Numerical study on interaction between initial seepage field and water injection: Analysis of radial seepage response in a single fracture

This study numerically investigates the influence of initial hydraulic boundary conditions and the fracture roughness on radial seepage characteristics during water injection in a single fracture. Using three-dimensional laser-scanned the rough fracture surfaces and COMSOL-based simulations, the interaction between the initial seepage field and injection-induced flow under varying hydraulic gradients (J = 0.001–1) and injection rates (1 × 10−10 to 1 × 10−6 m3/s) was analyzed. The efficacy of water injection is critically governed by the initial hydraulic gradient. Under the condition of J = 0.001, an injection rate of 1 × 10−8 m3/s suffices to transition the flow regime from parallel to radial. This shift results in a substantial increase in average velocity, which reaches up to 813.22% in the smooth and 1138.23% in the rough fracture. In contrast, under a high initial gradient (J = 1), injection induces only local perturbations, altering less than 5% of the flow domain in the fracture. The rough fracture exhibits greater susceptibility to injection showing 15%–20% higher flow line occupancy ratio (S) than the smooth fracture under the same conditions. Furthermore, nonlinear flow effects intensify with increasing J, reducing the equivalent hydraulic aperture in the fracture and limiting injection effectiveness.