Pore-scale simulation of counter-current spontaneous imbibition in natural fractured porous media

A comprehensive understanding of spontaneous imbibition mechanisms in fractured porous media is crucial for optimizing the development and sustainable utilization of geo-energy resources. This study employs advanced pore-scale simulation techniques to investigate counter-current spontaneous imbibition behavior in natural fractured porous media. Through digital rock physics technology, we accurately reconstructed both matrix and fracture structures from low-permeability sandstone cores. The study revealed two distinct snap-off phenomena in natural fractured porous media: (1) fracture-dominated snap-off, which plays a pivotal role in oil–water phase redistribution, and (2) matrix-dominated snap-off, which exhibits relatively minor effects on fluid phase redistribution. Three key geometric parameters significantly influence the spontaneous imbibition process in natural fractured porous media: the pore–throat ratio of matrix, the throat size distribution at the matrix–fracture junctions, and the fracture architecture. The relatively large pore–throat ratio in natural fractured porous media significantly inhibits the spontaneous imbibition capacity of water, consequently leading to reduced oil recovery efficiency. In addition, the systematic analysis focused on three critical controlling factors of fluid properties: wettability characteristics, viscosity ratio, and interfacial tension, with particular emphasis on their influence on interaction mechanism between fracture and matrix. Increasing interfacial tension or decreasing contact angle can promote deeper water penetration into the matrix, thereby improving oil recovery efficiency. Furthermore, our findings indicate that higher viscosity ratios enhance non-wetting phase mobility, facilitating more efficient spontaneous imbibition processes. These findings provide valuable insights for optimizing water injection strategies in unconventional reservoirs, such as oil shale formations.