Once CO₂ is injected into a reservoir for storage, it mixes with the fluid in the reservoir, and the properties of CO₂, brine, and the reservoir change over time. The density of CO₂-bearing brine is higher than brine; thus, CO₂-saturated brine sinks to the bottom. This process of density settling creates hydrodynamic processes that are necessary for CO₂–brine mixing and dissolution. The undissolved CO₂ is lighter and rises to the top of the mix by buoyancy.

2. Effect of Different Phases of CO₂–Brine on the Different Properties of Rocks

The rocks range from sandstone, mudrocks, claystone, shale to Carbonates. This is because each of these rocks has been used in CO₂ geological storage. The fine-grained and less permeable rocks such as shale, claystone, and mudrocks serve as stratigraphic traps, whereas the permeable sandstones and pervious Carbonates serve as the reservoirs in CO₂ geological storage. The effect of CO₂–brine on caprock is different from the effect on reservoir rocks because of the geological difference in their origin and difference in chemical composition and physical properties. There can be layers of claystone, mudrocks, or shale within a thick layer of sandstone or Carbonate rocks, and this makes the study of the CO₂–brine response of the fine-grained and less permeable rocks necessary. There is evidence to suggest that sandstones are better reservoirs for CO₂ geological storage compared with Carbonate rocks; for example, Hangx et al. [108]^[10] and Lamy-Chappuis et al. [98]^[11] showed that Carbonate rocks have a greater change in bulk modulus, strength, and porosity compared with siliciclastic rocks; they argued rocks that are rich in Quartz show minor changes due to the strong grain to grain contact, whereas calcites undergo significant dissolution and microstructural changes. Similarly, Alemu et al. [122]^[12] showed that carbonate-rich shale is more reactive compared with clay-rick shales while observing the dissolution of plagioclase, illite, and chlorites, the precipitation of Carbonates, and the formation of Smectite in Carbonate-rich rocks flooded with CO₂–brine. In their experiment, the clay-rich rocks did not show significant changes, but Analcime was deposited on the clay-rich shale that was flooded with CO₂–brine. Furthermore, Han et al. [97]^[13] confirmed that the capability of flow and storage in Carbonate rocks are significantly altered by chemical and physical reactions with CO₂–brine. Their experiment showed the disintegration of grains by dissolution and precipitation of minerals particles in contact with the CO₂–brine stream. These call for caution when Carbonates and calcite-bearing rocks are to be used for CO₂ geological storage. Primacy triggers of changes in the properties of rocks are temperature, pressure, and stress [92^[14][15],93], Other triggers include dissolution, precipitation and pores stress corrosion. A change in one property of the rock leads to change in other properties, such as the coupled nature of changes that can occur in a geosequestration site. For instance, Fuchs et al. [5]^[16] showed that an increase in the porosity of sandstone led to a reduction in fracture toughness, Xiao et al. [2]^[17] showed that decreased porosity due to precipitation led to a reduced risk of induced fracture, this is thought to be the case when a more stable mineral is precipitated. Additionally, Lamy-Chappuis et al. [125]^[18] showed that a 10% increase in porosity led to a corresponding change in the sonic velocity, the sonic velocity is indicative of the strength of the rock. Vialle and Vanorio [126]^[19] observed that increase in porosity and permeability of rocks flooded with CO₂–brine was matched with a decrease in P and S wave velocity. This knowledge implies that for any geosequestration site, there can be an index property that should be constantly monitored, from which the changes in other properties of the rocks can be evaluated. However, the index property is accurately measured and the relationship between the index property and the other properties that will be evaluated must be well understood and interpreted. All researchers reported a decrease in strength, bulk modulus, and elastic modulus, but an increase in porosity and permeability of the rocks due to CO₂–brine activity. However, researchers such as Peter et al. [94]^[20] and Xiao et al. [2]^[17] reported a decrease in porosity due to CO₂–brine activity, they explained that the CO₂–brine–rock reaction led to precipitation of minerals that clogged the pores and thus reduced the porosity. Han et al. [97]^[13], Olabode and Radonjic [121]^[21], and Delle and Sarout [1]^[22] also reported that induced precipitation leads to the closing of pores and micro-fracture. The difference in the change in strength, porosity, permeability, and elastic and bulk modulus recorded for the rocks used in the research reviewed may be due to the nature of the original rock and the minerals [115^[23][24],123], the nature of the pore fluid [115]^[23], physico-chemical condition [88,100,117]^[25][26][27] and the duration of chemical interaction between CO₂–brine–rock [127]^[28]. Pimienta et al. [95]^[29] found that dissolution of minerals in CO₂–brine increased with residence time, and Olabode and Radonjic [121]^[21] noted that with a long time of exposure, precipitation of minerals became dominant over dissolution in shales saturated with CO₂–brine. The duration of CO₂–brine residency is a very important factor that deserves more