Pore Structure Evolution and Its Impact on Fluid Flow in Shale: Insights from Numerical Simulation and Experimental Validation

With the rapid development of unconventional oil and gas resources, shale has become an increasingly important energy source. However, its ultralow permeability and complex pore structure pose major challenges for efficient hydrocarbon recovery. Pore structure evolution induced by stress variation and fluid-rock interactions plays a decisive role in controlling flow pathways, breakthrough behavior, and permeability, yet the underlying mechanisms remain poorly understood. In this study, pore-scale computational fluid dynamics (CFD) simulations, stress sensitivity experiments, and nuclear magnetic resonance (NMR) pore structure characterization were integrated to investigate the coupled mechanical and chemical effects on permeability evolution in shale. Two-phase displacement simulations using the Volume of Fluid (VOF) method revealed that narrowing pore throats concentrates flow into fewer pathways, leading to earlier breakthrough and reduced sweep efficiency. Stress sensitivity experiments demonstrate severe permeability degradation, with maximum damage reaching up to 95.01% and irreversible losses as high as 73.33% in felsic shale, indicating the strong susceptibility of pore networks to effective stress. NMR results further show that slickwater exposure alters pore structure by reducing the fraction of clay interlayer pores from approximately 61.8% to 58.9%, accompanied by clay swelling and pore throat blockage. Notably, although total porosity slightly increases after fluid exposure, permeability decreases, highlighting the dominant role of pore connectivity loss over pore volume change. This integrated pore-scale and experimental investigation provides new insights into the mechanisms linking structural evolution to permeability decline, offering practical guidance for stimulation fluid design and pressure management strategies to reduce formation damage and enhance recovery.