Geochemical modeling of near-surface, CO2 interactions: The critical element in cost-effective long- term monitoring GCCC Digital Publication Series #08-03e

Effective long-term monitoring of commercial carbon dioxide (CO2) geosequestration sites must involve multiple and integrated measurement, monitoring, and sampling techniques in order to identify and quantify CO2 leakage. Measuring changes in CO2 soil gas concentrations in the shallow sub-surface can be problematic due to naturally variable background levels, water table elevation changes, and interference from surface operations. This project focuses on modeling CO2 physical and chemical fate and transport in saturated near-surface zones (<100 m) under varying geological conditions to identify key geochemical variables that can be used in long-term monitoring programs to detect and track CO2 leakage. Numerical simulations were performed using the TOUGHREACT modeling suite to approximate the reactive transport of CO2 introduced at various depths below the water table. Results indicate that CO2 reaction products in groundwater can be used as surrogate measures of CO2 transport. Site-specific characterizations of soil, rock, and groundwater compositions are critical to quantifying interactions that will affect chemical reactions within the rock matrix and groundwater. Sensitivity analyses have been performed to investigate the impacts of soil and water chemistry on geochemical markers such as pH, alkalinity, and major cation concentrations. Results from this study indicate that modeling of the near-surface reactive transport of CO2 can be used to guide sampling efforts, thereby improving the efficiency of monitoring programs aimed at detecting CO2 leakage from geosequestration.