Complex wettability behavior triggering mechanism on imbibition: A model construction and comparative study based on analysis at multiple scales

Imbibition oil recovery improves the recovery rate of low-permeability fractured reservoirs in petroleum fields. However, there is a lack of research on complex wettability and comprehensive imbibition at different scales. This study adopted a fracture-controlled matrix unit to study the complex wettability and imbibition mechanisms at the pore and core scales. We propose a characterization method for complex wettability based on a two-dimensional fracture-controlled matrix unit core-scale numerical model and established mixed-wettability models. Based on the phase-field theory, the oil–water two-phase imbibition flow was simulated. The comparative study of numerical simulation results and microscopic experimental analysis indicates that Jamin's effect has consistently influenced imbibition at the core and pore scales, which hinders the imbibition process. Macroscopic wettability had the same influence at the core and pore scales. Notably, when θ = 90°, the fluid pressure in the fracture acts as a secondary driving force, such that 6.72% and 5.35% of the oil is still produced from the oil in the Y- and S-type fractures, respectively. The imbibition recovery rates of the Y- and S-type complex mixed-wettability cores were 38.23% and 27.85%, respectively, which are between the contact angles θ = 30° and 90°. Complex wettability pores can be divided into four types at the core scale: wetting type, sub-wetting type, mixed-wetting type, and non-wetting pore. This complex wettability behavior is triggered by the complex wettability of the pore walls. The flow phenomenon is shown as imbibition occurring continuously when the wetting phase meets wetting and sub-wetting type pores. When mixed-wetting type pores were encountered, the wetting phase proceeded along the wetting wall. Imbibition stops when a non-wetting pore is encountered. Furthermore, when the wetting phase flowed through the primary pores to the interconnected secondary pores, imbibition continues if wall wetting pores are encountered. When single-wall wetting pores are encountered, the wetting phase proceeds along the wetting wall. Imbibition stops when double-wall non-wetting pores are encountered. The inlet flow velocity at the core scale had a dual effect. This manifests as the contact time between the wetting phase and matrix wall, and the fluid pressure in the fracture as the driving force.