Accurate Vaporizing Gas-Drive Minimum Miscibility Pressure Prediction

Abstract

Summary Prediction of the minimum miscibility pressure (MMP) of the vaporizing gas drive (VGD) process is modeled by use of an equation of state (EOS) with different mixing rules joined with a newly formulated expression for the unlike three-body interactions between the injection gas and the reservoir fluid. The comparison of the numerical results with the available experimental data indicates that an EOS alone overestimates the MMP. When the EOS is joined with the correct version of the van der Waals mixing rules and the unlike three-body interaction term, however, the MMP will be predicted accurately. Introduction The ternary or pseudoternary diagram is a useful way to visualize the development of miscible displacement in EOR. The phase behavior of a reservoir fluid for which the exact composition is never known can be represented approximately on a triangular diagram by grouping the components of the reservoir fluid into three pseudocomponents. Such a diagram is called a pseudoternary diagram. The scope of this paper involves the use of the Peng-Robinson (PR) EOS, along with coherent mixing and combining rules derived from statistical mechanical considerations. Also discussed is the implementation of the three-body effects in the evaluation of the phase behavior of ternary systems and in the prediction of the MMP of simulated reservoir fluids. To support the application of the model, it was preferable to obtain phase-behavior data for true ternary systems, such as CO2/n-butane/n-decane and methane/n-butane/n-decane, which are rigorously described by ternary diagrams. Moreover, experimental vapor/liquid data for the above systems are available at pressures and temperatures that fall within the range of the majority of oil reservoirs. The usefulness of the PR EOS has been tested with limited success in predicting the phase behavior and MMP's of simulated reservoir fluids. With the PR EOS, an overprediction of the MMP of the methane/n-butane/n-decane system was observed and it was believed to be the result of limitations of the PR equation, which does not accurately predict the phase behavior of the methane/n-butane/n-decane system in the critical region. In addition, the prediction of the vapor/liquid coexistence curves of the C02/n-butane/n-decane systems was not satisfactory in all ranges of pressures and compositions.