Taehee Kim

Characteristics of Salt-Precipitation and the Associated Pressure Build-Up during CO2 Storage in Saline Aquifers

Mitigation and control of borehole pressure at the bottom of an injection well is directly related to the effective management of well injectivity during geologic carbon sequestration activity. Researchers have generally accepted the idea that high rates of CO2 injection into low permeability strata results in increased bottom-hole pressure in a well. However, the results of this study suggested that this is not always the case, due to the occurrence of localized salt precipitation adjacent to the injection well. A series of numerical simulations indicated that in some cases, a low rate of CO2 injection into high permeability formation induced greater pressure build-up. This occurred because of the different types of salt precipitation pattern controlled by buoyancy-driven CO2 plume migration. The first type is non-localized salt precipitation, which is characterized by uniform salt precipitation within the dry-out zone. The second type, localized salt precipitation, is characterized by an abnormally high level of salt precipitation at the dry-out front. This localized salt precipitation acts as a barrier that hampers the propagation of both CO2 and pressure to the far field as well as counter-flowing brine migration toward the injection well. These dynamic processes caused a drastic pressure build-up in the well, which decreased injectivity. By modeling a series of test cases, it was found that low-rate CO2 injection into high permeability formation was likely to cause localized salt precipitation. Sensitivity studies revealed that brine salinity linearly affected the level of salt precipitation, and that vertical permeability enhanced the buoyancy effect which increased the growth of the salt barrier. The porosity also affected both the level of localized salt precipitation and dry-out zone extension depending on injection rates. High temperature injected CO2 promoted the vertical movement of the CO2 plume, which accelerated localized salt precipitation, but at the same time caused a decrease in the density of the injected CO2. The combination of these two effects eventually decreased bottomhole pressure. Considering the injectivity degradation, a method is proposed for decreasing the pressure build-up and increasing injectivity by assigning a ‘skin zone’ that represents a local region with a transmissivity different from that of the surrounding aquifer.

Classification of springs of Jeju Island using cluster analysis of annual fluctuations in discharge variables: investigation of the regional groundwater system

The springs of Jeju Island have been studied since 1999. However, previous studies did not provide a systematic analysis of survey results. Thus, we performed a systematic analysis of the groundwater flow system using cluster analysis of discharge variables, i.e., discharge rate, electrical conductivity (EC), and temperature, at 121 springs. The 121 springs were categorized into nine major groups based on discharge rates and ten groups based on EC. Springs in groups 1, 2, and 3 in discharge cases had peak discharge rates in September, October, and occasionally in November, following the rainy season. Other springs showed complex seasonal variation in discharge rates. Springs in groups 4–1, 4–3, 5–2, 9–1, and 9–2 had peak discharge rates in winter (December–February). The results of classification by EC were directly correlated with the absolute values of and seasonal variation in EC. Based on EC, springs in group 1 had relatively low EC of > 500 μS/cm in general. The highest EC at each spring in group 1 was 110–18,000 μS/cm. Coastal springs generally had higher EC than did springs located inland. The relation between discharge rate and EC was of interest. At some springs, discharge rate and EC were proportional, especially in winter. The proportionality between discharge and EC cannot be explained by the conventional groundwater management model of Jeju Island. To explain this proportionality, we examined the results of previous studies with regard to geological features, well locations, hydraulic head and hydraulic gradient distributions, and vertical reduction in groundwater EC within the zone influenced by saline water. We then proposed a hypothetical regional conceptual model of groundwater flow for Jeju Island. More precise studies are needed to clarify the relationships among responses at surveyed springs, hydraulic features in coastal areas, and the regional conceptual model for groundwater circulation on Jeju Island.