**5.2 Osmosis**

According to Zhou et al. [81], earlier researchers viewed imbibition to mainly be the product of capillary pressure but findings from recent studies have challenged this position. Recent research shows osmosis contributes significantly to water imbibition and thus clay minerals and hydraulic fluid interactions especially for unconventional reservoirs which are often characterised by high clay mineral contents.

During osmotic imbibition of water into formation, formation clay minerals act as semi-permeable membranes through which fracturing fluids invade the matrix of the formation. Here, solutes from the concentrated formation fluids try to move into lower solute fracturing fluids but due to the semi permeable membrane formed by presence of clay, the solutes are unable to cross this barrier. Continuous accumulation of solutes near the semi-permeable membrane creates an attraction force that draws water into the formation in order to balance out the concentration differential.
