Modeling CO2 Storage in Aquifers with a Fully-Coupled Geochemical EOS Compositional Simulator

Abstract

Abstract This paper describes a fully coupled geochemical compositional Equation-of-State (EOS) compositional simulator for the simulation of CO2 storage in saline aquifers. The simulator (GEM-GHG) models the following phenomena:convective and dispersive flow in porous media;phase equilibrium between the oil, gas and aqueous phase;chemical equilibrium for reactions between the aqueous components andmineral dissolution and precipitation kinetics. For numerical robustness and stability, all equations are solved simultaneously. The simulator is applied to the simulation of typical field-scale CO2 sequestration processes, showing the migration of CO2(g) and CO2(aq), the dissociation of CO2(aq) into HCO3 and its subsequent conversion into carbonate minerals. Convection of high-density plumes of CO2-rich brine in conjunction with CO2 mineralization around the plumes is illustrated. Introduction Because of the climatic warming effect of CO2, CO2 storage is essential for reducing greenhouse effects. Gunter et al.1 provided a critical look at capacities, retention times, rates of uptake and costs for CO2 disposal in different classes of CO2 sinks in Canada. Sedimentary basins such as depleted oil and gas reservoirs and aquifers are potential sites for storage. Deep aquifers seem to be the most promising sites for CO2 storage2,3 as they are widely distributed, underlie most point sources of CO2 emission and are not limited by the reservoir size as in the case of depleted oil and gas reservoirs. Tanaka et al.4 discussed several structures for CO2 storage in Japan. These consist of (1) oil and gas reservoirs with neighboring aquifers, (2) aquifers in anticlinal structures, (3) aquifers in monoclinal structures on land and (4) aquifers in monoclinal structures offshore. Oil and gas reservoirs with neighboring aquifers in category 1 are still active and will be producing for some time in the future. When depleted, these reservoirs can be used for underground natural gas storage, instead of CO2 storage. Consequently, aquifers in categories (3) and (4) are the most attractive candidates for CO2 sequestration. Koide et al.5–6 provided additional discussions of the merit of storing CO2 in deep saline aquifers around the world in general and in Japan in particular. Baklid et al.7, Kongsjorden et al.8, and Chatwick et al.9 described the Sleipner Vest CO2 storage project in the North Sea. The rich gas of the Sleipner Vest Field contains sizable amounts of CO2 (9%). CO2 is removed using an activated amine and reinjected into an aquifer in the Utsira formation. Emberley et al.10 discussed the CO2 storage process in the CO2-EOR injection project in Weyburn, Saskatchewan, Canada. van der Meer11 reviewed significant milestones and successes achieved in underground CO2-storage technology over the past few years. All underground options including aquifer storage, EOR processes, CO2 storage in depleted gas and oil fields, and Enhanced Coalbed Methane are reviewed. He noted that Sleipner project has proven to be a successful storage project. CO2 has high density and high solubility in the aqueous phase at the high pressures that exist in deep aquifers. There are two ways in which CO2 can be trapped in aquifers:structural (or hydrodynamic) trapping andmineral trapping. The first process consists of trapping CO2 into a flow system with low flow velocity over geological periods of time. The second process converts CO2 to carbonate minerals and renders it immobile. The latter is very desirable as CO2 is sequestered in a form that is harmless to the environment. Wawersik et al.12 provide a comprehensive review of the physics and research needs related to the terrestrial sequestration of CO2 that highlight the importance of structural and mineral trapping. Geomechanics also plays an important role as the pressure increase due to the injection of CO2 may exceed the yield point of the cap rock or sealing faults. This will result in the undesirable leakage of CO2 into the environment.