How it works
The CHFR-Plus and CHFR-Slim cased hole formation resistivity tools induce a current into the casing, where it flows both up and down before returning to surface by a path similar to that of the current from many openhole laterolog tools. Most of the current remains in the casing, but a very small portion escapes into the formation. Electrodes on the tool measure the potential difference created by this leaked current, which is proportional to the formation conductivity.
Typical formations have resistivity values about 1 billion times that of steel casing. During a CHFR tool measurement, the currents escaping to the formation cause a voltage drop in the casing segment. Because casing has a resistance of a few tens of microohms whereas the leaked current is typically on the order of several milliamperes, the potential difference measured by the CHFR-Plus or CHFR-Slim tool is in nanovolts.
Measuring while stationary to reduced noise
Measurement is performed while the tool is stationary to avoid the noise introduced by tool movement. Contact between the electrodes and the casing is optimized by the design of the electrodes, which scrape through small amounts of casing scale and corrosion. Because the electrodes are in direct contact with the casing, the CHFR-Plus and CHFR-Slim tools are not limited to operations in conductive borehole fluids and operate reliably in wells with oil, oil-based mud, or gas in the casing.
The typical low-resistivity (1- to 5-ohm.m) cements used in well construction do not have a significant effect on cased hole resistivity measurement.