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This study tried to examine the feasibility of enzyme-induced calcium carbonate precipitation (EICP) for CO2 leakage prevention in a subsurface condition. For this, the effects of 1) concentrations of urease enzyme and reactants (urea and CaCl2), 2) initial pH (3–9), and 3) temperature (30–50 °C) on the precipitation of calcium carbonate were tested in the batch experiments. In addition, the core-plugging experiment was conducted to test the potential application of EICP to a porous medium with high permeability. The batch experiments showed that the 0.25 M urea was completely hydrolyzed by urease (above 1000 U/l). However, 0.4 M and 0.5 M urea were not completely decomposed because of the insufficient quantity of urease. Our result also showed that urea hydrolysis was not observed at pH 3, while the efficiency of calcite precipitation was not affected in the pH range of 5 to 9. Temperature also affected the hydrolysis of urea. The efficiency of calcite precipitation increased with increasing temperature from 30 °C to 50 °C. In addition, the result of the core-plugging experiment showed that the permeability of the sandstone core decrease by approximately 42% due to the precipitation of calcite in the pores. From our results, this study suggested that EICP can be a potential for CO2 leakage prevention from a storage reservoir within the pH and temperature ranges of this study. [ABSTRACT FROM AUTHOR]

The CO2-rich spring water (CSW) occurring naturally in three provinces, Kangwon (KW), Chungbuk (CB), and Gyeongbuk (GB) of South Korea was classified based on its hydrochemical properties using compositional data analysis. Additionally, the geochemical evolution pathways of various CSW were simulated via equilibrium phase modeling (EPM) incorporated in the PHREEQC code. Most of the CSW in the study areas grouped into the Ca-HCO3 water type, but some samples from the KW area were classified as Na-HCO3 water. Interaction with anorthite is likely to be more important than interaction with carbonate minerals for the hydrochemical properties of the CSW in the three areas, indicating that the CSW originated from interactions among magmatic CO2, deep groundwater, and bedrock-forming minerals. Based on the simulation results of PHREEQC EPM, the formation temperatures of the CSW within each area were estimated as 77.8 and 150 °C for the Ca-HCO3 and Na-HCO3 types of CSW, respectively, in the KW area; 138.9 °C for the CB CSW; and 93.0 °C for the GB CSW. Additionally, the mixing ratios between simulated carbonate water and shallow groundwater were adjusted to 1:9-9:1 for the CSW of the GB area and the Ca-HCO3-type CSW of the KW area, indicating that these CSWs were more affected by carbonate water than by shallow groundwater. On the other hand, mixing ratios of 1:9-5:5 and 1:9-3:7 were found for the Na-HCO3-type CSW of the KW area and for the CSW of the CB area, respectively, suggesting a relatively small contribution of carbonate water to these CSWs. This study proposes a systematic, but relatively simple, methodology to simulate the formation of carbonate water in deep environments and the geochemical evolution of CSW. Moreover, the proposed methodology could be applied to predict the behavior of CO2 after its geological storage and to estimate the stability and security of geologically stored CO2. [ABSTRACT FROM AUTHOR]

The objective of this study is to evaluate the impact of SO2-CO2-water-rock interaction on the alteration of a reservoir rock having Ca-deficient conditions and little buffering capacity and its implication for porosity change near the injection well from a CO2 storage pilot site, Republic of Korea. For our study, three cases of experimental and geochemical modeling were carried out (pure CO2, 0.1% SO2 in CO2, and 1% SO2 in CO2, resp.) under realistic geologic storage conditions. Our results show that SO2 accelerated water-rock interactions by lowering the pH. In the 1% SO2 case, pH remained less than 2 during the experiments because of insufficient buffering capacity. Sulfate minerals were not precipitated because of an insufficient supply of Ca. Because the total volume of precipitated secondary minerals was less than that of the dissolved primary minerals, the porosity of rock increased in all cases. Chlorite largely contributed to the decrease in total rock volume although it formed only 4.8 wt.% of the rock. Our study shows that the coinjection of a certain amount of SO2 at CO2 storage reservoirs without carbonate and Ca-rich minerals can significantly increase the porosity by enhancing water-rock interactions. This procedure can be beneficial to CO2 injection under some conditions. [ABSTRACT FROM AUTHOR]

The geochemistry of CO2-rich springs and wells was monitored as a natural analogue study of CO2 leakage into shallow aquifers. In result, within the relatively small study area, diluted CO2-rich waters (DCWs), concentrated CW (CCW), and ordinary groundwaters were observed. DCWs showed an exceptionally low pH (mean 4.8) and low EC (mean 150 μS/cm). The low pH and EC as well as the time-invariant geochemistry of DCWs were probably due to continuous CO2 inputs into the open system, which had a short reaction time and rarely consisted of reactive minerals. DCWs showed the low concentrations of SiO2 (mean 20.4 mg/L) and the average tritium concentration of 3.4 TU, which indicates the low CO2–H2O–rock interaction. In addition, δ 13 C CO 2 g in equilibrium with water was estimated using the mass balance equation and δ13CDIC measured in water samples. The results of the stable carbon isotope analysis showed that the CO2 originated from a deep-seated source to the shallow DCW aquifer (<80 m deep below the surface), whereas the deeper CCW aquifer was affected by soil organic CO2 near the surface. To detect the CO2 leakage from CO2 storage sites, the geochemistry of shallow aquifers should be monitored. This study result suggests that at least pH, EC, DIC, and carbon isotopes should be monitored because the monitoring of pH is helpful in an aquifer with low buffering capacity; while EC can be low despite CO2 leakage and the subsequently low pH at early stages, depending on the subsurface environment. Above all, this present study indicates that understanding the characteristics of aquifer conditions is of great importance for CO2 leakage detection. [ABSTRACT FROM AUTHOR]