Radiation induced conductivity of oil based mud around pads of electrical imaging tools
Methods, systems, devices, and products for making formation resistivity measurements including reducing the resistivity of the fluid proximate the electrode using ionizing radiation to induce a transient increase in electrical conductivity of the fluid for the resistivity measurement. The fluid may include oil-based mud. Methods include making the downhole measurement using the electrode during the transient increase. An electrode may be disposed on a pad having a bremsstrahlung assembly disposed thereon. Methods may include mitigating effects of an electrical resistivity of the fluid on the formation resistivity measurement by using the radiation to induce a transient increase in electrical conductivity of the fluid for the formation resistivity measurement. Methods may include using the ionizing radiation to generate at least one of: i) free ions; and ii) free electrons.
1. An apparatus for making a formation resistivity measurement of an earth formation in a borehole filled with fluid, the apparatus comprising:
an electrode configured to inject current into the formation via the fluid;
at least one processor configured to make a resistivity measurement indicative of the resistivity of the earth formation responsive to the injected current; and
a radiation source configured to reduce the resistivity of the fluid proximate the electrode using ionizing radiation to induce a transient increase in electrical conductivity of the fluid proximate the electrode for the resistivity measurement.
2. The apparatus of claim 1 , comprising a conveyance device configured to convey the electrode and the radiation source in the borehole.
3. The apparatus of claim 2 , wherein the radiation source comprises a source of at least one of i) x-rays; ii) gamma rays; iii) alpha radiation; iv) beta radiation; v) ultraviolet radiation; and vi) neutrons.
4. The apparatus of claim 3 , wherein the radiation source comprises a bremsstrahlung x-ray assembly.
5. A system for making a formation resistivity measurement of an earth formation in a borehole filled with fluid, the system comprising:
a tool comprising:
a tool body;
an electrode configured to inject current into the formation via the fluid;
a radiation source configured to reduce the resistivity of the fluid proximate the electrode using ionizing radiation to induce a transient increase in electrical conductivity of the fluid proximate the electrode for the resistivity measurement;
circuitry configured to perform a measurement cycle in a borehole intersecting an earth formation; and
a conveyance device configured to convey the tool in the borehole.
6. The system of claim 5 , wherein the measurement cycle comprises: i) applying the radiation over a first time interval; ii) making the resistivity measurement with the electrode while the electrical conductivity of the fluid proximate the electrode is in a target range; and iii) allowing the fluid to return to a nominal borehole conductivity by ceasing application of the radiation and waiting for a second time interval before repeating the measurement cycle.
7. The system of claim 5 , wherein the measurement cycle comprises: i) applying the radiation continuously over a time interval; ii) conveying the electrode in the borehole during the time interval; and iii) making a resistivity measurement with the electrode over the time interval such that the electrical conductivity of the fluid proximate the electrode is in a target range during the resistivity measurement; and iv) making additional resistivity measurements with the electrode over the time interval such that the electrical conductivity of the fluid proximate the electrode is in a target range during the additional resistivity measurements.
8. The method of claim 5 , further comprising a pad associated with the tool body, wherein the electrode and the bremsstrahlung assembly are each disposed on the pad.