A NOVEL METHOD FOR FORMATION DENSITY MEASUREMENT IN CASED WELLS

Density logging is a fundamental logging method of open-hole logging series. In cased wells, the formation is usually treated as a cylindrical multiple-layer model including borehole, casing pipe, cement, and original formation, producing more complicated logging response compared to open-hole wells. Several studies have indicated that it is feasible to measure formation density using a density logging tool in cased well under certain conditions, of which the key problem is to determine casing thickness, cement thickness and cement density. This study proposes a novel method to evaluate formation density behind casing using a three-detector density tool. Considering that the backscattered gamma is easily affected by the density behind the casing, especially when the casing thickness is small, a designed ratio of high and low energy window counts of the scattered gamma spectrum is used to calculate casing thickness, which improves the lower limit of the detection. After obtaining casing thickness, the cylindrical multiple-layer model of cased well can be simplified to a cylindrical three-layer model, consisting of cased borehole, cement, and the original formation. Subsequently, the analytical expressions of the thickness of the middle layer (cement) as well as the density of the outer layer (original formation) can be derived based on the logging geometric factor concept, G, which borrowing from electric logging. Consequently, the cement thickness and original formation density could be calculated independently. To demonstrate the feasibility of the proposed method, experimental data are measured in a group of calibration wells. The data processing results show that the proposed analytical model fit well with the experimental data. In addition, the application conditions of the novel method are discussed based on experimental data. Finally, several log examples illustrate that under suitable casing and cement conditions, the formation density calculated with the proposed method in cased borehole is in good agreement with the corresponding open-hole density.