Pore-scale modeling of dynamic imbibition process in fractured porous media under self-adjusting pressure boundary condition

Hydraulic fracturing is essential for developing unconventional oil and gas resources, yet the imbibition process within fracture–matrix systems during shut-in remains unclear. In this study, X-ray computed tomography was employed to reconstruct the pore structure of tight sandstone with embedded tree-shaped fractures, generating fractured porous media. A novel self-adjusting pressure boundary condition was proposed by setting two virtual buffers at the inlet and outlet to simulate pressure diffusion. Pore-scale simulations were conducted to examine the effects of initial pressure difference, wettability, viscosity ratio, and interfacial tension on dynamic imbibition. Results show that pressure release from the inlet to the outlet buffer gradually reduces the driving force for flow, shifting pore-scale displacement from viscous- to capillary-dominated flow. Two distinct local imbibition behaviors, including counter-current and forced imbibition, were simultaneously observed in the fractured medium, and their kinetic mechanisms were analyzed. Increasing the initial pressure difference from 2 × 10⁵ Pa to 5 × 10⁵ Pa enhanced water infiltration into fractures and matrix, raising water saturation from 0.30 to 0.62. Although fluid properties have limited influence on the overall water saturation profile, they significantly affect local imbibition processes, thereby altering oil–water distribution patterns, which may have notable implications for subsequent depressurization-induced production.