Pressure responses and acoustic emission characteristics during simultaneous and sequential propagation of multiple hydraulic fractures in shale

Deep shale gas reservoirs in the southern Sichuan Basin feature well-developed geological discontinuities, including bedding planes and natural fractures. The interaction between Hydraulic fractures (HFs) and these weak planes can cause complex pressure responses and microseismic event patterns during multi-fracture propagation. This study introduces a novel experimental method for multi-stage and multi-cluster hydraulic fracturing in a horizontal well. The simultaneous and sequential HF propagation are analyzed by integrating real-time injection pressure monitoring and acoustic emission (AE) event localization. Characteristic pressure and AE response patterns are identified for different HF propagation behaviors. Results show that multi-cluster HFs within a stage may not initiate simultaneously, with multiple pressure peaks and each associated with high-energy AE events. The initiation pressure and AE energy of previously initiated clusters are higher than those of subsequently initiated clusters. For multi-fracture sequential propagation, subsequent HFs may initiate at an angle to the wellbore due to the stress shadow from previous fractures, eventually coalescing with them. Moreover, under poor cementing quality, longitudinal HFs may initiate and the injection pressure decreases slightly after peak pressure, followed by a gradual increase with weak fluctuations. The pressure drops when the HF crosses a natural weak plane, with high-energy AE events generated at the intersection and low-energy events distributing along the weak plane. When the fracturing fluid seeps along natural discontinuities, the pressure curve fluctuates intensely and periodically, with low-energy AE events along the weak plane. The conclusions provide a guidance for evaluating fracture geometries through interpreting operation pressures and microseismic monitoring results.