Analytical Calculation Of Minimum Miscibility Pressure By Using Stability Analysis In Reduced Variable Space

Abstract

Abstract Miscible gas injection is one of the most efficient methods in enhanced oil recovery and Minimum Miscible Pressure (MMP) is a vital parameter in miscible injection projects. In this paper a new method of MMP calculation is presented which is based on stability analysis in reduced space variables and consists of a fast and robust algorithm which guarantees that increasing the number of components of oil and injection gas has no effect on the volume of calculations; nevertheless the accuracy of calculation does not change. It is illustrated that using stability analysis in reduced space for miscibility development and consequently in MMP calculation causes that the number of equations for "C" components to reduced marvelously to "m" equations (m is less than or equal to 6). Thus in addition of accuracy and fastness this method offers that there is no need for splitting, lumping or pseudo component definition. Comparisons in examined cases demonstrate that this claim is correct. By this method of MMP calculation, the hardships in phase calculation due to existence of the ragged shape of Gibbs free energy function and consequently the existence of several minimum points and difficulty in finding a proper solution for nonlinear Ratchford-Rice equations especially near the critical point and also complication in analytical tie line intersection approach for MMP calculation are obviated. The analytical algorithms for calculating MMP for general multi component systems are tested against slim tube test data for real crude oil systems. It is shown that for all the crude oil systems considered, the displacement is a condensing/vaporizing gas drive. The MMP calculation results indicate that the analytical stability analysis approach can be used to calculate MMP accurately for systems with an arbitrary number of components, provided that an appropriate equation-of-state based on phase behavior characterization is available. This approach is significantly faster than the conventional compositional simulation approach. It is applicable to all systems with any type of displacement mechanism, and hence it can be used in areas that call for rapid calculation of MMP.