Quantitative dependence of CH₄-CO₂ dispersion on immobile water fraction

Enhanced Gas Recovery (EGR) involves CO₂ injection into natural gas reservoirs to both increase gas recovery and trap CO₂. EGR viability can be determined by reservoir simulations; however, these require a description of fluid dispersion (mixing) between the supercritical CO₂ and natural gas. Here, this dispersivity (α) in sandstone rock plugs as a function of residual water fraction is quantified. To ensure the accuracy of such data, a novel core flooding experimental protocol that ensured an even spatial distribution of water was designed, minimized erroneous entry/exit contributions to mixing, and minimized dissolution of the CO₂ into the water phase. Dispersivity was found to increase significantly with water content, although the differences in α between sandstones were eliminated upon the inclusion of residual water. This enabled development of a correlation between α and water content and, hence, between the dispersion coefficient and Peclet number that is readily incorporable into reservoir simulations.