Experimental works have proved that imbibition under forced pressure (FP, the difference between hydraulic fluid pressure and original pore pressure) has a positive effect on fossil hydrogen energy development in tight oil reservoir. However, the knowledge of the influence of imbibition under forced pressure (forced imbibition, FI) on fracturing fluid flowback and water retention is still limited. In this paper, experiments were designed and conducted to reveal the mechanism of fracturing fluid flowback and water retention under imbibition effect in tight sandstones. As a comparison, unconsolidated sandstones were also investigated in this study. Core samples were divided into two categories: the imbibition ones (treated by imbibition) and the filtration ones (treated by displacement), in accordance with the real oil-water distribution after well shut-in. An imbibition core and a filtration core were stitched together to conduct a flowback experiment. During the experiment, fluid distribution in different pore sizes was monitored continuously by using a low-field nuclear magnetic resonance device. Results show that the flowback recovery in tight sandstones is much lower than that in unconsolidated sandstones due to the difference of pore structure. Meanwhile, forced imbibition leads to higher oil recovery than spontaneous imbibition (SI) in tight sandstones since forced pressure enhances water imbibition. The water imbibed into small-macro and macro pores contributes the major flowback recovery, but the imbibed water in micro pores is rather difficult to displace and finally retains in these pores during the flowback process. Overall, forced imbibition not only enhances oil recovery but also increases water retention, explaining the mechanism of well productivity increase by using the shut-in method in the field. This study can help to clarify the influence of forced imbibition on fracturing fluid flowback and enhancement of fossil hydrogen development, and further provide guidance for flowback designs.

实验工作证明，在强迫压力（FP，液压流体压力与原始孔隙压力之差）下的吸水作用对致密油储层中化石氢能的开发具有积极作用。但是，关于在强迫压力下的吸水（强迫吸水，FI）对压裂液回流和保水的影响的认识仍然有限。本文设计并进行了实验，揭示了在致密砂岩的吸收作用下压裂液返排和保水的机理。作为比较，在这项研究中还研究了未固结的砂岩。岩心样品分为两类：吸收样品（通过吸收处理）和过滤样品（通过置换处理），根据井关闭后的实际油水分布情况。将吸收芯和过滤芯缝合在一起以进行回流实验。在实验过程中，使用低场核磁共振设备连续监测不同孔径的流体分布。结果表明，致密砂岩的返排采收率由于孔隙结构的不同而大大低于未固结砂岩。同时，由于强迫压力会增强水的吸收作用，因此与致密砂岩中的自发吸收作用（SI）相比，强迫吸收作用会导致更高的采收率。吸入小孔和大孔的水有助于主要的返排回收，但是微孔中的吸水很难置换，并最终在回流过程中保留在这些孔中。总体而言，强制吸水不仅提高了采油率，而且增加了保水率，这说明了在现场使用封闭方法提高了油井生产率的机理。这项研究可以帮助阐明强制吸水对压裂液返排和增强化石氢发育的影响，并进一步为返排设计提供指导。