**5. Discussion**

In summary, those studies on osmosis-associated imbibition provide a good understanding of the fluid flow in unconventional formations. Osmosis is a critical mechanism of fluid movement especially in clay-rich tight formations, and it can enhance the fluid extraction process as an additional force to other forces, such as capillary pressure. However, further studies are required to investigate the impacts of osmosis on the fluid imbibition process to maximize production for field applications. Here, some discussions on the practical implications of osmosis-associated imbibition are summarized.

The common perception is that water imbibition could cause water blockage and clay swelling, which decrease the permeability of the formation. There were many experiments and studies to indicate the decrease due to water blockage [108,120–123] and clay swelling [124,125]. There was also an argumen<sup>t</sup> about whether the decrease is temporary or permanent. On one hand, the damage could be eliminated by increasing drawdown pressure or applying alcohol and alcohol surfactant. Therefore, the damage is temporary to the formation [15,126–130]. However, on the other hand, in some formations,

additives of alcohol have little effect on gas permeability, so that water blockage can be permanent [131–134].

In recent studies, it was found that osmosis-associated imbibition could improve permeability in unconventional formations. In shale, the experiments established the relationship between the amount volume of imbibition and permeability. It was found that imbibition in shale could cause natural fractures instability and regrowth, thereby causing a significant permeability increase [25,70,135,136]. While positive capillary pressure is the main sucking force for strong imbibition in water-wet rocks, the imbibition volume and rate can be significantly boosted by osmotic effect and lead to more natural fracture reactivations and regrowth. However, that conclusion requires further investigation, because those results were taken from the experiments under non-reservoir conditions. In another word, it is not clear how many fractures can be reopened under reservoir high pressure due to osmosis-associated imbibition. Hence, more studies are necessary to determine the relationship between osmosis-associated imbibition and formation permeability under reservoir conditions for unconventional formations.

In addition, there are studies that investigate the influence of osmosis-associated imbibition on water recovery and production in shale. Those studies provided some explanations on the field observations that extended shut-in time after fracturing treatment often results in a lower percentage of water recovery and but higher production in shale [137–141]. One of the potential reasons is that the near wellbore water blockage is mitigated deeper to the reservoir due to strong imbibition enhanced by osmosis during the extended shut-in. However, more evidence is required to establish a more accurate relationship between osmosis-associated imbibition and water recovery and production.

Therefore, the findings through further investigation between osmosis-associated imbibition and permeability change, water recovery, and production will be beneficial for the hydraulic fracturing design and reservoir management, such that the injection amount and fluid types of fracturing treatment and the shut-in time and flowback rate can be specially designed to minimize the water blockage effect but maximize natural fracture reactivations and well production performance.
