Numerical simulation of abnormal response characteristics of hydraulic fracturing pumping pressure in strongly heterogeneous volcanic reservoirs

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Volcanic reservoirs exhibit significant heterogeneity characteristics influenced by volcanic activity and tectonic evolution. The unclear mechanisms underlying abnormal pressure fluctuations during the pre-pad fluid injection stage in fracturing operations severely constrain reservoir stimulation effectiveness. This study establishes a hydraulic fracture propagation model for heterogeneous reservoirs using the three-dimensional discrete element method, analyzing fracture propagation behaviors and pressure response characteristics to identify dominant factors controlling abnormal pressure responses. Results indicate that bedding, high-angle natural fracture, and localized heterogeneous region with high mechanical strength all impede hydraulic fracture propagation, resulting in pressure surges with maximum increases of 20%, 25%, and 158%, respectively. Due to weak-plane normal stress constraints and additional tortuosity friction, pumping pressure exhibits a continuous rise without decline during hydraulic fracture penetration of weak plane. In contrast, variation in fracture propagation difficulty leads to a rise followed by an abrupt drop in pumping pressure when fracture penetrates localized heterogeneous region. Pumping pressure shows strong sensitivity to Young’s modulus and fracture toughness of localized heterogeneous region. Consequently, the interaction between the hydraulic fracture and localized heterogeneous region with high mechanical strength is the dominant controlling factor leading to abnormal pressure responses during the pre-pad fluid injection stage in volcanic reservoirs.