Theoretical and experimental study of controls on CO2 dissolution and CH4 outgassing rates

Understanding CO2 dissolution is significant to the development of CO2 geological storage. It allows us to more accurately predict the extent of the CO2 plume. It also provides a better estimate of the trapping mechanisms to assist in interpreting the monitoring observations and to assess the risk of CO2 leakage. When CO2 is injected into methane-saturated brine aquifers, the dissolution process has been observed to be accompanied by CH4 outgassing from the pore water, which affects the properties of the gas phase and in turn CO2 migration. Currently available commercial and academic multiphase flow simulators (such as CMG-GEM and TOUGH2) assume instantaneous equilibrium of CO2 dissolution within a single grid cell, which introduces the maximum amount of dissolution and methane outgassing. The incomplete mixing due to the presence of a porous media and subsurface heterogeneity reduces CO2 dissolution rates by orders of magnitude compared to the well-mixed case. In this study, we investigate the effects of mixing-controlled CO2 dissolution and of methane outgassing on CO2 migration in the subsurface with the help of numerical simulations and laboratory experiments. We first developed numerical pore-scale models to demonstrate that the mass transfer rate of CO2 depends on the gas saturation and the aqueous CO2 concentration and that the mass transfer rate can be written as a power-law function of the CO2 saturation. Based on the mass transfer rate obtained from the pore-scale models, we then simulated the two-phase Darcy's flow in a column test considering the dynamic dissolution process. Results show that the CO2 migration is affected by the mass transfer rate into the aqueous phase. Even within a small scale of several meters, which is the typical grid size used in simulation of carbon storage in the field, the assumption of instantaneous equilibrium may not be valid. When the mass transfer time scale is much larger than the advection time scale, the CO2-water flow acts as an immiscible flow, and the amount of CO2 dissolved within the advection time scale is negligible. As the mass transfer rate increases, the flow approaches the scenario with instantaneous equilibrium. Modeling and experiment presented here confirm the well-known observations that CO2 dissolution into pore water introduces a retardation effect on CO2 breakthrough. When the mass transfer rate of CO2 into pore water increases, the CO2 breakthrough is delayed, decreasing the extent of the CO2 plume. (C) 2014 The Authors. Published by Elsevier Ltd.