*2.2. Measurement of the scCO2 Storage Ratio for the Conglomerate and Sandstone Cores*

When scCO2 was injected, water filling the void spaces of the reservoir rock was not fully replaced by the scCO2 because of the difference in interfacial tension between water and the scCO2 (or difference in wettability) [24]. Thus, the amount of scCO2 that could be stored in the subsurface reservoir rock was less than the total void space of the rock, and was affected by various physico-chemical parameters. For selection of an optimal CO2 storage site, the scCO2 storage capacity of rock from that specific reservoir should be estimated, based on the real scCO2 storage ratio. Moreover, the potential amount of storage for each kind of reservoir rock should also be determined before subsurface injection of the scCO2. Laboratory scale measurement of the scCO2 storage ratio, displacing water from the pore spaces of Janggi sandstone and conglomerate cores under the simulated scCO2 injection conditions was performed. The experimental conditions were maintained (50 ◦C and 100 bar) to simulate the CO2 storage conditions underground. The sandstone and conglomerate cores were cut (4.2 cm in diameter and 5–7 cm in length) and their cut surfaces were polished using powdered diamond paper to maintain a uniform scCO2 or water injection pressure on the cut surface. A high-pressure stainless-steel cell was developed to measure the amount of scCO2 stored in the pore spaces of each core after scCO2 injection. It is difficult to measure the scCO2 remaining in the pore space of the rock core after scCO2 injection because of the leakage of the injected scCO2 at the cylinder surface boundary between the cell inner wall and the rock-core wall surface. The high-pressure cell was designed with two different walls; the inner wall was composed of a thick rubber layer (1 cm thick) and the outer wall of stainless steel. The space between the inner and the outer wall of the cell was sealed with pressurized water, which was injected from outside the cell. The surface of the inner rubber wall was in tight contact with the rock core cylinder surface, when the water pressure in the space was much higher than the scCO2 injection pressure (Δp > 100 bar). The rock core top and bottom head surfaces were held using a screw-type steel holder with a hole in the middle for scCO2 or water injection/drainage in the rock

core. It was possible to shut off the bypass of injected scCO2 or pore water through the boundary between the core cylinder surface and the cell inner wall, allowing the scCO2 (or water) to flow only through pore spaces within the rock core. Figure 2 shows photographs of the high-pressure cell and the schematic diagram for the cross-section of the cell used in the experiment.

**Figure 2.** Photograph of the high-pressure cell used for the experiment to measure the supercritical CO2 (scCO2) storage ratio.

For the experiment, each core was fully dried at 50 ◦C in an oven and then weighed. The dried core was fixed by two core holders inside the high-pressure stainless-steel cell. The outer wall of the high-pressure cell was covered by a heating jacket to maintain the cell wall at the cell temperature of 50 ◦C. Distilled water was injected into the sealed space between the inner wall and the outer wall of the cell by a syringe pump (Isco-D260; Teledyne Isco, Inc., Nebraska, NE, USA), which was maintained at 300–350 bar. Then, the distilled water was flushed through the core at 100 bar (the injection pressure) for three pore volumes of the core to fully saturate the core with water. Next, scCO2 was injected through the influent opening into the cell to displace water from the pore spaces of the core at 110 bar (Δp = 10 bar between the injection pressure and the pore water pressure in the core), while more than two pore volumes of scCO2 were flushed from the core at 110 bar (assuming that displacement of water by the scCO2 was successful within a few days). All of the effluent water was stored in a small stainless storage cell and its mass was weighed to measure the amount of water displaced by the scCO2 in the rock core. A high-pressure stainless-steel chamber (5 L capacity) was connected to the effluent of the cell to consider the boundary condition of the reservoir rock when the scCO2 was flushed from the rock core in the experiment. The water in the pores was compressed as the pore pressure increased due to scCO2 injection and enough water or scCO2 volume had to be provided in the chamber for the replacement of all the scCO2 during the experiment. All of the high-pressure cells were maintained at 50 ◦C and 110 bar, after the CO2 injection, to simulate the subsurface CO2 storage conditions. Figure 3 shows the procedure of the experiment for scCO2 exchange in the rock core.

**Figure 3.** Schematic of the experiment to measure the scCO2 storage ratio.

When the amount of water drained from the core was measured, the scCO2 storage ratio for the specific rock core under the scCO2 injection condition (in this case, 100 bar and 50 ◦C) could be calculated using Equation (1).

$$\text{The scCO2 storage ratio } \left(\%\right) \text{ for the rock } \left(\varepsilon\right) = \left(1 - \frac{W\_s - W\_{out}}{W\_s}\right) \times 100\tag{1}$$

where *Ws* is the volume of water saturating the core and *Wout* is the water volume displaced by scCO2 during the scCO2 injection.

From Equation (1), the scCO2 storage capacity of the conglomerate and the sandstone formations in the Janggi Basin were estimated via Equation (2) with the volume of the stratum, the average porosity, the specific gravity of the scCO2, and the storage ratio of scCO2 [3,14].

$$\text{The storage capacity (ton)} = \sum (V \times \rho \times \rho \times \varepsilon),\tag{2}$$

where *V* is the volume of the conglomerate or sandstone layer (estimated from the previous geological survey data), ϕ is the average porosity, ρ is the specific gravity of the scCO2, and ε is the scCO2 storage ratio. The feasibility of the Janggi Basin as a CO2 reservoir formation was evaluated based on the scCO2 storage capacity of rudaceous sandstone and conglomerate drill core samples from the Janggi Basin (total of six cores).
