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Rock spontaneous imbibiton is the process of wetting phase fluid within the pore space spontaneously exhausting and driving the non-wetting phase, which is one of the important mechanisms for tight reservoirs to improve recovery. Due to the complexity of porous media characteristics and fracture morphology and other factors, the researches on imbibiton and mass transfer laws between fractures and pores have not yet been fully elucidated. In this paper, based on the phase field method and fluid motion equations, a pore-scale dynamic imbibiton and suction numerical model was established to analyze the mass transfer mechanism between fractures and pores within complex pore structures and the relationship with the recovery rate. The results show that: (1) the imbibiton process mainly covers three key stages inside the pore space: rapid penetration of the fracture, interaction between the fracture and the pore space, and gradual advancement in the pore space (i.e., repulsion process). A faster injection rate will hinder the imbibiton process, and result in more residual oil retention. (2) There is a specific critical fracture width, and when the fracture width is about 40 times the average pore size, the recovery rate will fluctuate up and down in a certain range. As the critical fracture width decreases, the positive correlation between the fracture dimensionless number and the recovery rate is shown. (3) Fracture systems of different complexity have different effects on fluid transport. As the critical fracture width decreases, the impact of different fracture complexity on fluid mobilization is different. Specifically, with the increase of fracture complexity, the wave range of imbibiton effect become larger. The decrease of crack width will exacerbate the phenomenon of oil droplet aggregation, which will significantly slow down the recovery rate and cause clogging problems in the small pore area. (4) The number increase of the system open boundaries can effectively enhance the contact area of the wetting phase, which can maximize the dynamic utilization of the pore space, and form a synergistic seepage drive mechanism. The optimal imbibiton recovery was achieved under the four-sided open (AFO) condition, while the worst recovery was achieved under the one-sided open (OEO) condition. At the same dimensionless time, TEO and OEO show higher normalized recovery rates due to the strong non-homogeneous effect of the open number of end faces and spatial distribution model, while the recovery change curves of the remaining three boundary conditions show relatively concentrated trends.
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