**4.2 Application of MO-NFs to CO2 Sequestration**

As of 2018, 70% of the global warming was subsequent to the release of greenhouse gases (GHG) to the atmosphere, with fossil resources contributing to up to 37.1 billion metric tons. The total concentration in carbon dioxide, CO2, in the atmosphere was reported to hit its highest level ever (407 ppm) million. Great efforts should be invested to reduce CO2 concentration to an acceptable value. Carbon dioxide capture, utilization and storage (CCUS) technology of which Carbon capture and storage (CCS) technologies have a potential to reduce CO2 emissions to the atmosphere due to the huge global capacity for underground storage [81]. With 21 large-scale CCS projects operating worldwide, the volume of storable CO2 is estimated to be up to 37 Mtpa.

Yet more CCS projects are needed to reach the Paris 2 °C target, which is partly due to the leakage of the stored CO2 through the faults of the formation within which the gas is trapped [82, 83]. A typical CCS project encompasses the capture, the compression and transport, and the injection in the designed formation. The success of a gas storage depends primarily on the trapping mechanisms occurring during CO2 containment.

A trapping mechanism refers a process (either physical or chemical), which improves the sequestration of CO2. Among the different known trapping mechanisms, three processes stand out including residual, solubility and mineral trapping [84]. During the residual trapping, CO2, injected at its supercritical state, displaces the fluids as it moves through the porous rock. As CO2 moves upward due to the buoyancy difference, some of the CO2 will be left behind as disconnected droplets within the pore throats, which are immobile.

This mechanism, however, is challenged by the faults present the geological formation (cap rock). The fault could crack due to the over-pressurization of the aquifer leading to a leak in CO2 Solubility trapping involves the dissolution of supercritical CO2 in the salty water (brine), which leads to a fluid denser than the native fluids. From the difference in buoyancy, the resulting fluids force CO2 to sink at the bottom of formation over time. The problem, in here, is that not only the solubility of CO2 in brine is low, but it reaches quickly its saturation causing thereby an over pressurization of the aquifer.

Mineral trapping, which is the slowest of the processes, is the final phase. It results from the geochemical reactions of carbonic acid (H2CO3) and the native minerals of the formation. This trapping mechanism is dependent on the rock minerals, the pressure of the gas, temperature and porosity of the host formation [85]. However, if mineral trapping is hastened, it may weaken the cap rock and the overlying formation causing a serious leak in CO2. From above, it appears that the extent to which the CO2 reacts with the formation water (dominated by its solubility) will vary according to factors such as pressure, temperature, the solubility of CO2, the fluid and fluid/rock chemistry. The selection of a proper MO-NF could enhance the trapping mechanisms, and ultimately ensure an efficient CO2 sequestration.

This is potentially achieved by injecting a nanofluid that buffers the acidity within the host formation (**Figure 14b**), but more importantly will yield a gel-like material (**Figure 14a**), denser than the resident fluid in the host formation.

In this study, it was found that formulating a nanofluid from Si-NP and polyvinyl alcohol under CO2 bubbling would lead to the formation of silicated gels. Increasing the load in Si-NP yields a rigid gel (**Figure 15**).

The results suggest that condensation of SiO2-NF depends rather on the load in Si-NP than the concentration in PVOH. However, further investigations are required to understand the extent to which the host formation-fluid chemistry alters the solubility of CO2, and the host formation parameters (fluid chemistry, temperature, and pressure) alter the gel formation.
