Modeling Long-Term CO₂ Storage in Aquifer with a Black-Oil Reservoir Simulator

This paper presents results of modeling long-term CO₂ storage in a shallow saline aquifer with a commercial black-oil reservoir simulator. Realistic CO₂/water phase behavior (pVT properties) covering all pressure, temperature and compositional conditions accounted for during the simulations have been used. The pressure and temperature in the aquifer is above the CO₂ supercritical conditions giving rise to the existence of a two-phase fluid system of CO₂ as a supercritical fluid ("gas") and CO₂ dissolved in the aqueous phase. The objective was to model scenarios of CO₂ storage in aquifer with emphasis on the sensitivity of CO₂ distribution in the deposit with respect to critical CO₂ saturations during the injection period and to residual CO₂ saturation for water reentering CO₂ filled volumes (hysteresis in fluid saturations). The re-distribution of water occurs after stop of CO₂ injection due to gravity segregation of dense CO₂ saturated water and CO₂-free water. The impact of various reservoir parameters has been studied, including average permeability, vertical to horizontal permeability ratio (k_v/k_h), relative permeability, and capillary pressure. For the saturation functions the main focus has been on end points and hysteresis effects. It is observed that storage of CO₂ as residual gas is most important for low k_v/k_h ratios.