A derivative-less approach for generating phase envelope and applications to equations of state
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Petroleum engineers are interested in the phase envelope that describes the characteristics of the pore fluids. These phase envelopes are normally established by laboratory measurements involving expensive PVT equipment and highly trained personnel. By using an Equation of State (EOS) based computer model that requires some hydrocarbon composition data, these envelopes can be generated almost instantly. An algorithm for generating a complete phase envelope using computer modeling was suggested by Michelsen (1980). His procedure, based on EOS modeling, may sometimes suffer from convergence issues, especially near the critical region. In addition, his procedure requires the partial derivatives of fugacity. In this work a phase behavior model has been developed that incorporates some modifications to Michelsen’s algorithm avoiding the need for calculating these derivatives, which in turn put fewer requirements on the data and computation. These modifications allowed running the model successfully for different hydrocarbon systems without any convergence issues. By not having to deal with the partial derivatives of fugacity, the new model can work with any existing EOS based phase behavior model. After validating the phase behavior model against commercial software, the model was used for simulating a slim-tube experiment to calculate the Minimum Miscibility Pressure (MMP) using the Metcalfe et al. (1978) procedure. The results from computer modeling are then compared with the results from an experimental study performed by Amao (2011). The results match closely, demonstrating the power of EOS modeling.