Cased-Hole Sonic Technique for Stress Determination and Optimization of Oriented Hydraulic Fracturing Operations

摘要

Shear-wave anisotropy gives a measure of stress anisotropy intensity and the associated fast-shear azimuth gives the maximum horizontal stress direction. This information is very useful in optimizing oriented hydraulic fracturing operations, wellbore stability, sand management, well placement and other applications. In open holes, this fracture or stress induced anisotropy can be effectively measured using cross-dipole sonic logging. However, determining shear-wave anisotropy in cased holes has been hindered by two factors. The first is the concern of the effect of casing and cement on the cross-dipole measurement, and the second is the lack of an effective device to measure the tool's orientation inside casing. This paper presents a solution to this problem using recently developed sonic logging technology in conjunction with Gyro data acquisition. Cased-hole Sonic and Gyro data were acquired in four wells for oriented fracturing optimization over zones with no or poor production. Gyro data was used as a reference for the sonic waveform rotation in order to determine the fast shear azimuth direction and maximum horizontal stress orientation. The cased-hole sonic monopole and dipole data quality was excellent, even over double casing sections. It was found that the reservoir sands are anisotropic;however, the shear-wave anisotropy magnitude is low. This is the first ever cased-hole Sonic + Gyro acquisition survey in India. The results showed that we can now confidently determine shear-wave anisotropy behind casing with the new sonic technology, even in low anisotropic formations with accuracy down to ~1%. The Sonic derived horizontal stress information was then used effectively to optimize the oriented fracturing operations. Low tortuosity while fracturing in the studied wells proved perfect direction for oriented fracturing. After fracturing, the production gain was substantial. Therefore, such cased-hole sonic technique can be useful in generating stress-maps of the reservoirs, which can then be used in various applications, such as oriented fracturing, sand control, reservoir characterization, fracture evaluation behind casing, wellbore stability and well placement.