# US 2014/0197839 A1 · Apparatus and Method for Balancing Impedance of a Resistivity Measuring Tool > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 2014/0197839 A1 - Document ID: US20140197839A1 - Status: Published application - Kind: A1 - Application number: 14/217,040 - Filed: 2014-03-17 - Published: 2014-07-17 - Earliest claimed: 2008-07-22 - Assignees: Schlumberger Technology Corporation - Applicants: Schlumberger Technology Corporation - Inventors: Richard Bloemenkamp - Web page: https://patentgrep.com/patent/US20140197839A1 ## CPC - G01V 3/20: operating with propagation of electric current - E21B 49/00: Testing the nature of borehole walls; Formation testing; Methods or apparatus for obtaining samples of soil or well fluids, specially adapted to earth drilling or wells - G01V 3/24: using AC - G01V: GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS (means for indicating the location of accidentally buried, e.g. snow-buried, persons A63B29/02) - E21B: EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS ## Abstract Techniques related to making resistivity measurements of an underground formation surrounding a borehole involve using a tool having an insulating pad mounted on the conductive body of the tool. The pad has a current injector electrode and a current return electrode electrically isolated from each other and mounted on a face of the pad. The pad is positionable in use to be adjacent the wall of the borehole. At least two impedance ratios are determined. The first is the ratio of the electrical impedance between the current injector electrode and the tool body and the electrical impedance between the current return electrode and the tool body. The second is the ratio of the electrical impedance between the current injector electrode and the formation, and the electrical impedance between the current return electrode and the formation on the other hand. The tool is arranged such that in operation, the two impedance ratios are substantially equal and there is substantially no potential difference between the tool body and the formation. ## Claims ### Claim 1 An apparatus for making resistivity measurements of an underground formation surrounding a borehole, comprising: a conductive tool body; a pad mounted on the conductive tool body, wherein the pad comprises: an insulating front face; at least one current injector electrode mounted on the front face; at least one current return electrode mounted on the front face, wherein the at least one current return electrode is electrically isolated from the at least one current injector electrode; and a back face conductively connected to the conductive tool body; wherein the tool body, the pad, the at least one current injector electrode, and the at least one current return electrode are arranged such that: (a) the ratio of the electrical impedance (ZLI) between the at least one current injector electrode and the tool body and the electrical impedance (ZLR) between the at least one current return electrode and the tool body; and (b) the ratio of the electrical impedance (ZMI) between the at least one current injector electrode and the formation, and the electrical impedance (ZMR) between the at least one current return electrode and the formation differ by 25% or less of the larger of the two impedance ratios (ZLI/ZLR, ZMI/ZMR). ### Claim 2 (depends on 1) The apparatus of claim 1, wherein a potential difference between the tool body and the formation is 25% or less than a potential difference between the at least one current injector electrode and the at least one current return electrode. ### Claim 3 (depends on 1) The apparatus of claim 1, wherein the tool body, the pad, the at least one current injector electrode, and the at least one current return electrode are arranged such that: (a) the ratio of the electrical impedance (ZLI) between the at least one current injector electrode and the tool body and the electrical impedance (ZLR) between the at least one current return electrode and the tool body, and (b) the ratio of the electrical impedance (ZMI) between the at least one current injector electrode and the formation, and the electrical impedance (ZMR) between the at least one current return electrode and the formation, differ by 10% or less of the larger of the two impedance ratios (ZLI/ZLR, ZMI/ZMR). ### Claim 4 (depends on 3) The apparatus of claim 3, wherein a potential difference between the tool body and the formation is 10% or less than a potential difference between the at least one current injector electrode and the at least one current return electrode. ### Claim 5 (depends on 1) The apparatus of claim 1, wherein the tool body, the pad, the at least one current injector electrode, and the at least one current return electrode are arranged such that: (a) the ratio of the electrical impedance (ZLI) between the at least one current injector electrode and the tool body and the electrical impedance (ZLR) between the at least one current return electrode and the tool body, and (b) the ratio of the