**3. Geological storage of CO2**

CCUS is considered a viable option to reduce CO2 emission, sustain exploration and production of fossil fuel for the short to medium term and eventually transition to a full green energy in the long term [9, 10]. Among the proposed CO2 emission reduction strategies, CCUS provides the highest emission reduction potential [11]. Generally, CCUS involves the (1) capture of CO2 from large industrial emission sources and direct air capture points (2) the transportation of the captured gas to utilization, conversion, or storage facilities and (3) the utilization of the gas as feedstock in industrial processes, conversion to other products or the injection of the gas into geological storage facilities. In terms of geological storage, the injected CO2 may be stored in depleted oil and gas reservoirs, deep saline reservoirs, unmineable coal seams or injected into active oil and gas reservoirs for Enhanced Oil Recovery (EOR) [12, 13].

CCUS system, although simple in concept, would require significant investment of capital, new technology and time [14]. Besides, many current policies also need to be revised and new legal and regulations framework has to be introduced that require support from local authorities, governments and international bodies [15]. Investigation by the International Energy Agency (IEA) have shown that CCUS can contribute up to about 14% reduction in global greenhouse gas emissions required to limit global warming to 2°C by 2050 [16].

#### **3.1 Storage in saline aquifers**

Saline aquifer refers to a deep, large geological formation consisting permeable sedimentary or carbonate rock types that are saturated with formation water or brines, non-potable water, containing high concentration of dissolved salts [17]. It is buried under a layer of non- or low-permeability rocks that serve as a cap rock to prohibit the fluid flowing upwards to the surface. The saline aquifer can be located both onshore and offshore and normally found at depth greater (more than 800 metres) than aquifers that contain potable water [18]. Deep saline formations have enormous potential for CO2 storage in terms of volumetric storage capacity [19, 20]. On a global scale, deep saline reservoirs have the capacity to hold between 20 and 500% of the projected CO2 emissions by 2050 [9, 21, 22]. Thus, worldwide CO2 storage potential of deep saline reservoirs ranges from 400 to 10,000 Gt CO2. Deep saline aquifers, usually at depths between 700 and 1000 m, hold large quantities of high salinity formation brines [23].

Although the natural content of these reservoirs has no direct commercial value, the chemical composition of the formation brine makes them suitable for CO2 mineralization. In deep saline aquifers, the injected CO2 could be sequestered through hydrodynamic trapping where the gas is trapped beneath a caprock, residual trapping where the rock contains residual saturation of CO2, solubility trapping where the gas dissolves in the formation brine and mineral trapping where CO2 reacts with Ca, Fe or Mg to form stable carbonate precipitates [24, 25]. Lack of additional economic benefits except carbon tax incentives in some countries, makes CO2 storage in saline aquifers less attractive to the oil and gas industry.