research. Undissolved and mobile CO₂ is predicted to be in the reservoir for thousands of years [128]^[30]. However, most experiments have been completed within days or weeks, due to experimental limitations. It is necessary to determine the resident time needed for the different Phase CO₂–brine to have an impact on the properties of the rocks. Peter et al. [109]^[31] saturated samples of rocks with different Phase CO₂–brine for 7 days and concluded that the impact of the resulting CO₂–brine on the properties of the rock started gradually from the first day and increased as the concentration of the acidic brine increased. Pimienta et al. [95]^[29] studied the effect of residence time on the dissolution and integrity of rocks flooded with CO₂ and found that the pore brine acidifies just after 2 h of exposure leading to calcite dissolution, a significant increase in the calcium ions of the brine concentration and commensurate changes in rock physical properties such as porosity and permeability. In a scCO₂ fracturing experiment, Zou et al. [129]^[32] observed that the CO₂–brine–rock reaction occurs rapidly (less than 0.5 h). Olabode and Radonjic [121]^[21] had reported a substantial change in the pH of effluent from shale flooded with CO₂–brine only after 3 days of flooding; the change in pH of the effluent was higher in the earlier days. Results from Pimienta et al. [95]^[29], Peter et al. [109]^[31]; Zou et al. [129]^[32] and Olabode and Radonjic [121]^[21] are short-termed and show that the impact of CO₂ on the properties of rock starts immediately and progresses with time. There is a need to carry out a long-term investigation. Hangx et al. (2015) and Espinoza et al. (2018), used samples from natural CO₂ analog sites, and provide insights into the long-term effect of CO₂ on rocks. Both studies report a reduction in strength and agreed on the role of cement size alteration as a control for chemo-mechanical changes, the dissolution of cement led to an alteration of cement size and consequent increase in porosity, reduction in strength, vertical compaction, and lateral stress. However, the conditions at CO₂ analog sites may not apply to geological CO₂ storage. This entreviewy indicates that CO₂–brine–rock interaction is site-specific as the process can be easily affected by many factors that are bound to be different at different reservoirs. Supercritical CO₂ is the most popular phase of CO₂ that has been used in geological storage research. This is because CO₂ is injected in supercritical conditions into the reservoir. Given the dynamic pressure-temperature condition of the reservoir, the phase of CO₂ will change; therefore, there is a need to investigate the impact of other phases of CO₂ in geological CO₂ storage. Peter et al. [94]^[20] and Peter et al. [109]^[31] evaluated the effect of different Phase CO₂–brine on deformation rate, deformation behavior, bulk modulus, compressibility, strength, stiffness, porosity, and permeability of reservoir rocks. Changes in pore geometry properties, porosity, and permeability of the rocks under CO₂ storage conditions with different Phase CO₂–brine were also evaluated using digital rock physics techniques. Microscopic rock image analysis was also applied to provide evidence of changes in micro-fabric, the topology of minerals, and the elemental composition of minerals in saline rocks resulting from different Phase CO₂–brine that can exist in saline CO₂ storage reservoirs. In this paper, ScCO₂ refers to supercritical CO₂, whereas gCO₂ refers to gas-phase CO₂. It was seen that the properties of the reservoir that are most affected by the scCO₂–brine state of the reservoir include an increase in secondary fatigue rate, decrease in bulk modulus and shear strength, change in the topology of minerals caused by precipitation of fines, and agglomeration of grains, as well as change in shape and flatness of pore surfaces. The properties of the reservoir that is most affected by the gCO₂–brine state of the reservoir include an increase in primary fatigue rate, stress-induced decrease in permeability, porosity, and change in the topology of minerals. For all samples, the roundness and smoothness of grains as well as smoothness of pores increased after compression, whereas the roundness of pores decreased. Change in elemental composition in rock minerals in CO₂–brine–rock interaction was seen to depend on the reactivity of the mineral with CO₂ and/or brine and the presence of brine accelerates such change. Additionally, Lei and Xue [102]^[33] reported that the highest reduction in P-velocity and strength was seen in the sandstone sample saturated with supercritical CO₂ compared with those saturated with gaseous, liquid CO₂. These results show that the phase of CO₂ affects the nature of the impact of CO₂–brine on the properties of the rocks. All CO₂ geological storage research that has been reported in this entreviewy is conducted under defined conditions and for a short time, different reservoirs have different conditions, and the condition of the reservoir changes over a long time, this imposes a limitation on experimental geological storage research as a slight change in reservoir condition can have far-reaching impact on the storage process. It is advised that CO₂ geological storage research be conducted as a dynamic process in which different possible scenarios can be examined. Additionally, CO₂ geological storage sites need to be explicitly studied and continuous monitoring of changes is recommended.