Research on focal mechanism of microseismic events and the regional stress during hydraulic fracturing at a shale play site in southwest China

Abstract

We propose a waveform matching inversion method to determine the focal mechanism of microseismic events recorded by a single well observation system. Our method employs the cross-correlation technique to mitigate the influence of anisotropy on the S-wave. Then by conducting a grid search for strike, dip, and rake, we match the observed waveforms of P- and S-wave with the corresponding theoretical waveforms. A synthetic test demonstrates the robustness and accuracy of our method in resolving the focal mechanism of microseismic events under a single well observation system. By applying our method to the events that have been categorized into two clusters based on spatial and temporal evolution recorded during the hydraulic fracturing operation in the Weiyuan shale reservoir, we observe the two clusters have distinct focal mechanism and stress characteristics. The events in remote cluster (cluster A) exhibits consistent focal mechanisms, with a concentrated distribution of P-axis orientations. And the inverted maximum principal stress direction of cluster A aligns with local maximum principal stress direction (SHmax). It implies events in cluster A occur in a uniform stress condition. In contrast, the other cluster (cluster B) near the injection well exhibits significant variation in focal mechanisms, with a scattered distribution of P-axis orientations. And the inverted maximum principal stress direction deviates from local maximum principal stress direction (SHmax), indicating that events in cluster B occur in a complicated stress condition.