Study on the Formation Mechanism of Fracture Network in Hydraulic Fracturing of Shale Reservoirs

With the global energy structure transition and increasing demand for environmental protection, shale gas, as an important unconventional natural gas resource, plays an increasingly important role in China’s energy strategy. However, the ultra-low permeability of shale gas reservoirs requires reliance on hydraulic fracturing techniques to enhance production. In this study, we used the Longmaxi Formation shale reservoir in Chuannan, China, to assess the stress sensitivity of the permeability of rock samples by using transient pressure-pulse permeability tests combined with aging tests, as well as to analyze the fracture extension behavior by constructing a model of hydraulic fracture extension within intersecting closed joints. The results show that permeability decreases with increasing pore pressure at low pore pressure conditions (2-6 MPa), while permeability increases with increasing pore pressure at high pore pressure conditions (8-24 MPa). Further analysis reveals the expansion behavior of the fracture network within the intersecting closed joints, where the possibility of forming a complex fracture network is higher at low (0-11°) and medium (11°-38°) intersection angles, and the cohesion (2 MPa) and tensile strength (2 MPa) of the joints, as well as lower friction coefficients (0.2) and geostress differentials (0-5 MPa), are favorable for the expanding this range. When the ground stress difference increases to 5 MPa, the fracture extension pattern reverts to a three-range pattern, indicating that the increase of the ground stress difference has a significant effect on the fracture extension pattern. This study provides an important theoretical and experimental basis for understanding the formation of fracture networks during hydraulic fracturing of shale gas reservoirs, which is of great practical significance for guiding the efficient development of shale gas in China.