Patent FR-2846996-A1 (FR)
Title: Determining resistivity of geological formation traversed by tubed shaft comprises forming resistivity diagraphs with and without tubing, and identifying zone where diagraphs match
Inventor: DUBOURG ISABELLE
IPC: G01V3/20
Filed: 2002-11-08
Abstract: Resistivity diagraphs are made of the formation with and without the tubing, and at least one zone where the two diagraphs match is identified. A model of the formation is constructed from the non-tubed diagraph and the tube and shaft characteristics and this is compared to the tubed diagraph. A geometric factor k is deduced conditioning the resistivity when the diagraphs do not match  The resistivity of a geological formation is determined by: (1) making at least one resistivity diagraph of the formation before tubing; (2) making at least one resistivity diagraph of the tubed shaft using a tool; (3) identifying at least one zone in the formation where the two diagraphs match; (4) constructing a model of the formation by parametric inversion from the non-tubed diagraph and the characteristics of the shaft and tubing; (5) calculating the response of the tool to this model; (6) comparing the modelled response to the diagraph of the tubed shaft in the calibration zone, and evolving if necessary a geometric factor k conditioning the resistivity when the comparison criteria are not satisfied; (7) deducing the geometric factor k; and (8) calculating the resistivity of the formation using the tubed resistivity diagraph and at least the geometric factor for at least one zone of the formation separate from the calibration zone. Several calibration zones are determined with different resistivities. In addition there can be a stage evaluating the resistivity of the cement between the tubing and the shaft by comparing the model and the diagraph in a low resistivity calibration zone and if necessary including the cement resistivity in the model in stage (8). The offset current can also be evaluated by comparing the model and the diagraph in a high resistivity calibration zone and if necessary including it in the model in stage (8). The model is constructed also using results from diagraphs after tubing is there are a number of them. Before stage (4), there is a recalibrating stage to check that the resistivity diagraphs with and without tubing are at the same depth. A preliminary estimation of the geometric factor k, the offset current (Ioff) and the cement resistivity (Rcem) are made. The model includes two concentric regions with different resistivities separated by an interface, one near the shaft and one further away. At least one diagraph is taken which, knowing the salinity of the near region, allows the resistivity of the near region (Rxo) to be deduced and then the resisitivity of the further region (Rt) and the position of the interface (di) to be calculated using the model in at least one separate calibration zone. An Independent claim is included for determining the salinity of the water and/or the saturation in water found in a homogenous formation with a shaft through it by making a diagraph of section capture in the tubed shaft and combining the results with the resistivity determined using one of the above processes to determine the salinity and/or saturation.