MODELING OF CO2-BRINE-ROCK REACTIONS IN A MIXED CARBONATE-SILICICLASTIC AQUIFER

Equilibrium geochemical simulations were conducted using Geochemist's Workbench to evaluate the impact of fluid-rock reactions on the CO₂ sequestration-potential of the Rose Run sandstone of eastern Ohio. Representative carbonate and siliciclastic mineral assemblages were assessed to investigate reactions with the Rose Run's mixed lithology. A wide range of CO₂ -brine-rock proportions was considered in order to simulate reaction experiments and the displacement of water in the aquifer by injected CO₂.

For the carbonate case, equilibrium mineral assemblages remained the same for the range of added supercritical CO2, but pH decreased with increasing CO₂ to brine ratios. Some of the CO₂ was trapped as bicarbonate and as dissolved CO₂ (aq). The proportion that remained free increased as the CO₂ to brine ratio increased. For the siliciclastic case, geochemical reactions converted some of the CO₂ to carbonate minerals such as siderite, calcite, and dawsonite, absorbing more of the CO₂ than for the comparable carbonate case. For higher CO₂ to brine ratios, all the reactive minerals were consumed by reactions with the CO₂ such that free CO₂ remained in the system at equilibrium. Generalizing these results to a front of injected CO₂ displacing water, at the nose of the displacement profile where the saturation of CO₂ is small, all the CO₂ can be consumed by dissolving into water or reacting with the rock. Close to the injection site, there will be free CO₂.