CO2 injection has proven to be a promising technology in enhancing oil recovery in low-permeablility carbonate reservoirs, especially in miscible flooding. How to demonstrate near-miscible/miscible mechanisms and their influence on production performance is a difficult issue to deal with in a compositional simulation. In this paper, considerable research has been conducted on compositional models, especially miscible gas flooding models, by identifying the near-miscible/miscible state and its influence on relative permeability.

This paper describes a multi-component, three-phase, compositional model for simulating miscible CO2 flooding problems. An EOS equation is constructed for phase equilibrium and property calculations. Gibbs stability model is developed to determine whether a hydrocarbon mixture at a particular temperature and pressure is more stable in a single-phase state or in a two-phase state. An interfacial tension (IFT) weighted relatively permeability model is developed to calculate the relative permeability in a near-miscible state. With the help of the Gibbs and Kr models, miscible CO2 flooding in a carbonate reservoir is simulated to evaluate the miscible and immiscible state and demonstrate the production performance.

The Gibbs stability model determines the tangent plane of the Gibbs energy surface at the mixture composition and parallel tangent planes at possible incipient phase compositions. If any of the parallel tangent planes lie below the tangent plane of the mixture composition, a two-phase state will exist. The simulation shows that a Gibbs stability model in CO2 injection can identify whether the hydrocarbon mixture (oil) and CO2 at reservoir pressure and temperature is in one phase state (miscible) or in a two-phase state (immiscible). The IFT weighted Kr model determines an interpolation between immiscible and miscible states using a weighting function, which is a function of IFT with a specified critical gas/oil IFT to control the contribution of the near-miscible effect on kr. The compositional simulation shows that CO2 becomes miscible with a hydrocarbon mixture under the condition of CO2 multi-contact with the hydrocarbon mixture in the reservoir pressure and temperature. Three areas exist between the injector and the producer which are the immiscible area (IFT: about initial value), near-miscible area (IFT: 0.5~2) and miscible area (IFT: lower than 0.5). The residual oil saturation of the miscible CO2 flooding area is around 5%. Oil recovery is enhanced compared with the previous hydrocarbon gas injection.

CO2 injection represents a promising technology to improve production performance and to enhance oil recovery for the carbonate reservoir mentioned in the paper.

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