The effects of micro-fractures and mixed wettability on oil/water imbibition in porous media

Imbibition is widely recognized as an effective process for enhancing oil recovery in shale reservoirs. However, shale formations often feature complex multi-scale laminar structures, including micro-fractures, and distinct wettability characteristics in organic and inorganic pores. To better understand the impact of micro-fractures and mixed wettability on water imbibition, we developed a pore-scale model incorporating mixed wettability and micro-fractures to simulate countercurrent imbibition. The results indicate that: (1) Counter-current imbibition exhibits complex flow characteristics and can be divided into three stages. In the early stage, two equivalent oil-water phase interfaces form, and an external force exceeding resistance is required to displace the oil phase. In the middle stage, a continuous oil phase gradually develops in the main channel, with only one phase interface. In the later stage, driving forces and resistance approach mechanical equilibrium, allowing some oil droplets to be expelled into the main channel. (2) The mixed wettability of shale pores amplifies oil phase trapping and capillary fingering during water imbibition. A concentrated distribution of oil-wet pores increases adhesion forces between the oil and pore walls, resulting in pronounced oil trapping. Conversely, a concentrated distribution of water-wet pores accelerates water flow due to capillary forces, enhancing the fingering effect. (3) Micro-fractures effectively connect matrix pores distant from the main channel. As the bifurcation angle increases, imbibition recovery initially rises and then decreases. However, mixed wettability significantly restricts the flow conductivity of micro-fractures, highlighting the importance of considering its influence in reservoir studies.