CO2 FATE IN GEOLOGICAL RESERVOIRS WITH THEORETICAL CONSTRAINTS ON SYSTEM d13C(CO2) AND CO2/3HE

CO₂ is an important greenhouse gas in the Earth system. The fate of CO₂ gas in geological reservoirs is a major concern in many environmental and geochemical issues such as global warming, CO₂ sequestration, and ocean acidification. Using the ratio of CO₂/³He combined with δ¹³C of CO₂ gas to trace CO₂ fate has been proposed to be an applicable geochemical tool. However, it remains vague under what conditions this geochemical tracer system is effective; it is still uncertain how to determine the initial ratio of CO₂/³He which is needed to calculate the proportion of CO₂ loss relative to the total system. In this presentation we develop a new method based on simple mass balance and Rayleigh fractionation, derive analytical equations to simulate differentiation of CO₂ and ³He, and compute CO₂ loss due to dissolution of CO₂ into groundwater and precipitation of calcite. The uncertain initial ratio of CO₂/³He can introduce large errors into calculation of CO₂ loss. Spatial heterogeneous distributions of CO₂ and 3He and mixing of Helium from different sources have significant influences on the effectiveness of the tracer system. Finally, an example is given to apply our methodology and the limits of applicability are discussed for the system of CO₂/³He combined with d¹³C of CO₂ gas.