Hydro-mechanical behavior of dual-scale natural fractures: Insights from analytical modeling and microseismic analysis in shale

Understanding how natural fractures at multiple scales respond to pore pressure changes is essential for interpreting microseismicity and predicting stimulation performance in unconventional reservoirs. This study integrates analytical modeling, surface-seismic–based fracture characterization, and microseismic analysis to investigate how dual-scale natural fractures—small-scale distributed fractures and large-scale discrete networks—control hydraulic-fracturing behavior in the Weiyuan shale gas reservoir, Sichuan Basin, China. Field-calibrated fracture constitutive models and slip- and dilation-tendency analyses are used to evaluate stress- and scale-dependent hydro-mechanical responses and associated deformation modes. Spatial microseismic clustering is quantified using Ripley’s K-function, and b-value analysis characterizes fracture activation mechanisms and their temporal evolution. Modeling results show that initial aperture is the primary geometric control on fracture stiffness and closure behavior. Small-scale fractures deform mainly through normal-stress–driven opening and closure, whereas large-scale fractures undergo shear dilation, enhancing permeability. Field observations confirm the reactivation of large-scale, unfavorably oriented fractures. Microseismic clustering patterns reflect the geometry, density, and deformation modes of pre-existing fractures, delineating the spatial extent of stimulated zones and the connectivity of induced fracture networks. Spatial variations in b-value distinguish shear reactivation of large-scale fractures from fluid-driven tensile propagation in small-scale fracture zones, while temporal trends capture evolving deformation mechanisms during stimulation. Together, these results demonstrate that fracture scale, density, and deformation mode exert primary control on hydro-mechanical behavior and microseismic response, providing a mechanistic framework for designing geomechanically informed stimulation strategies in naturally fractured shale reservoirs.