Investigating the influence of CO2 injection and reservoir cores on the phase behavior of two low-permeability crude oils: Experimental verification and thermodynamic model development

CO2 flooding can be used to increase oil recovery and also achieve CO2 sequestration. Knowledge of the phase behavior of CO2-crude oil mixtures in reservoir cores during CO2 sequestration is important for predicting reservoir performance and future processing needs. In this work, a new experimental system for measuring the bubble point pressures of oils in low permeability reservoir cores was built. The saturation pressures and gas-oil volume ratios of two crude oil samples with different amounts of CO2 injection (13 mol%, 18 mol%, 26 mol%, 34 mol%, 41 mol%, 49 mol%) in both a PVT cell and three low permeability reservoir cores with different permeabilities (3.4 mD, 1.0 mD, 0.5 mD) were measured. The experimental results show that the dissolution of CO2 substantially increased both the saturation pressure and gas-oil volume ratio of the oils. There was only a slight decrease (Delta P similar to 0.87 bar) of saturation pressure for the oil 1 + 13 mol% CO2 sample in the 3.4 mD core than in the PVT cell. Increasing the amount of CO2 dissolution in the oil resulted in Delta P quickly increasing. Meanwhile, Delta P increased quickly with the decrease of the core permeability. For the oil 1 + 34 mol% CO2 sample in the 0.5 mD core, Delta P reached similar to 22.85 bar. For the first time, tests showed the dissolution of CO2 substantially increased the influence of reservoir cores on the phase equilibrium of oils. A new thermodynamic model based on the PT equation of state (EOS) was further developed to describe the phase behavior of oils in both the PVT cell and reservoir cores. To model the saturation pressures of oils in reservoir cores, considering the influence of core pores on the phase behavior of oils, a new idea of adding the ratio between the molecular diameter of oil components with the mean pore diameter of the core to the alpha(T) function of the PT EOS was proposed. The calculated results are consistent with the experimental data.