LAUSR

Abstract A small percentage of the fracturing fluid pumped can be recovered after multi-stage fracturing stimulation in tight reservoirs. One cause for this can be the numerous branch fractures formed at different scales during large volume fracturing. Understanding the retention mechanism and flowback law of fracturing fluid in branch fractures is a key to solving this problem. In this study, we used a leaser macroscope, a low-field nuclear magnetic resonance (NMR) instrument, and a permeability device to scan fracture surface morphology and conduct fracturing fluid retention and permeability regain experiments. Meanwhile, retention mechanisms and the flowback process of fracturing fluid in branch fractures during flowback is considered. The experimental results show that the fracturing fluid present in branch fractures can be divided into free movable water (FMW), restricted movable water (RMW) and retained water (RW). The average proportion of FMW, RMW, and RW in Lucaogou and Longmaxi shale reservoirs in our work are about 0.16: 0.14: 0.7 and 0.1: 0.1: 0.8, respectively. The rougher the surface and the greater the tortuosity of the fracture are, the higher the retention percentage of fracturing fluid in branch fractures is. The main mechanisms of fracturing fluid retention in branch fractures include water film, capillary retention on the fracture surface, and matrix imbibition. The critical T2 of FMW and RMW in branch fractures in the Lucaogou shale reservoir are 25.3 ms and 10.2 ms, respectively. However, they are 64 ms and 3.7 ms in the Longmaxi shale reservoir. During the flowback, FMW and partial RMW gradually flowback to the surface. After this, the permeability regains with the decrease of water retention in branch fractures. This study contributes to understanding fracturing fluid retention mechanism in tight reservoirs and provides guidance for the optimization of fracturing flowback.