Patent ID: 7260478

Claim:
Method for determining, in a geological formation crossed by a cased well, the resistivity of the formation beyond the casing comprising the following steps: a) carrying out at least one resistivity log of the formation in the non-cased well before casing, b) carrying out at least one resistivity log of the formation in the cased well by means of a tool, c) identifying at least one calibration zone of the formation in which the resistivity given by the log in the cased well and that given by the log in the non-cased well has remained substantially constant, d) constructing a model of the formation by a parametric inversion method from the results of the log in the non-cased well and the characteristics of the well and the casing, e) calculating the response of the tool to said model, f) comparing the response of the tool to said model and the resistivity log in the cased well in the calibration zone while changing, if necessary, in the model, a geometric factor k conditioning the resistivity as long as the comparison criterion is not satisfactory, g) deducing when comparison criterion is satisfactory, the geometric factor k of the model, h) determining the resistivity of the formation beyond the casing by calculation using the resistivity log in the cased well and at least the geometric factor k deduced for at least one zone of the formation different to the calibration zone, and i) performing one of recording and logging of the determined resistivity, and of updating the model of the formation based on the determined resistivity.