Monitor the process of shale spontaneous imbibition in co-current and counter-current displacing gas by using low field nuclear magnetic resonance method

• NMR was used to monitor spontaneous imbibition of distilled water in shale, sandstone and volcanic rocks.
• No preferential pores appear during spontaneous imbibition in sandstone and volcanic rocks.
• Preferential pores (micro-pores) appear during spontaneous imbibition in shale rocks.
• Abundant micro cracks generated during imbibition contribute to the auto-removal of shale APT.
• Counter-current imbibition has less damage than co-current imbibition in shale during initial stage.

Large scale fracturing fluid is injected into the formation to produce fractures for the effective development of shale reservoir. However, the flow back rate of fracturing fluid is often less than the half of the injected liquid, which causes large number of fracturing fluid retaining in the shale reservoir, thus aqueous phase trapping (APT) appears. But after well was shut in for a period of time, the APT can be auto-removed. The experiments that monitored the process of shale spontaneous imbibition in co-current and counter-current displacing gas combined with nuclear magnetic resonance (NMR) were performed. Results show that no matter whether the spontaneous imbibition of sandstone and volcanic rock happened in co-current or counter-current displacing gas, the water content increases in the whole pores range gradually, and no preferential pores for spontaneous imbibition appear. The water content increases with convex curve in the early period of both conditions. Simultaneously, in the process of the experiments no apparent micro cracks appeared on the surfaces of the sandstone and volcanic rock. However, shale has some special characteristics in spontaneous imbibition of both co-current and counter-current displacing gas, which may contribute to the auto-removal mechanism of shale reservoir APT. During the experiments of shale, lots of micro cracks appeared on the surface of the sample gradually. The liquid absorbed into the shale sample fills the micro pores firstly. Subsequently the water takes up the space of mesopores slowly. The liquid in the large pores of shale is too small to be detected, so the water content change in these pores couldn't be distinguished clearly. In the early period, the water content of shale increases with convex curve in the co-current displacing gas, while the water content of shale increases linearly in the counter-current displacing gas. Thus, the counter-current spontaneous imbibition condition is beneficial to protect the reservoir. The results of our study contribute to not only explaining the auto-removal mechanism of shale reservoir APT, but also fixing the optimal flow-back time after hydraulic fracturing.