**1. Introduction**

Eco-friendly plans and policies to reduce CO2 emission are being driven forward around the world. In developed countries, CO2 capture and sequestration (CCS) technology is partially commercialized and the total amount of subsurface CO2 storage has been on the rise [1–4]. Since the early 2000s, the government of South Korea has been working on several projects to determine the optimal CO2 storage sites on the Korean peninsula and the Janggi Basin. This basin is located in the southeastern part of the East Sea and is currently being evaluated for its potential to be one of the best onshore or offshore storage sites in Korea [5,6]. From geophysical and geological surveys, it is has become clear that the Miocene Janggi Basin consists of four small blocks (Guryongpo, Ocheon, Noeseongsan, and Yeongamri basins) [7]. The Noeseongsan block contains rudaceous sandstone and conglomerate layers that are considered promising for CO2 storage sites more than 800 m deep; these also have mudstone and dacitic tuff layers above them that are able to serve as stable shield layers [8,9]. The Korean government has a plan to inject a hundred thousand metric tons of CO2 in a pilot-scale onshore CO2 storage test site in 2030 and the Janggi Basin is considered a suitable place for the scCO2 storage test site. During stratigraphic analysis in 2015 and 2016, four sites in the Janggi Basin were drilled by the Korea Institute of Geoscience and Mineral Resources (KIGAM), and continuous drill cores to 1200 m in

depth were collected at each site. From previously collected well logging data and the geo-structural interpretation results, the Janggi conglomerate formation in the Janggi Basin can be divided into four lithofacies. These consist of conglomerate and rudaceous sandstone from gravelly braided stream deposits, coarse sandstone deposited in mouth-bar or delta, muddy sandstone and shale deposited in floodplain environments, and mudstone as lacustrine deposits [8]. The western part of the basin is mainly composed of thick conglomerate and rudaceous sandstone lithofacies, which are available for use as a CO2 storage reservoir, whereas the mudstone and muddy sandstone lithofacies constitute the eastern part of the basin. The western part of the Janggi Basin has no large faults and is considered to be an optimal CO2 storage area. This area is now estimated to be at least about 10 km2, assuming that the practical volume of the CO2 storage volume in the Janggi Basin deeper than 800 m is about 0.025 km3 [8,9].

The estimation of the CO2 storage capacity of geological storage reservoirs is essential to determine reasonable CO2 storage site candidates and is directly dependent on the practical amount of scCO2 in non-aqueous phase, which can be stored in the pore spaces of the reservoir rock after scCO2 injection. The more water displaced by scCO2 in the void spaces of the reservoir rock, the more CO2 could be stored there. There are no exact definitions for scCO2 storage capacity yet, although there are several definitions of storage capacity from previous studies [10–18]. In recent research, the scCO2 storage capacity was generally defined as the proportion of the volume of scCO2 stored after injection, in relation to the pore volume of the CO2 reservoir rock [19,20]. Geological exploration and numerical simulation to determine the representative amount of scCO2 that could be stored within the pore spaces of a specific reservoir formation have been major subjects in CO2 sequestration (CS) studies.

However, the pore volume saturated with water was not fully replaced by scCO2 while injecting scCO2 into the reservoir rock and the practical scCO2 storage capacity for the specific reservoir rock can often be overestimated. The scCO2 displacement of water from pore spaces of the rock during scCO2 injection can be influenced by various parameters—Not just physical properties such as pore size, heterogeneity of the pore network, and injection pressure, but also mineralogical and geochemical reactivity. Thus, the best way to determine the amount of scCO2 storage possible in a specific rock formation is direct measurement of scCO2 displacement of water under scCO2 injection conditions. This occurs using rock cores at a laboratory scale, from which the results are extended to macro scale, including the entire reservoir formation [21,22].

The scCO2 storage ratio (%) (or "scCO2 displacement efficiency (%)") of a reservoir formation is one of the most general parameters used in laboratory work for evaluating the CO2 storage amount of a formation [3,10,19,20]. The scCO2 storage ratio is defined as the fraction of the amount of scCO2 occupying pore spaces after scCO2 injection into a reservoir formation. It can be directly measured under scCO2 injection conditions simulated on a laboratory scale. In 2016, Wang et al. and Kim et al. [19,20] carried out direct laboratory measurement of the scCO2 storage ratio for possible reservoir rock cores, presenting the possibility that it might be used to estimate the scCO2 capacity for the specific reservoir. However, this process was still in the experimental stages and the number of studies on scCO2 storage capacity based on direct measurement in the laboratory are very limited. The CO2 storage capacity for a specific reservoir formation can be calculated by multiplying the scCO2 storage ratio by the total void volume of the formation, ignoring the amount of dissolved CO2. There are many benefits of direct measurement of the scCO2 storage ratio because it represents the substantive amount of CO2 retained in a specific storage formation, under the scCO2 injection condition. The estimation of the practical amount of CO2 storage for a specific reservoir rock could be made possible by using both the scCO2 storage ratio and the reservoir volume acquired from geological survey, which has almost never been tried before. In this study, laboratory experiments were performed to measure the amount of scCO2 displacing water from the pore spaces of the sandstones and conglomerate cores sampled from 800–1000 m depth in the Janggi Basin, which is classified as an available CO2 storage reservoir in Korea. The scCO2 storage capacity of the Janggi Basin was calculated

quantitatively, according to the measured scCO2 storage ratio and with additional geophysical data on the spatial domain of the Janggi Basin.

The scCO2 sealing capacity of the cap rock is another major parameter used to select successful CO2 storage sites because it correlates to the leakage of scCO2 during the anticipated duration of CO2 sequestration. Even if a CO2 storage site has enough scCO2 storage capacity, it has to be excluded from suitable CO2 storage sites if the scCO2 leakage safety of the cap rock in the site is not assured. Layers of mudstone and dacitic tuff are repeated above the rudaceous sandstone and conglomerate layers in the Janggi Basin and it is assumed that they can play the role of cap rock to prevent the upward movement of scCO2 from deeper reservoir rock [9]. In 2017, the initial capillary entry pressure of scCO2 into the cap rock core surface, determined when the scCO2 began to infiltrate the rock, was successfully measured in the laboratory [23]. In this study, several experimental conditions for the capillary entry pressure measurement such as the boundary condition of the high-pressure tank and the scCO2 injection time on the core surface were modified, to more realistically simulate the scCO2 leakage at the boundary between the reservoir and the cap rock. More information for the comparison of the experimental conditions can be drawn from [23]. The scCO2 sealing capacity of the mudstone and dacitic tuff in the Janggi Basin was evaluated based on their initial scCO2 capillary entry pressures under simulated scCO2 injection P-T conditions. The change of mineralogical composition of the reservoir and cap rock after 90 d of scCO2–water–rock reaction at 50 ◦C and 100 bar was also investigated by XRF analysis. This was done to observe the effect of scCO2-related geochemical reactions on the sealing capacity. From the experimental results on the scCO2 storage ratio for the reservoir rock, on the initial scCO2 capillary entry pressure for the cap rock, and from mineralogical analyses for rock cores, the feasibility of the Janggi Basin as an available pilot-scale CO2 storage test site where a hundred thousand metric tons of CO2 could be injected was evaluated.

This study presents a novel and reliable method by which to select a successful CO2 storage site based on both quantitative scCO2 storage ratio and capillary entry pressure under CO2 sequestration conditions, as well as on geochemical analyses. The results of this study will also provide ideas for further quantitative research about the CO2 storage capacity and CO2 leakage safety based on practical measurements of the scCO2 storage ratio and initial scCO2 capillary entry pressure.

#### **2. Materials and Methods**
