Closed-form van der waals critical point for petroleum reservoir fluids



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The prediction of critical points is of great practical importance because the classification of petroleum reservoir fluids as a dry gas, gas condensate, volatile oil, and crude oil depends largely on the knowledge of the critical properties of the reservoir fluid. Also, the critical pressure and critical temperature of reservoir fluids are important properties for describing the reservoir fluid phase behavior, predicting volumetric properties of reservoir fluids and designing supercritical fluid processes.

Previous work for determining critical pressure, and critical temperature for reservoir fluids include, empirical correlations, corresponding states method, and pseudo-critical property methods. The generality of these previous correlations is limited to the range of conditions and parameters used in the establishment of the correlations. Methods based on the Gibbs criteria have also been used with Redlich-Kwong and Peng-Robinson equations for prediction of critical properties. However, the Gibbs criteria have not been applied to predicting critical properties of reservoir fluids.

A closed-form equation is developed for predicting the critical properties (Tc, Pc) of complex reservoir fluids by using the Lawal-Lake-Silberberg (LLS) equation of state with the criticality criteria established by Nobel Laureates van der Waals (VDW) in 1873. By inverting the parameters of the LLS EOS in terms of the mixing parameters that are based on the constituent substances and composition of the reservoir fluids, experimental critical pressures and temperatures are predicted with interaction parameters expressed in terms of molecular weight ratios of the binary constituent of reservoir fluids.

The prediction results of critical pressures and temperatures based on the VDW criticality criteria show that experimental data consisting of 85 reservoir fluid mixtures are within average absolute percent deviation of 3% to 5% of the measured critical pressures and temperatures. In contrast to the previous work, this research project provides an accurate method for computing the critical properties of reservoir fluids and it is easy to use because the parameters of the criticality equation are readily available. This project is useful for unifying near-critical flash routine with phase equilibria of the compositional reservoir models. The project is also very attractive for establishing reservoir models that are based on the critical composition convergence pressure concept.