Methods and apparatus for measurement of electronic properties of geological formations through borehole casing

A method for measuring electrochemical phenomena of formations adjacent to a cased borehole including polarization effects, the skin effects of the casing, dielectric constant effects, and the resistivity of formations, comprising the steps of:

• causing a sinusoidally time varying current at a first predetermined frequency of operation to enter the casing in the proximity of the formation to be tested;
• selectively receiving at least a portion of said current at a point in contact with the earth's surface and spaced apart from the casing;
• detecting the voltage level at a plurality of at least three spaced apart voltage measuring points along the casing in response to said current;
• measuring the amplitudes and phases of the differential voltages between a plurality of discrete pairs of said spaced apart voltage measuring points in response to said current at the first predetermined frequency, the measured differential voltages including a first voltage output indicative of the differential voltage between a first discrete pair of said voltage measuring points and a second voltage output indicative of the differential voltage output between a second discrete pair of voltage measuring points;
• and
• repeating said measurements at a second predetermined frequency of operation,
• thereby providing a method to measure electrochemical phenomena of formations including polarization effects, the skin effects of casing, dielectric constant effects, and the resistivity of formations.

A method for monitoring the integrity of a cement layer adjacent to a metal pipe which is located within a drill hole in a geological formation comprising the steps of:

• causing a time varying current to enter the pipe in the proximity of the portion of the cement layer to be tested;
• selectively receiving at least a portion of the current at a point sufficiently remote from the current entry point to insure that a majority of the current introduced at the current entry point enters the geological formation;
• detecting the magnitude and phase of the time varying voltage level at a plurality of at least three spaced apart voltage measuring points along the casing;
• measuring the magnitudes and phases of the time varying differential voltages between a plurality of discrete pairs of said spaced apart voltage measuring points, the measured differential voltages including a first voltage output indicative of the differential voltage between a first discrete pair of said voltage measuring points and a second voltage output indicative of the differential voltage between a second discrete pair of voltage measuring points, to measure electrochemical phenomena of formations including polarization effects, the skin effects of casing, dielectric constant effects, and the resistivity of formations;
• and
• interpreting the measured differential voltages to predict cement integrity.

An apparatus for measuring electrochemical phenomena of formations adjacent to a borehole casing including polarization effects, the skin effects of casing, dielectric constant effects, and the resistivity of formations, comprising:

• introduction means for selectively causing a time varying measuring current to enter the casing at a point in the proximity of the specific portion of the formation of interest;
• receiving means for receiving at least a portion of the current at a point sufficiently remote from the current entry point to insure that a majority of the current introduced by said current introduction means enters the formation from the casing;
• voltage measuring means for detecting the magnitude and phase of the time varying voltage level at a multiplicity of at least three spaced apart voltage measuring points along the casing adjacent the specific portion of the formation of interest;
• and
• differential means for simultaneously measuring the magnitudes and phases of the time varying differential voltages between a plurality of discrete pairs of said voltage measuring points to provide information about the specific formation of interest, said differential means producing a first differential voltage output indicative of the differential voltage between a first discrete pair of voltage measuring points and a second differential voltage output indicative of the differential voltage between a second discrete pair of voltage measuring points, said information including polarization effects, the skin effects of casing, dielectric constant effects, and the resistivity of formations.

A method for measuring electrochemical phenomena of a formation while drilling a borehole through the formation using a drill having a drill bit and a metal drill stem extending uphole from the bit, the method comprising the steps of:

• drilling through the formation;
• applying a measurement current to the drill stem;
• selectively receiving at least a portion of the current at a point in electrical contact with the earth that is spaced apart from the borehole;
• detecting the voltage level at a plurality of at least three spaced apart voltage measuring points along the drill stem;
• measuring the differential voltages between a plurality of discrete pairs of the spaced apart voltage measuring points;
• and
• interpreting the measured differential voltages to predict formation characteristics;
• said measured electrochemical phenomena including polarization effects, the skin effects of the drill stem, dielectric constant effects, and the resistivity of formations.