Methods and apparatus for measurement of the resistivity of geological formations from within cased boreholes

A method for measuring the resistivity of a geological formation adjacent to a borehole having a metallic borehole casing, the method comprising the steps of:

• (a) conducting current from a first current conducting electrode in electrical contact with the interior of the borehole casing to a second current conducting electrode in electrical contact with the surface of the earth;
• (b) measuring the total current conducted from said first current conducting electrode to the second current conducting electrode;
• (c) measuring the differential current conducted into the adjacent geological formation from a finite, predetermined length of the borehole casing;
• (d) measuring the local casing potential of the finite, predetermined length of the borehole casing adjacent to the geological formation to be measured;
• (e) determining the outside diameter of the borehole casing;
• wherein said method provides sufficient measurements and information to allow calculation of the resistivity of the geological formation adjacent to the finite, predetermined length of the borehole casing.

A method of measuring the resistivity of a particular geological formation of interest adjacent to a borehole having a metallic borehole casing, the method comprising the steps of:

• (a) conducting current from a first current conducting electrode in electrical contact with the interior of the borehole casing to a second current conducting electrode in electrical contact with the surface of the earth while simultaneously measuring the total current conducted between said first and second electrodes;
• (b) measuring the differential current conducted into the particular geological formation of interest from a finite, predetermined length of the borehole casing;
• (c) measuring the average local potential voltage of the finite, predetermined length of the borehole casing;
• (d) measuring the resistance of the predetermined length of the borehole casing;
• (e) determining the outside diameter of the casing;
• wherein said method provides sufficient measurements and information to allow calculation of the resistivity of the particular geological formation of interest adjacent to the finite, predetermined length of the borehole casing.

An apparatus for measuring the resistivity of particular formations adjacent to a borehole extending from the earth's surface into remote geological formations, the borehole having a metallic casing, said apparatus comprising:

• (a) a first electrode disposed within and engaged in electrical contact with the interior of said metallic casing at a depth from the surface of the earth;
• (b) an earth contacting electrode in electrical contact with the surface of the earth remote from said borehole;
• (c) a plurality of at least three voltage measuring electrodes vertically disposed within said metallic casing, said voltage measuring electrodes being in electrical contact with said metallic casing adjacent to a particular formation of interest;
• (d) means for generating an alternating current;
• (e) conducting means for selectively applying said alternating current to said first electrode in a manner requiring a majority of the current to pass through the earth to said earth contacting electrode, thereby causing at least a portion of the current to pass through the particular formation of interest;
• and
• (f) measuring means for simultaneously measuring the voltage differences between a plurality of discrete pairs of said voltage measuring electrodes and processing means for providing an output signal indicative of the electrical resistivity of the particular formation of interest based at least in part upon the simultaneous voltage differences detected by said measuring means.

An apparatus for measurement of the resistivity of formations adjacent to a well bore that is surrounded by a metallic casing comprising:

• a plurality of at least three vertically spaced apart voltage measuring electrodes disposed at substantially equal intervals within said well bore, each said voltage measuring electrode being in electrical contact with said metallic casing;
• a first current conducting electrode disposed within and engaged in electrical contact with said metallic casing vertically on a first side of said voltage measuring electrodes;
• a second current conducting electrode disposed within and engaged in electrical contact with said metallic casing vertically on a second side of said voltage measuring electrodes which is separated vertically from said first current conducting electrode by a minimum distance of separation;
• means for generating an alternating current;
• conducting means for conducting said alternating current from said first electrode to said second electrode and choosing the minimum distance of separation between said first and second electrodes to exceed 100 meters which therefore causes at least a portion of the current passing between said first and second current conducting electrodes to be conducted through the adjacent geological formations;
• and
• detecting means for simultaneously measuring the voltage difference between discrete pairs of said voltage measuring electrodes while said alternating current is being applied to said first electrode, said detecting means including a first and a second differential amplifier for detecting said voltage differences to provide information that is indicative of the voltage differences between discrete pairs of said voltage measuring electrodes and is responsive to the resistivity of formations adjacent to said metallic casing.

An apparatus for measurement of the resistivity of formations adjacent to a well bore that is surrounded by a conductive casing comprising:

• a plurality of at least three vertically spaced apart voltage measuring electrodes disposed at substantially equal intervals within said well bore, 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 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 voltage measuring electrodes;
• an earth contacting electrode in electrical contact with the surface of the earth;
• means for generating an alternating current;
• means for selectively applying said alternating current to said first electrode in a manner requiring at least a portion of the current to pass from said first electrode through said formation of interest to said earth contacting electrode;
• conducting means for selectively applying said alternating current to said first electrode in a manner that requires a majority of the current to pass through the portion of the casing between said first and second current conducting electrodes;
• and
• detecting means for simultaneously measuring the voltage difference between discrete pairs of said voltage measuring electrodes while said alternating current is being applied to said first electrode, said detecting means including a first and a second differential amplifier for detecting said voltage differences to provide information that is indicative of the voltage differences between discrete pairs of said voltage measuring electrodes and is responsive to the resistivity of formations adjacent to said casing.

A method for measuring the resistivity of materials adjacent to a metallic borehole casing comprising the steps of:

• causing a current to enter said casing at an entry point on said casing within said borehole in the proximity of the materials of interest;
• selectively receiving at least a portion of said current at a point on the earth's surface remote from said borehole;
• detecting the voltage level at a plurality of at least three vertically spaced apart points along the walls of said casing;
• measuring the differential voltage between discrete pairs of said spaced apart points, for the purpose of measuring electrical properties of the materials adjacent the metallic borehole casing to infer the resistivity of the materials of interest.

A method for measuring the resistivity of a formation adjacent a borehole surrounded by a metallic casing using an apparatus having a first and a second current conducting electrode disposed within and engaged in electrical contact with said casing, a plurality of at least three voltage measuring electrodes vertically disposed at substantially equal vertically spaced apart intervals within said borehole between said current conducting electrodes, and an earth contacting electrode in electrical contact with the surface of the earth remote from said borehole, the apparatus having a calibrating state wherein said second current conducting electrode completes a current conducting circuit with said first electrode, and a measuring state wherein said earth contacting electrode completes a current conducting circuit with the first electrode during the measurement state, said method comprising the steps of:

• causing a current to enter said casing at said first current conducting electrode;
• (selecting either a measuring state or a calibrating state, wherein said second current contacting electrode completes a current conducting circuit with said first electrode during said calibrating state and wherein said earth contacting electrode completes a current conducting circuit with said first electrode during said measuring state;)
• obtaining a first voltage output that reflects the voltage difference between a first discrete pair of said voltage measuring electrodes;
• obtaining a second voltage output that reflects the voltage difference between a second discrete pair of said voltage measuring electrodes;
• selecting a calibrating state and balancing said first and second voltage outputs by proportionally amplifying said first voltage output to equal said second voltage output;
• and
• selecting a measuring state and detecting a third voltage output that reflects the voltage difference between said first and second voltage outputs, said third voltage output being useful for inferring the resistivity of the formation of interest.