Hydraulic fracture propagation geometry and acoustic emission interpretation: A case study of Silurian Longmaxi Formation shale in Sichuan Basin, SW China

Abstract

A series of laboratory fracturing experiments was performed on samples mined from an outcrop of the Silurian Longmaxi Formation shale in the Sichuan Basin, using a true triaxial fracturing simulation system. To reveal the characteristics of acoustic emission (AE) response in hydraulic fracture (HF) propagation, the HF propagation geometry obtained by specimen splitting and CT scanning technology was compared with the interpretation results of AE monitoring. And the difference of hypocenter mechanism between hydraulically connected and unconnected regions was further discussed. Experimental results show that the AE events distribution indicates well the internal fractures geometry of the rock samples. Numerous AE events occur and concentrate around the wellbore where the HF initiated. Sparse AE events were presented nearby bedding planes (BPs) activated by the HF. AE events tended to be denser where HF geometry was more complex. The hydraulically connected region was obviously distinct with the spatial distribution of AE events, which resulted in the overestimation of stimulated reservoir volume (SRV) based on micro-seismic mapping result. Both tensile and shear events occurred in the zone connected by the HFs, while only shear events were observed around BPs which were not hydraulically connected. Thus, the hydraulically connected and unconnected region can be identified in accordance with the hypocenter mechanism, which is beneficial to improve the accuracy of SRV evaluation. Key words: shale, bedding plane, hydraulic fracture, propagation geometry, acoustic emission, CT scanning