Study of resistivity of geological formation around borehole with metal casing, comprises applying current to casing, shunting current, and deducing leakage current from difference of voltage drops

摘要

The resistivity of a geological formation around a borehole (10) fitted with a metal casing (11) is studied by applying an electrical current to the casing to cause current leak into the formation; shunting the current by an amplifier; and deducing the leakage current from the difference between the voltage drops on the casing. Study of the resistivity of a geological formation around a borehole fitted with a metal casing, comprises applying an electrical current to the casing to cause current leak into the formation at a given level. The current is shunted by an amplifier in contact with the casing on either side of the measurement level. The amplifier is organized so that the current flowing along the casing at the level is small compared with the shunt current. The difference between the voltage drops on adjacent sections of the casing on either side of the measurement level is determined, and the leakage current is deduced from it. An Independent claim is also included for a downhole apparatus for studying the resistivity of a geological formation around a borehole fitted with metal casing, comprising an electrical power source (24); and two amplifiers, one for maintaining the current flowing along the casing, and the other for measuring the voltage drop that results from the leakage.