electrical impedance (ZMI) between the at least one current injector electrode and the formation, and the electrical impedance (ZMR) between the at least one current return electrode and the formation, differ by 1% or less of the larger of the two impedance ratios (ZLI/ZLR, ZMI/ZMR). ### Claim 6 (depends on 3) The apparatus of claim 3, wherein a potential difference between the tool body and the formation is 1% or less than a potential difference between the at least one current injector electrode and the at least one current return electrode. ### Claim 7 (depends on 1) The apparatus of claim 1, wherein the electrical impedance (ZLI) between the at least one current injector electrode and the tool body are arranged to have a predetermined value. ### Claim 8 (depends on 1) The apparatus of claim 1, wherein the electrical impedance (ZLR) between the at least one current return electrode and the tool body are arranged to have a predetermined value. ### Claim 9 (depends on 1) The apparatus of claim 1, wherein the pad is mounted on the tool body by at least one arm providing a conductive path between the back face of the pad and the tool body. ### Claim 10 (depends on 1) The apparatus of claim 1, wherein the pad comprises one current injector electrode located between at least two current return electrodes. ### Claim 11 (depends on 1) The apparatus of claim 1, wherein the at least one current return electrode is configured to wrap around a portion of the at least one current injector electrode. ### Claim 12 (depends on 1) The apparatus of claim 1, further comprising an electrical component between the at least one current return electrode or the at least one current injector electrode and the back face of the pad so that there is substantially no potential difference between the tool body and the formation. ### Claim 13 (depends on 1) The apparatus of claim 1, wherein the apparatus is tuneable such that the ratio of the electrical impedance (ZLI) between the at least one current injector electrode and the tool body and the electrical impedance (ZLR) between the at least one current return electrode and the tool body can be adjusted by connecting a variable electrical component to at least one of the tool body, the pad, the at least one current injector electrode, and the at least one current return electrode. ### Claim 14 (depends on 1) The apparatus of claim 1, wherein the ratio of the electrical impedance (ZLI) between the at least one current injector electrode and the tool body and the electrical impedance (ZLR) between the at least one current return electrode and the tool body is determined prior to use of the apparatus based on an estimation of at least one of the electrical impedance (ZMI) between the at least one current injector electrode and the formation, and the electrical impedance (ZMR) between the at least one current return electrode and the formation. ### Claim 15 (depends on 1) The apparatus of claim 1, wherein the apparatus is adapted to make resistivity measurements at one or more frequencies above 100 kHz or to make resistivity measurements with transient signals having spectral components above 100 kHz. ### Claim 16 A method comprising: providing a tool having a pad comprising at least one current injector electrode and at least one current return electrode mounted on a front face of the pad, the at least one current injector electrode and the at least one current return electrode being electrically isolated from each other; positioning the pad at a distance from a formation in a borehole; determining a ratio of an electrical impedance (ZMI) between the at least one current injector electrode and the formation, and the electrical impedance (ZMR) between the at least one current return electrode and the formation; determining the ratio of the electrical impedance (ZLI) between the at least one current injector electrode and the tool body, and the electrical impedance (ZLR) between the at least one current return electrode and the tool body. ### Claim 17 (depends on 16) The method of claim 16, comprising comparing the ratio of impedance (ZMI/ZMR) and the ratio of impedance (ZLI/ZLR). ### Claim 18 (depends on 17) The method of claim 17, comprising determining whether ratio of impedance (ZMI/ZMR) and the ratio of impedance (ZLI/ZLR) differ by 25% or less of the larger of the two impedance ratios. ### Claim 19 (depends on 16) The method of claim 16, comprising estimating, prior to positioning the pad at the distance from the formation in the borehole, the electrical impedance (ZMI) between the at least one current injector electrode and the formation, and the electrical impedance (ZMR) between the at least one current return electrode and the formation. ### Claim 20 (depends on 19) The method of claim 19, wherein determining the ratio of the electrical impedance (ZLI) between the at least one current injector electrode and the tool body, and the electrical impedance (ZLR) between the at least one current return electrode and the tool body is based on the estimation.