Mineral veins are pervasive in the subsurface, and their interaction with hydraulic fractures is modulated by injection protocol, thereby influencing fracture network complexity and associated microseismic source mechanisms. To capture these processes, a hydro-mechanically coupled Discrete Element Method (DEM) framework incorporating full moment tensor decomposition is employed to simulate hydraulic stimulation in a veined granite model containing two heterogeneous, parallel shear veins. Hydraulic fractures initiate in a tensile-dominated mode and propagate preferentially along the direction of the maximum principal stress. However, their subsequent evolution is governed by vein orientation. Low approach angles (30° and 60°) promote fracture deflection and interface reactivation, generating pronounced shear-dominated microseismic events. In contrast, vertical veins (90°) are predominantly crossed by tensile fractures. Injection protocols exert additional control on both fracture evolution and source mechanism characteristics. Monotonic and stepwise injection promote a higher proportion of double-couple (DC)-dominated events, indicating enhanced shear reactivation, whereas cyclic injection produces a higher proportion of non-DC mechanisms. Cyclic injection also results in the lowest fracture breakdown pressure (approximately 2-7% lower than monotonic injection), enhances pore pressure diffusion, reduces main fracture aperture, and slows the accumulation of hydraulic energy and microseismic events. Although comparable moment magnitude distributions are observed across injection protocols, cyclic injection maintains the lowest seismicity rate over prolonged injection durations. These findings elucidate fracture-vein interaction mechanisms and injection strategy, providing mechanistic insights for optimizing hydraulic stimulation and mitigating injection-induced seismicity in fractured reservoirs. • Injection protocol influences microseismic source mechanisms through fracture-vein interactions. • Low vein approach angles promote localized shear reactivation, whereas vertical veins favor tensile crossing. • Cyclic injection reduces breakdown pressure and enhances pore-pressure diffusion, limiting main fracture aperture. • Cyclic injection maintains the lowest seismicity rate despite similar moment magnitudes.
Ling et al. (Mon,) studied this question.