In-situ Stress Magnitude and Azimuth from Well Log Measurements. Topical Report,May 1-December 31, 1993

摘要

The tropical report includes the technical results obtained under GRI Contract5093-221-2567, Part 2, Task 6 'In Situ Stress Azimuth From Well Logs.' The primary objective of this part of the contract is to prove the feasibility of determining maximum and minimum in situ stress azimuth and magnitude from well log velocity measurements. The report includes two technical sections: (1) the investigation of flexural wave splitting in a borehole surrounded by rock with stress-induced anisotropy, and (2) a rock physics section that shows how one can relate well log velocity measurements to the actual state of stress around the borehole. The first technical section contains the review of the existing results, and new theoretical solutions on flexural wave propagation and splitting in a borehole surrounded by an anisotropic formation. The conclusion is that a flexural wave with given polarization splits into two flexural waves, a slow one and a fast one, that are polarized along the minimum and maximum horizontal stress directions, respectively. At high frequencies (>4 kHz), ellipticity of borehole may bias any estimates of anisotropy derived from flexural wave splitting. Therefore, reliable well log measurements of in situ stress azimuth should be obtained at frequencies of about 1 kHz. At these fequencies, a wavelength is on the order of 1 m. Placing a source and a receiver (receivers) a few meters apart should be enough to allow for the splitting to occur. The second technical section presents a discussion of laboratory and theoretical rock physics results illustrating the velocity-stress connection, and summarize a new formulation for parameterizing these relations. A strategy is provided for transforming log-derived velocity data into estimates of the maximum and minimum horizontal stresses.