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 using an instrument, the method comprising the steps of:

• calibrating the instrument to compensate for variations in the casing's electrical properties;
• applying a first time varying current to the casing in the proximity of the specific portion of the formation 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 formation along the length of the borehole casing;
• detecting the voltage level at a plurality of at least three spaced apart voltage measuring points along the casing, wherein the disposition of voltage measuring points defines a portion of the casing that is adjacent to the specific portion of the formation to be tested;
• measuring the magnitudes of the differential voltages between a plurality of discrete pairs of said spaced apart voltage measuring points and the phases of the differential voltages with respect to the applied current to measure electrochemical phenomena of the formations, 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;
• proportionally amplifying the first voltage output by a calibration parameter determined by said calibrating step to compensate for variations within the casing;
• measuring a third voltage output indicative of the difference between the proportionally amplified first voltage output and the second voltage output, said third voltage output being indicative of the differential in the current conducted into formation between said first and second discrete pairs of voltage measuring points;
• repeating said measurements using multiple different signals as the applied current;
• using said measurements to infer information about the electrochemical properties of the formations;
• and
• wherein said calibrating step includes the substeps of causing a calibration current to flow through the casing between a first calibration point disposed on a first side of the voltage measuring points and a second calibration point disposed on the opposite side of the voltage measuring points, and comparing the first and second voltage outputs in response to the calibration current, the calibration parameter being a function of the proportional magnitudes of said first and second voltage outputs in response to said calibration current.

An apparatus for measuring electrochemical phenomena of predetermined specific formations adjacent to a borehole casing comprising:

• introduction means for selectively applying a time varying measuring current to the casing at a current entry point in the proximity of the specific portion of the formation of interest;
• receiving means for receiving at least a portion of the measuring current at a point sufficiently remote from the current entry point to insure that a majority of the current introduced by said introduction means enters the formation from the casing;
• voltage measuring means for detecting the time varying voltage level at a multiplicity of at least three spaced apart voltage measuring points along the casing, wherein the disposition of voltage measuring points defines the sections of the casing that is adjacent to the specific portion of the formation to be measured;
• and
• differential means for simultaneously measuring the magnitudes and the 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.

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 time varying 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 voltage differentials between a plurality of discrete pairs of the spaced apart voltage measuring points and the phase differences between the applied current and the detected voltages;
• and
• interpreting the measured differential voltages to predict formation characteristics.

An apparatus for measuring electrochemical properties of formations adjacent a borehole extending into the formations and having a metallic casing, the apparatus comprising:

• a first current conducting electrode disposed within and engaged in electrical contact with the interior of the casing;
• a current receiving electrode positioned sufficiently remote from the first current conducting electrode to insure that a majority of the current passing between the first current conducting electrode and the receiving electrode enters the formation from the casing;
• a plurality of at least three spaced apart voltage measuring electrodes disposed within and engaged in electrical contact with the casing on a first side of the first current conducting electrode;
• generating means for selectively applying a time varying measuring current to the casing at the first current conducting electrode in the proximity of the specific portion of the formation of interest;
• first differential means for measuring the voltage differentials between discrete pairs of said voltage measuring electrodes;
• and
• second differential means for measuring the time shift between the measuring current applied at the first current conducting electrode and the signals measured by said voltage measuring electrodes.

An apparatus for measurement of the electronic properties of formations adjacent to a wellbore that is surrounded by a conductive borehole casing comprising:

• a plurality of at least three vertically spaced apart voltage measuring electrodes disposed at substantially equal intervals within said wellbore, each said voltage measuring electrode being in electrical contact with said casing and positioned adjacent to a formation of interest;
• a first current conducting electrode disposed within and engaged in electrical contact with said casing on a first vertical side of said voltage measuring electrodes;
• a second current conducting electrode disposed within and engaged in electrical contact with said casing on a second vertical side of said voltage measuring electrodes opposite the first current conducting electrode;
• an earth contacting electrode in electrical contact with the surface of the earth;
• conducting means for selectively applying a first time varying current to said first electrode in a manner requiring at least a portion of the first current to pass from said first electrode through said formation of interest to said earth contacting electrode and for applying a second time varying current to said first electrode in a manner that requires a majority of the second current to pass through the portion of the casing between said first and second current conducting electrodes;
• measurement means for measuring said first and second applied currents;
• first detector means for measuring the voltage difference between a first pair of said voltage measuring electrodes and producing a first output indicative thereof;
• second detector means for measuring the voltage difference between a second pair of said voltage measuring electrodes and producing a second output indicative thereof;
• third detector means for measuring the voltage differential between said first and second outputs;
• and
• comparison means for comparing the time delay between the applied current and the measured voltage differences to provide information concerning the electronic properties of formations.

