Study on Dynamic Phase Behavior of Oil and Gas System during CO2 Flooding

CO₂ flooding is one of the most promising EOR methods. Due to the complicated compositional change during the displacement process, the effect of CO₂ injection on phase behavior of oil and gas system is still not clear so far. It can great limits the forecast of development effect and composition optimization of injection gas. In this paper, taken an offshore oilfield as an example, based on compositional grouping, PVT test and slim tube test, phase behavior of oil and gas system during impure CO₂ flooding was studied by PVT Sim 20. Results show that under reservoir conditions, the thickness of oil layer can greatly influence the composition distribution of the reservoir fluids. Thus, the phase behavior can be significantly different. For the initial stage of CO₂ flooding, two phase region of the phase diagram continuously extend. The cricondenbar increases obviously and the cricondentherm decreases. Critical point moves towards top left. With the CO₂ flooding proceeds, the two phase region begins to shrink when the gas oil ratio achieves 2:1. The cricondenbar increases slightly and the cricondentherm decreases significantly. Moreover, with the mixing of contaminated gases, both the two phase region enlarges and the cricondenbar increases greatly. However, the cricondentherm changes slightly.