**7. Summary of advances and research gaps topics for future consideration**

In this section, recent research advancements are summarised. Review of recent studies shows that geochemical and geo-mechanical impacts from interactions between clay minerals and/or formation and fracturing fluids are being assessed more closely to help solve problems associated with reduced permeabilities during post-fracturing flowback. Geo-mechanical response of formations due to differences in temperatures of fracturing fluid and subsurface formations have also become a focus area for researchers. These advancements have unlocked new areas of research which will be explored in the near future. Other researchers have also focused on developing methods to measure the extent to which geochemical and geo-mechanical impacts are controlled by certain mechanisms during interactions between formations and fracturing fluid. Recent studies assessed for the purpose of this review are as follows:

### **7.1 Fracture face damage**

Though water-blocking effect is known to be one of the causes of permeability loss following hydraulic fracturing, mechanisms by which this occurs are not well understood. Elputranto et al. [108] simulated this phenomenon to study the main forces that drive it. They concluded that the fundamental driver of high water saturations held near the fracture-matrix interface may be due to capillary end effects. These act near the interface between fracture and formation matrix to increase the water saturation beyond the saturation caused by imbibition. Elputranto et al. [108] therefore suggested that capillary end effect is a significant mechanism that must be considered when assessing potential of water-blocking effect in a formation. Future research may focus on experimental validation of this simulation work.

In order to effectively diagnose the predominant mechanism of face damage in fractures in tight sands, Li et al. [109] proposed a new experimental method. In their work, two mechanisms are suggested as mostly being responsible for fracture face damage; high capillary pressure and swelling of water sensitive clays. Li et al. [109] integrated pressure transmission and pressure decay methods to determine the predominant cause of fracture face damage. They concluded that their method is able to distinguish the cause of the key mechanism in fracture face damage. Though the method was effective in tight sand, it has yet to be tested on shales and other unconventional reservoir rock samples with high clay mineral content. Future research should focus on investigating the scope of application of this method for shale formations and other unconventional reservoir rocks with high clay compositions.
