岩石自发渗吸是孔隙内润湿相流体自发排驱非润湿相的过程,是致密油藏提高采收率的重要机理之一,由于多孔介质特性以及裂缝形态等因素的复杂性,目前对裂缝与孔隙间的渗吸传质规律的研究尚未完全阐明.本文基于相场法和流体运动方程,建立孔隙尺度动态渗吸数值模型,分析复杂孔隙结构内部裂隙与孔隙之间的传质机理及其与采收率的关系.结果表明:(1)渗流过程在孔隙内部主要涵盖 3 个关键阶段:裂隙的快速渗透,裂隙与孔隙间的相互作用(即渗吸现象),以及孔隙中的逐步推进(即驱替过程).较快的注入速率会阻碍渗吸过程的进行,从而导致更多的残余油滞留.(2)存在特定的临界裂缝宽度,当裂缝宽度约为平均孔径大小 40 倍时,采收率会在一定范围上下波动.较小的裂缝宽度使得流体在孔隙和裂缝间的流动通道更为狭窄,这增强了毛细管力对油滴的驱动作用.随着临界裂缝宽度的减小,裂缝无量纲数与采收率之间展现出正相关关系.(3)不同复杂度的裂缝系统对流体运移产生不同的影响.随着临界裂缝宽度的减小,不同复杂程度的裂缝对流体动用产生的影响呈现出差异性.随着裂缝复杂程度的增加,渗吸作用波及范围越大.裂缝宽度的减小会加剧油滴的聚集现象,进而显著减缓渗吸速率,并在小孔隙区域引发堵塞问题.(4)系统开放边界数的增加可有效提升润湿相接触面积,实现孔隙空间的最大化动态利用,进而形成协同渗吸驱动机制.四边开放(AFO)条件下的渗吸采收率为最优,而单边开放(OEO)条件下的采收效果最差.在相同无量纲时间下,TEO和OEO因开放端面数量与空间分布模型的强非均质效应,呈现更高归一化采收率,而其余三种边界条件采收率变化曲线呈现相对集中趋势.

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.