Abstract Effect of formation water composition on predicting CO2 behavior: A case study at the Nagaoka post-injection monitoring site
Applied Geochemistry
Saeko Mito, Ziqiu Xue, Tatsuya Sato

Carbon dioxide capture and storage (CCS) is one of the options for reducing CO2 emissions to the atmosphere from large-scale point sources. Stored CO2, initially present mainly in a supercritical state, reacts with formation water and minerals in saline aquifers and, some of it alters to dissolved CO2 (e.g., CO2aq, HCO 3 ) and to mineral CO2 (e.g., CaCO3) over the years. Geochemical reactions play an important role in increasing the security of CO2 storage. Understanding the processes of CO2 trapping is important in evaluating the risks involved and in obtaining public acceptance of its geological storage. In this geochemical modeling study, formation water composition is used in sensitivity analyses of CO2 behavior in a saline aquifer. Three types of formation water compositions were examined; (1) a NaCl solution at an assumed salinity, (2) a solution in equilibrium with the minerals in a reservoir and (3) actual formation water collected from the reservoir. Higher salinity water resulted in an earlier arrival time of CO2 at an observation well. Equilibrated water was used to estimate contributions of CO2 trapping mechanisms in long-term predictions. The estimated proportion of trapped CO2 phases (gas, aqueous and mineral phase) was almost the same as when actual water was used for the initial water composition in the calculations. However, using actual formation water will provide better understanding of chemical composition changes, especially in short-term prediction.

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