Tracing CO2 leakage and migration using the hydrogeochemical tracers during a controlled CO2 release field test

Abstract

A critical environmental issue in carbon capture and storage (CCS) is potential CO2 leakage, which accompanies geochemical reactions with aquifer materials. To investigate the hydrogeochemical effects of CO2 leakage on shallow groundwater at the early stage of CO2 leakage and to evaluate a hydrochemical or isotopic tracer for CO2 migration, a controlled CO2 release experiment was performed in a siliciclastic aquifer at the Environmental Impact Evaluation Test (EIT) site, South Korea. After the baseline survey of hydrochemical and carbon isotope (delta(CDIC)-C-13) compositions, CO2-infused water was injected at a rate of 5.5 m(3)/day for 26 days at-22 m below ground level in the-40 m thick heterogeneous aquifer whose pressure gradient was increased to approximately 10 times the natural gradient to make a flow path along monitoring wells. The arrival of CO2 plume was determined at each monitoring well by the decrease in pH and the increase in the partial pressure of CO2 (PCO2) and EC. delta(CDIC)-C-13 decreased at the arrival of CO2 plume and showed high correlations with log PCO2 since the delta(CDIC)-C-13 of injected CO2 (-24.7%o) was distinct from that of ambient groundwater (-16.7%o) and little carbon sources existed in the aquifer. The spatial and temporal evolution of hydrochemical and isotopic compositions observed using a monitoring well network indicated that the CO2 plume migrated along a preferential pathway overwhelming induced pressure gradient due to water table mounding at injection and that the plume sank to some degree probably due to its large density. Concentrations of hydrochemical elements displayed three types of behavior: (1) pulse-like with rapid increases at the arrival of CO2 plume and decreases despite the continuous injection of CO2 similar to EC (HCO3, Ca, Mg, Na, K, Sr, and Ba), (2) pH dependent with relatively slow increases and decreases in concentrations (SiO2 and Mn), and (3) rapid increases but slow decreases (Li). The hydro -chemical variations indicated the dissolution of a limited amount of reactive minerals such as calcite, followed by cation exchange at the early stage of CO2 leakage in siliciclastic aquifers. Based on the study result, Li was an effective hydrogeochemical tracer to monitor the migration of CO2 in siliciclastic aquifers as well as pH, EC, and delta(CDIC)-C-13.