**2.4 Geological CO2 storage**

Recent surge in research on interaction between clay minerals and CO2 in the subsurface is due to the advent of the concept of CO2 capture, utilisation and storage (CCUS). The CCUS technology may also incorporates enhanced hydrocarbon recovery when CO2 injection is done in depleted oil and gas reservoirs. The importance of clay-fluid reactions is seen in the fact that most target storage reservoirs and accompanying seal formations have high clay mineral contents. In addition, reactions that cause immobilisation of CO2 and the ability of caprock to withstand pressures resulting from CO2 plume beneath are directly related to the amounts and types of clay minerals in the formation [21, 22]. Many studies have thus been conducted to investigate clay-fluid interaction in the context of understanding their implications on combined enhanced hydrocarbon recovery and CO2 storage projects [23–29].

Olabode and Radonjic [27] studied the reaction between CO2-Brine and caprock formation to assess the impact of mineral precipitation patterns on caprock integrity at elevated temperature and pressure conditions. They concluded that, precipitation of minerals could cause the sealing of micro-pores in caprocks thus enhancing their ability store CO2 within the subsurface. Hui Du et al. [30] also studied the sealing properties of caprock at nano and micro-scale with the premise that the durability of the caprock is directly affected by the nanostructures and microstructure of these rocks.
