Mechanisms of fracturing fluid spontaneous imbibition behavior in shale reservoir: A review

Spontaneous imbibition behavior gained popularity several decades ago which led to widely exploited application in fractured reservoirs. Recently the imbibition behavior of unconventional reservoirs has attracted a good number of researchers and field engineers to pay attention and research more on how to enhance recovery through imbibition behavior. Despite the numerous studies that have been conducted in this area, it is still controversial whether spontaneous imbibition behavior is applicable on reservoir stimulation especially for shale reservoir. In this paper, the recent works on the spontaneous imbibition behavior in unconventional reservoirs particularly for shale has been reviewed. This paper is divided into four main sections, the first section presents the review of the research progress in wettability, the imbibition scale model, and the pore-throat structure with regard to the capillary force and its related influencing factors. Moreover, the shortcomings of current research methods are presented. The second section involves the water-rock interaction examination including the ion exchange between the fracture fluid and reservoir as well as the evolution of the rock physical properties during well shut-in. The third section presents the review of the numerical simulation methods that have been used to study imbibition in the recent years focusing on the flow characteristics at the micro scale. In the final section, the future research directions from the current conducted researches have been suggested. Our research proposes that the thermal-hydro-mechanical-chemistry multi-field coupling effect should be considered in numerical simulations. It can provide a basis and reference for quantifying the effect of imbibition stimulation during the shut-in time.