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

The mineralogical changes of the mudstone and the dacitic tuff after 90 days of scCO2–water–rock reaction were investigated by XRF analyses and their results are shown in Table 4. The results indicated that even after 90 d of reaction, the proportions of the major constituents of the two kinds of rock were not significantly changed, except for a minor decrease of SiO2 and CaO. These originated from increased dissolution of calcite, Ca–feldspar, and Ca-bearing silicates, which were similar to results of previous studies [37–39]. It is suggested that the mudstone and the dacitic tuff in the Janggi Basin is likely to maintain significant stability against scCO2-involved geochemical reactions during CO2 storage.

**Table 4.** XRF analysis of mudstone and dacitic tuff before and after 90 days of scCO2–water–rock reaction.


Results from the measurement of the initial scCO2 capillary entry pressure (Δp) for the mudstone and the tuff core are shown in Figure 5. For all of tuff cores, the scCO2 began to intrude into the rock core at 115 bar (Δp = 15 bar) and continuous scCO2 injection into the core occurred at a Δp higher than 20 bar. This suggests that the initial scCO2 capillary entry pressure (Δp) of the dacitic tuff ranged from 15 to 20 bar, under the conditions of 100 bar and 50 ◦C. In the tuff cores, 8–10% of the void space (0.7–0.9 mL) was filled by scCO2 at a Δp higher than 20 bar. In a previous study (Kim et al. 2019), the average initial scCO2 capillary entry pressure of the sandstone and the conglomerate in the Janggi Basin was lower than 10 bar (mostly < 5 bar), which was less than one-third that for the dacitic tuff. For the mudstone cores in the Janggi Basin, the scCO2 did not penetrate the core surface and the stored scCO2 was less than 0.005 mL, even when the injection pressure was 250 bar (Δp = 150 bar) for 30 d. This suggests that the initial scCO2 capillary entry pressure for the mudstone core was much higher than 150 bar (10 times higher than for the tuff). Based on the initial scCO2 capillary entry pressure, the mudstone formation in the Janggi Basin was much more suitable than the tuff formation as a shield against scCO2 leakage from the reservoir rock.

**Figure 5.** Initial scCO2 capillary entry pressure (Δp) and the volume of scCO2 stored in mudstone and dacitic tuff cores.

#### **4. Conclusions**

Determining an optimal storage site and choosing the main parameters to be considered has been one of the main issues for the geological sequestration of CO2. During the last two decades, evaluation of the storage capacity and the leakage safety has been considered essential for optimal storage site selection. However, previously, only rough estimations were made of the reservoir and the capping rock using conventional parameters such as porosity and permeability, or by using large-scale information acquired from geophysical exploration and geological field observation. The quantitative evaluation of CO2 storage capacity and of the risk of CO2 leakage has been very limited, even at a laboratory scale. Recently, a direct technique for the measurement of the scCO2 storage ratio and the initial scCO2 capillary pressure of the rock was developed, but its application to CO2 storage site selection has almost never been attempted in previous studies. As mentioned earlier, the Janggi Basin was selected as a feasibility testing site to store at least 100,000 metric tons of CO2 before starting large-scale injection (more than 1,000,000 metric tons) on the Korean peninsula. For the testing site, it was important that the CO2 storage capacity of the Janggi Basin was, at the least, larger than 100,000 metric tons. This study presents an easy and effective technique by which to evaluate the CO2 capacity of reservoir rock and the leakage safety of the cap rock, and finally, to successfully select the Janggi basin as an optimal CO2 storage site in Korea. The scale-up estimation of the CO2 storage capacity for the conglomerate and the rudaceous sandstone in the Janggi Basin of Korea was performed on the basis of direct measurement of the scCO2 storage ratio. The safety risk of scCO2 leakage for the cap rock of dacitic tuff and mudstone in the Janggi Basin was also quantitatively evaluated by measuring the initial scCO2 capillary entry pressure.

The experimental results successfully demonstrated that the conglomerate and sandstone formations of the Janggi Basin are suitable as a geological storage test site, for injection of a hundred thousand tons of CO2 from the viewpoint of storage capacity. If the amount of dissolved CO2 in the pore water could be considered, a more precise estimate of the CO2 storage capacity for the specific reservoir formation could be estimated; related research in this area is already in progress.

It was also verified that the mudstone formation in the Janggi Basin is adequate to prevent the seepage of buoyant scCO2 from the reservoir site because its initial capillary entry pressure (Δp) was higher than 150 bar. From the XRF analysis before and after the experiment, reliable evidence for the geochemical stability of the tuff and the mudstone was also provided. These quantitative measurements of the scCO2 storage ratio and the initial scCO2 capillary entry pressure applied in this study could be used to determine practical CO2 storage sites and could also provide meaningful information for future decisions regarding scCO2 injection conditions.

This study focused on the hydromechanical measurement of the scCO2 in the pore spaces of rock core, over just a few months and, thus, did not deal with the effect of geochemical reactions (CO2–water–rock) on the scCO2 capacity and the capillary entry pressure with a long-term view. Recent research indicates that physical or chemical changes in the properties of rock involved in CO2 sequestration could arise from mineralogical or geochemical reactions within a shorter time after CO2 injection, than previously expected. For successful selections of optimal CO2 storage subsurface sites, the geochemical stability of reservoir or cap rocks should be considered an important parameter, along with the storage capacity and the capillary entry pressure.

**Author Contributions:** M.L. and J.P. conceived and designed the methodology and the experiments; J.P., M.L., and S.K. performed the experiments and the data analyses; S.W. and M.Y. contributed materials and data interpretation; M.L. wrote the paper to prepare the submission.

**Funding:** This research was supported by the Korea Agency for Infrastructure Technology Advancement (KAIA) grant funded by the Ministry of Land, Infrastructure and Transport (Grant 19CTAP-C151965-01) and by the grant (2019002470002) from Korea Ministry of Environment as "The SEM (Subsurface Environmental Management) projects".

**Acknowledgments:** The authors would like to express their gratitude to the anonymous reviewers for their critical comments and advice.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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