Analytical Calculation of Minimum Miscibility Pressure


Yun Wang







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The analytical theory of one-dimensional, dispersion-free displacement of oil by injection gas with nc components has shown that the behavior of the displacement is completely controlled by a sequence of key tie lines: those that extend through the initial oil and injection gas compositions and nc - 3 tie lines known as crossover tie lines. The theory also demonstrates that if any one of the key tie lines becomes a critical tie line, the dispersion-free displacement becomes piston-like, and multicontact miscibility develops. Therefore, the minimum miscibility pressure (MMP) is calculated as the lowest pressure at which any one of the key tie lines becomes a critical tie line.

A comprehensive study of the geometry of the key tie lines is presented in this dissertation. It is shown that key tie lines can be geometrically represented by a series of intersecting tie lines. This simple geometric property makes possible the systematic determination of all key tie lines for a multicomponent system. The justification for the geometric property is examined in detail.

Simple systems with constant K-values are used to understand the geometry of the key tie lines and develop algorithms for finding crossover tie lines. Closed-form solutions for nontie-line paths for three- and four-component systems with constant K-values are obtained and entropy conditions for nontie-line shocks are derived to explore the behavior of the solutions further. Algorithms for finding crossover tie lines in single-component gas injection are then developed, followed by the extension to systems with an arbitrary number of components present in both oil and injection gas, where a set of general tie line intersection equations is solved to find all crossover tie lines simultaneously.

The analytical algorithms for calculating MMPs for general multicomponent 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 tie line intersection approach can be used to calculate MMPs accurately for systems with an arbitrary number of components, provided that an appropriate equation-of-state based phase behavior characterization is available. The tie line intersection 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 MMPs.

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Copyright 1998, Yun Wang: Please note that the reports and theses are copyright to their original authors. Authors have given written permission for their work to be made available here. Readers who download reports from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the author, Yun Wang.

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