Numerical Simulation of CO2 Storage in Aquifers without Trapping Structures

Abstract

Abstract The numerical simulations for the CO2 storage using a conceptual, generic, simple 3D aquifer model without trapping structures were carried out in this study. They showed that the mudstone barriers, which have relatively poor sealing efficiency, prevent the upward migration of CO2 and help its lateral distribution. This suggests that the heterogeneities in the formation, particularly the distribution and properties of low permeability rocks, are very important to predict the movement of the injected CO2. Even the gently dipping structures lead to increased updip migration of injected CO2 due to buoyancy effect. When CO2 comes in contact with fresh unsaturated formation water, its dissolution into water phase occurs. If injected CO2 is not large in quantity the dissolution can still store the injected CO2. When a large amount of CO2 is injected into the aquifer the gas relative permeability hysteresis is a key for the long-term CO2 storage. The trapping of CO2 as a residual phase in the post-injection period strongly reduces the distance of updip migration, though it subsequently leads to decreased dissolution into the formation water. The simulation study suggests that heterogeneous formations that do not have structural closures and mudstones with relatively poor sealing efficiency can be a target for long-term CO2 geological storage, if these effects are understood precisely and utilized appropriately. Introduction Storage of CO2 in oil and gas reservoirs and aquifers with trapping structures and proven seal has been considered for reducing greenhouse effects. However, the potential capacity for CO2 storage in those structures is not large1, 2. Aquifers in the formations that are composed of weakly consolidated sandstone and mudstone are possible attractive candidates for CO2 storage because they are widely distributed in the vicinity of common industrial sources of CO2. These formations usually dip at low angles. The movement of the injected CO2 may be affected by the properties of mudstone3. The dissolution of CO2 into formation water is one of the important trapping mechanisms. The trapping mechanism of CO2 as residual phase due to gas relative permeability hysteresis may occur in those formations4, 5, 6, 7. We conduct 3D numerical simulations to know how they work in the aquifers without trapping structures.