Tangent Plane Criteria for Phase Stability Computation for System with Hydrocarbon and Aqueous Phase Components

CO2 injection in depleting oil reservoirs provides the dual benefit of increasing oil recovery as well as sequestration. Phase changes occur during the injection and the injected CO2 can react with aqueous phase components, especially high ionic concentration brines typically found in the Middle East hydrocarbon reservoirs. Compositional simulations using equation of state (EOS) models are used to represent changes in phase behavior of hydrocarbon components accompanying CO2 injection. However, most of these models account for CO2 dissolution in the aqueous phase using a Henry’s law approximation valid only at low pressures. These models do provide the ability to track the changes in CO2 as well as other ions in the aqueous phase following the injection of CO2. A new tangent plane criterion to evaluate stability of each phase at equilibrium conditions has been developed using the unified Gibbs free energy function. This criteria accounts for both hydrocarbon and aqueous components described using EOS and activity coefficient models, respectively. A Gibbs free energy module for compositional simulations has been developed that includes this stability criterion along with a free energy minimization algorithm. This can be integrated in any reservoir simulator for varying reservoir fluid compositions.