Matric Potential-Driven Evolution of Methane Permeability in Rough-Walled Fractures of Unsaturated Shales

The gas permeability of fractures in shales provides a crucial foundation for evaluating subsurface fluid flow in numerous resource and environmental engineering projects. Water exists in natural shales, but its role in the evolution of fracture gas permeability remains incompletely understood. This study involved the preparation of two fractured shale cores, each with an individual fracture, and the use of high-precision laser scanning to reconstruct the three-dimensional morphology of rough-walled fractures. To determine how water affects gas permeability, the cores underwent saturation to five distinct matric potentials under a given confining stress and the coexistence of water and gas, and their methane permeabilities were measured at various gas pressures utilizing the steady-state flow method. The experimental findings reveal that fracture apertures follow a long-tailed normal distribution and show a scale dependence, and at the core scale, the average aperture across the fracture width exhibits a power function variation as the fracture length extends. The existence of water within fractures severely impedes gas migration, with the observed sharp reduction in the methane permeability by one to three orders of magnitude resulting from the loss in the effective aperture and available flow area due to capillary condensation as well as the swelling and dispersion of clay minerals. Gas flow through rough fractures displays a high degree of heterogeneity and noticeable slip, gas slip exponentially intensifies with increasing matric potential, and the slippage factor and intrinsic permeability exhibit a power law relationship. An analytical model considering the coupled effects of slip flow and heterogeneous fracture structures is proposed to describe gas flow in rough fractures. A comparison of the model-predicted and experimental permeabilities verified the model validity. Based on the analytical model, the matric potential-driven evolution of fracture gas permeability is quantitatively determined, which can be described as a function of exponential form. This relationship can be applied to accurately predict the variation trend of fracture permeability in underground engineering.