Effect of Reservoir Anisotropy on Carbon Dioxide Dissolution-Induced Density-Driven Convection

Sequestering carbon dioxide by injecting it into non-potable aquifers overlain by suitable cap rock is being widely considered and field tested to reduce greenhouse gas emissions to the atmosphere. Being less dense than local brine, supercritical carbon dioxide introduced into the aquifer will tend to spread beneath the cap rock. Numerical and experimental investigations have shown that beneath the buoyant carbon dioxide layer, carbon dioxide dissolution into the brine will increase the density of the brine, resulting in an unstable condition in which denser brine overlies less dense brine. This instability can lead to dissolution-induced density-driven convection of the carbon dioxide laden brine, enhancing dissolution and the permanence of the sequestration. We have performed a series of visualization and quantitative laboratory experiments with homogeneous, heterogeneous, and anisotropic media to examine this process. Our visualization experiments have shown dense finger formation and flow into the underlying brine. These experiments have led us to quantitative measurements of density-driven convection from carbon dioxide dissolution under a variety of conditions. These conditions include high and low permeability porous media, layered media, and fluids including pure water and various brines. Density-driven convection was quantified by measuring the carbon dioxide uptake rate for the systems, and comparing it to the predicted diffusive uptake rate. Uptake rates exceeding the diffusive uptake rate are indicative of convection. Our results show the effects of density-driven convection were smaller for lower permeability and higher salinity conditions, whereas strong convection occurred for pure water in a highly permeable medium. Quantitative tests investigating the effect of anisotropy were based on visualization test results and layered systems were used in the experiments. Detailed experimental results and interpretation of these tests will also be discussed.