*3.1.2 Water blocking*

During fracturing fluid injection, water invasion occurs in the formation due to high capillary forces. High surface tension in pores and the hydrophilic characteristic of rock surfactant result in a significant water block in the formation, illustrated in **Figure 4**. This effect will be detrimental to gas productivity [30]. The increase of water saturation due to water trapping or water blocking considerably decreases the relative permeability of the gas [31].

As compared to conventional fluid, CO2-based fluid can assist the clean-up of injected liquid phase during flowback. As the pressure decreases, the expansion of the gaseous phase assists the flow to the surface and provides a rapid fracture and reservoir clean-up which accelerates the onset of the production phase after speeding up the flowback phase [30].
