Single-well microseismic focal mechanism inversion under shear-tensile source constraint
LIU Lei1, SONG Weiqi1, YANG Xiaohui2, HU Jianlin1, DONG Lin1, YU Zhichao3
1. School of Geosciences, China University of Petro-leum(East China), Qingdao, Shandong 266580, China; 2. SINOPEC Geophysical Research Institute, Nanjing, Jiangsu 211103, China; 3. Institute of Oil & Gas, School of Earth and Space Sciences, Peking University, Beijing 100871, China
Abstract：The microseismic monitoring data of hydraulic fracturing contain the information of focal mechanism, which can be used to describe the fracture type and stress state of the working area and is thus of great significance to the evaluation of fracturing effect and the guidance of subsequent fracturing scheme. Due to the small monitoring angle and single measure direction, full moment tensor inversion is hard to perform in single-well observation systems. In this study, comprehensively utilizing fracturing holes, logging and location data, we design a target function based on P- and S-wave energy and first-arrival polarity matching from three-component microseismic recordings under the constraint of the focus to shear-tensile dislocation model. Further, we realize the single-well microseismic focal mechanism inversion with full-space grid search. Synthetic data test shows that the inversion method has good accuracy and noise resis-tance. Specifically, the inversion accuracy of each parameter is more than 81% under the noise-free condition and is still higher than 51% in the pre-sence of 10dB noise. The anti-noise abilities of different parameters are in the order of tensile angle>dip angle>strike angle>slide angle. Finally, we apply this method to actual microseismic data of shale fracturing. The inversion results show that microseismic events of the fracturing are mainly formed by rock rupture and slip caused by one pair of couple or two pairs with different magnitudes, and the anti-tensile fracture formed by compression is more than tensile fracture formed by expansion.
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