*2.3. Measurement of the Initial scCO2 Capillary Entry Pressure for Mudstone and Dacitic Tu*ff

When scCO2 is injected into reservoir rock, it is distributed as a separate phase from water in pore spaces and begins to move slowly upward from the lower part of the reservoir rock, due to buoyant force, because of its lower density as compared to water. Some of the injected scCO2 might reach the boundary between the cap rock and the reservoir rock during continuous injection. At the early stage of scCO2 injection, the amount of scCO2 reaching the boundary is not much and the cap rock can prevent the intrusion of scCO2 because of its low permeability. As the amount of scCO2 at the boundary and its buoyant pressure increase because of continuous scCO2 injection, advective and the diffusive intrusion of scCO2 into the cap rock occurs. This pressure initiates seepage of the scCO2 from

the reservoir rock, threatening the leakage safety of the CO2 storage site. The sealing capacity of the cap rock is directly dependent on the initial scCO2 seepage (or intrusion) pressure on the cap rock surface [22]. The direct measurement of the initial scCO2 capillary entry pressure of the cap rock was performed at a laboratory scale, to evaluate the scCO2 shielding capacity of the cap rocks in the study area. The mudstone and dacitic tuff rock cores sampled during the deep drilling expedition (from 700–800 m) at the onshore site in the Janggi Basin were used for the experiment (Figure 1).

Rock cores without cracks or fractures were cut (4.2 cm diameter, 5–6 cm length) and were then fully dried at 50 ◦C in an oven for 7 d. Each rock core was fixed in the high-pressure stainless-steel cell (which was used in the same way as in the previous experiment, Section 2.2). Each core was saturated with distilled water at 100 bar of pore water pressure. The effluent of the cell was connected to a large tank filled with 3 L of water and 2 L of scCO2 at 100 bar and 50 ◦C, simulating the subsurface scCO2 injection conditions. The initial scCO2 capillary entry pressure into the rock core head (higher than 100 bar: Δp = injection pressure − 100 bar), was controlled at the influent port with the regulator of the core holder in the cell bottom, until the scCO2 began to penetrate the rock. The scCO2 injection pressure was set at 110 bar and the injection pressure was increased by 10 bar until the scCO2 began to penetrate the rock core head. At the outset, the scCO2 injection pressure on the core head surface was set at 110 bar (Δp = 10 bar) and any scCO2 intrusion into the core was observed for 10 days. If no scCO2 intrusion occurred, the injection pressure was increased by 10 bar for 10 more days, to monitor any scCO2 intrusion into the core. This process was repeated until the scCO2 began to penetrate the rock core head. When the scCO2 begin to intrude and the scCO2 injection pressure started to decrease, the scCO2 injection pressure was maintained until scCO2 was flushed from the end of the rock core. This pressure was regarded as the initial scCO2 capillary entry pressure (Δp) of the rock core. The scCO2 shield capacity of each kind of cap rock core (dacitic tuffs and mudstones here) was evaluated by comparing their initial scCO2 capillary entry pressures (Δp).

The mineralogical changes of mudstone and tuff were also measured to evaluate their geochemical stability for 90 d of the scCO2–water–rock reaction under CO2 storage conditions (100 bar and 50 ◦C). The rock core was pulverized using a mortar, and ten grams of powdered rock materials were mixed with 100 mL of distilled water in a high-pressure stainless-steel cell (capacity of 150 mL), in which the inner wall was coated with a Teflon layer. The void spaces in the cell were filled with scCO2 using a syringe pump. Then scCO2–water–rock reactions were allowed to occur in the cell at 100 bar and 50 ◦C, simulating the subsurface storage conditions. The total reaction time was 90 d and XRD analysis was conducted before and after the reaction to identify any mineralogical changes of the cap rock due to the scCO2–water–rock reaction.

#### **3. Results and Discussion**
