Fluid-solid coupling mechanisms in the evolution of hydraulic fracture networks in large-scale true triaxial tight sandstone

The mechanism of hydraulic fracture propagation and fracture network formation in tight sandstone is essential for deep energy development. To investigate the influence of injection rate on hydraulic fracture behavior in tight sandstone containing natural fractures, this study conducted large-scale true triaxial fracturing tests on specimens measuring 400 mm × 400 mm × 400 mm from the tight sandstone in the Puguang area of Sichuan. By integrating three-dimensional acoustic emission monitoring, isotope tracing, and fractal analysis of fractures, we systematically analyzed the fracture propagation process under three injection rates (100, 200 and 300 mL/min). The results indicate that: (1) The high quartz content (40.5%) and high brittleness index (64) of the reservoir provide a geological basis for the formation of complex fracture networks. (2) As the injection rate increases, the hydraulic fracture growth rate escalates from 19.95% at 100 mL/min to 67.01% at 300 mL/min, while the fractal dimension increases from 1.102 2 to 1.267 5. The fracture morphology transitions from a single dominant direction to a complex radial network. (3) Increasing the injection rate raises the maximum borehole pressure from 68 MPa to 100 MPa, with cumulative acoustic emission energy surging by 415.1% when the rate increases from 100 mL/min to 200 mL/min, while the increase slows to 6.1% when the rate rises from 200 mL/min to 300 mL/min, indicating that energy release tends to saturate at high injection rates. (4) Acoustic emission events are concentrated along the direction of maximum principal stress (at angles ≤ 45° to the wellbore). The time-series curve of the fractal dimension exhibits a three-stage characteristic: decline–rise–fluctuating decline, revealing the evolution of damage from disordered initiation to ordered connectivity. (5) Theoretical analysis indicates that breakdown pressure is influenced by the competing effects of injection rate and fluid infiltration. At low injection rates, fluid infiltration predominates, resulting in lower breakdown pressure, while at high injection rates, the injection rate dominates, leading to higher breakdown pressure. This study elucidates the influence of injection rate on hydraulic fracture propagation in tight sandstone and provides a crucial theoretical basis for optimizing parameters in deep volumetric fracturing.