Process for determining the resistivity of a formation through which a well equipped with a casing passes

Process for determining the resistivity (Rt) of a geological formation (9) surrounding a well (10) equipped with a casing (11) consisting of several casing segments (11.i, 11.s) following each other, in which two successive casing segments (11.i, 11.s) have an overlapping part (1), the cement (3) located between the casing (11) and the formation (9) and in the overlapping part (1) between two adjacent casing segments (11.i, 11.s),

• in which a current is injected into the casing (11) to cause a leakage current (Ifor) into an area (8) of the formation (9) for which measurements are required, offset from the overlapping part (1), the leakage current (Ifor) in the measurement area (8) is determined and is used to deduce the measured resistivity (Rm) of the formation in the measurement area (8),
• wherein a current is injected into the casing (11) to cause a current leakage (Icem) in the cement (3) of the overlapping part (1), the leakage current (Icem) in the cement (3) in the overlapping part (1) is determined, and is used to deduce the measured resistivity (Rcem) of the cement (3), and
• the measured resistivity (Rm) of the formation is corrected using a factor to take account of the measured resistivity (Rcem) of the cement (3) to obtain the resistivity (Rt) of the formation (9).