An apparatus for measurement of the electronic properties of selected portions of formations of interest adjacent to a wellbore that is surrounded by a conductive metallic casing comprising:

• an upper voltage measurement electrode, a middle voltage measurement electrode, and a lower voltage measurement electrode vertically spaced apart at substantially equal intervals within said wellbore, each said voltage measuring electrode being in electrical contact with said casing and positioned adjacent to a formation of interest;
• a first current conducting electrode disposed within and engaged in electrical contact with said casing a first distance vertically above said upper voltage measurement electrode;
• a second current conducting electrode disposed within and engaged in electrical contact with said casing a second distance vertically below said lower voltage conducting electrode;
• a third current conducting electrode disposed within and engaged in electrical contact with said casing a third distance vertically above said upper voltage measurement electrode;
• an earth contacting electrode in electrical contact with the surface of the earth;
• conducting means for selectively applying a first time varying current to said first current conducting electrode in a manner that requires a first portion of the current to pass from said first electrode through said formation of interest to said earth contacting electrode, selectively applying a second time varying current to said first current conducting electrode in a manner that requires a majority of the current to pass through the portion of the casing between said first and second conducting electrodes, and selectively applying a third time varying current to said first current conducting electrode in a manner that requires a second portion of the current to pass through said formation of interest to said third current conducting electrode;
• measurement means for measuring said first, second and third time varying currents;
• a first detector means for measuring the voltage difference between the upper pair of said voltage measuring electrodes;
• a second detector means for measuring the voltage difference between the lower pair of said voltage measuring electrodes;
• a third detector means for providing a voltage differential between said first and second detector means;
• and analysis means to measure said first and second portions of the current conducted through the formations;
• thereby providing an apparatus capable of investigating various selected portions of formations.

An apparatus for measurement of the electronic properties of selected portions of formations adjacent to a wellbore that is surrounded by a conductive borehole casing comprising:

• a first set of at least three vertically spaced apart voltage measuring electrodes separated vertically by a first distance of separation within said wellbore, each said voltage measuring electrode being in electrical contact with said casing and positioned adjacent to a formation of interest;
• a second set of at least three vertically spaced apart voltage measuring electrodes separated vertically by a second distance of separation within said wellbore, each said voltage measuring electrode being in electrical contact with said casing and positioned adjacent to a formation of interest;
• a first current conducting electrode disposed within and engaged in electrical contact with said casing vertically on a first side of said sets of voltage measuring electrodes;
• a second current conducting electrode disposed within and engaged in electrical contact with said casing vertically on a second side of said sets of voltage measuring electrodes;
• an earth contacting electrode in electrical contact with the surface of the earth;
• conducting means for selectively applying a first time varying current to said first electrode in a manner requiring at least a portion of the first current to pass from said first electrode through said formation of interest to said earth contacting electrode and for applying a second time varying current to said first electrode in a manner that requires a majority of the second current to pass through the portion of the casing between said first and second current conducting electrodes;
• means for measuring said first and second time varying currents;
• a first detector means for measuring the voltage difference between a first pair of said first set of voltage measuring electrodes;
• a second detector means for measuring the voltage difference between a second pair of said first set voltage measuring electrodes;
• a third detector means for measuring the voltage difference between a first pair of said second set of voltage measuring electrodes;
• a fourth detector means for measuring the voltage difference between a second pair of said second set of voltage measuring electrodes;
• thereby providing an apparatus capable of investigating various selected portions of formations.

An apparatus for measuring electrochemical phenomena of a predetermined specific formation adjacent to a section of borehole casing comprising:

• introduction means for selectively applying a time varying measuring current to enter the casing at a point in the proximity of the predetermined specific 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;
• and
• voltage measuring electrode means for detecting the magnitude and phase of the time varying voltage level between at least two spaced apart voltage measuring points along the casing, said disposition of electrodes defining the section of casing which is adjacent to the specific formation to be measured to provide information about the predetermined specific formation of interest.

A method for measuring the electrochemical phenomena of formations adjacent to a cased borehole, comprising the steps of:

• causing a time varying current to enter the casing in the proximity of a specific formation 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 formation along the length of the borehole casing;
• and
• detecting the magnitude and phase of the time varying voltage level at a plurality of at least two spaced apart voltage measuring points along the casing, said disposition of spaced apart points being adjacent to the specific formation to be tested, to determine information about electrochemical phenomena of formations.

A method for measuring electrochemical phenomena of formations adjacent to a cased borehole comprising the steps of:

• causing a first total time varying current to enter the casing in the proximity of a particular geological formation to be tested thereby causing a first portion of said first total time varying current to pass through a first portion of the formation to be tested;
• selectively receiving said first current at a first point sufficiently remote from the current entry point to insure that a first portion of said first total time varying current passes through said first portion of the formation to be tested;
• causing a second total time varying current to enter the casing in the proximity of the particular geological formation to be tested thereby causing a second portion of said second total time varying current to pass through a second portion of the formation to be tested;
• selectively receiving said second current at a second point, which is different than said first point, and which is sufficiently remote from the current entry point to insure that a second portion of said second total time varying current passes through said second portion of the formation to be tested;
• and
• measuring the magnitudes and phases of the time varying voltage levels at a plurality of at least three spaced apart voltage measuring points along the casing in response to said first and second total time varying currents, said disposition of electrodes defining the extent of the formation adjacent to the casing to be tested, thereby providing information about electrochemical phenomena of formations.

A method for measuring the resistivity of specific geological formation adjacent to a predetermined portion of a steel casing surrounded by cement, the method comprising the steps of:

• (a) measuring the electrical resistance properties of the predetermined portion of the casing to determine thickness variations of the said portion of the casing thereby providing a first compensation step;
• (b) measuring the electrical resistance properties of the cement adjacent to the said predetermined portion of the casing thereby providing a second compensation step;
• (c) causing a time varying current to enter the casing in the proximity of the specific geological formation 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 formation along the length of the borehole casing;
• and
• measuring the differential current conducted into formation from said predetermined portion of said casing thereby providing a measurement of the resistivity of specific geological formations adjacent said specific portion of said casing thereby providing a measurement step;
• and
• (d) using the electrical resistance properties from said first and second compensation steps to correct measurements of the resistivity obtained during said measurement step to provide improved information about the resistivity of specific geological formations adjacent to said predetermined portion of said casing.