Microseismic Focal Mechanism Inversion Considering Shale Anisotropy

Abstract

Summary Microseismic monitoring is a key technology in unconventional oil and gas exploration, mainly including events detection, source imaging and focal mechanism inversion. Through the microseismic source mechanism study, information such as fracturing scale, direction and in-situ stress state, can be quantitatively obtained, thus providing reliable basis for the exploration. Most of the source inversions assumed the media is isotropic. However, hydraulic fracturing is usually conducted in stratified sedimentary strata which often show strong anisotropy. So lack of the consideration of shale anisotropy may cause errors in inversion results. In this paper, we take account of the shale anisotropy effect during microseismic focal mechanism inversion and study its impact on the inversion results. During forward modeling, the Moment Tensor source model is adopted and the synthetic waveforms are calculated using the finite difference method. Then we perform the inversion following the ideas of Cut-and-Paste (CAP) method in order to reduce the influence of imperfect model and source location. Numerical tests were used to analyze the dependence of microseismic source inversion on data and model (velocity and shale anisotropy). We also applied the inversion method to a real microseismic dataset and calculated the source focal mechanisms of several microearthquakes.