**1. Introduction**

Imbibition is defined as a movement in which the wetting fluid occupies pore space through the displacement of the non-wetting fluid. Osmosis refers to a process of water molecules' spontaneous movement through a semi-permeable membrane, such as clay, from a low-salinity to high-salinity region against concentration gradient [1]. Previously, the major mechanism of imbibition was narrowed to capillary pressure; however, osmosis due to molecule diffusion and the semipermeable membrane effect has been overlooked.

In unconventional formations, such as shale and tight sandstone, the imbibition exists when the injected working fluid contacts the formation rock during hydraulic fracturing and water flooding. The capillary pressure is considered as one of the driving forces for imbibition. Since this force is in inverse proportion to the pore size, it is particularly significant in the formation of nanopores and micropores. Thus, unconventional formation with smaller pore size usually has much higher capillary pressure and larger imbibition effect. In recent years, the imbibition effect was considered as a potential explanation for the low percentage of flow back after hydraulic fracturing in the shale gas reservoir. Roychaudhuri et al. [2], Makhanov et al. [3,4], and Zhou et al. [5,6] proved that a large volume of the fracturing fluid can be imbibed by the shale samples. Imbibition has also been studied as part of the research related to water flooding to achieve a sweep area as large as possible [7].

**Citation:** Zhou, Z.; Li, X.; Teklu, T.W. A Critical Review of Osmosis-Associated Imbibition in Unconventional Formations. *Energies* **2021**, *14*, 835. https://doi.org/ 10.3390/en14040835

Received: 3 December 2020 Accepted: 21 January 2021 Published: 5 February 2021

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Osmosis requires a semi-permeable membrane and concentration differences. Clay can act the membrane because of its salt-exclusionary behavior. In tight clay-rich formations, such as shale, clay distributes on the wall of the porous space so that the membrane has a high efficiency to exclude the passage of salt ions, which is called membrane efficiency. In addition, due to the salinity differences between the injected fluid and formation brine, the imbibition process in clay-rich formations is often associated with osmosis. Fakcharoenphol et al. [8] and Zhou et al. [9,10] both indicated the osmosis effect on water flooding and hydraulic fracturing.

Therefore, besides capillarity, it is necessary to study the effect of osmosis on imbibition for water flooding and hydraulic fracturing in unconventional formations. In this paper, previous studies are reviewed and summarized for the osmosis-associated imbibition. The details and conclusions are described to provide insights about osmosis-associated imbibition in petroleum engineering applications.
