**5.1 Hydration imbibition and fluid retention**

Imbibition describes the displacement of immiscible fluids from within the formation matrix. In the context of this topic, the fluid within the formation matrix is hydrocarbon and brine, whereas the invading fluid is the hydraulic fracturing fluid, mostly water. The displacement described above occurs at times where the fracturing fluid comes into contact with the formation face creating disequilibrium. In order to gain equilibrium, fracturing fluid is drawn into the matrix spontaneously, without the application of any form of pressure. This phenomenon is known as spontaneous imbibition. Handy [65] defined it as the process in which a fluid is displaced by another fluid within a porous medium due to the effect of capillary forces alone. Other researchers like Bear [66], Bennion et al. [67], Hoffman [68] and Dutta [69] have also interrogated the mechanisms of hydration imbibition.

Imbibition of water into shale matrix has been identified as the major water retention mechanism when using water-based hydraulic fracturing fluids [70, 71]. Research into the controlling factors of imbibition of water in fracturing fluid into formation matrix revealed it to be the function of several parameters which are briefly discussed in the following:

### *5.1.1 Fluid and rock properties*

Fluid and rock properties have been identified as determinants of the amount and rates of imbibition. Ma et al. [72] reported that when water displaces oil and gas within the formation, the rate of imbibition becomes directly proportional to the viscosity of water. Pore sizes of formation inversely affect imbibition since smaller pore sizes generate greater capillary pressures and thus higher imbibition.
