift and relative attenuation computed by the said circuit means; and

,

a nomogram for obtaining the resistivity and relative dielectric constant of the formation from the phase shift and relative attenuation.

6. The apparatus of claim 5 and, in addition, two additional receiver coils, with said additional receiver coils being coupled to said phase sensitive detector and amplifier network, to said circuit means and also to said recording mechanism to measure, compute, and record additional phase shifts and relative attenuations between all adjacent pairs of receiver coils;

and additional computing means for utilizing the said recorded phase shifts and relative attenuations to determine the resistivity and dielectric constant of the invaded zone of the formation, the radius of the invaded zone, and the resistivity and dielectric constant of the non-invaded zone of the formation.