ABSTRACT
A new equilibrium coefficient or K-value correlation useful in compositional simulation studies was developed to reduce computing times in phase equilibrium calculations while retaining good accuracy in the calculated results. This correlation is based on the convergence pressure concept. Unlike previous works, the mixture critical pressures are correlated from binary critical pressures, the convergence pressures are correlated by treating the mixture as a pseudo-binary mixture, and the functional form of this correlation was derived by considering the asymptotic behavior of K-values. In the low pressure region, it reduces to the Chao-Seader correlation. In the near critical region, it conforms to the critical exponent theory. The correlation is related to the overall fluid composition explicitly, and requires no iteration.
This correlation has been tested for three different reservoir fluid systems. The K-values calculated using the new correlation are in good agreement with those using the calibrated Peng-Robinson Equation of State (P-R EOS). The average deviation is less than 3% for the reservoir fluid systems studied. Slim tube displacement simulations, using CO2 and nitrogen as the displacing fluid, were also performed for the three reservoir fluid systems to validate the correlation developed. The predicted oil production rates using the new correlation to calculate phase equilibria are in good agreement with those using P-R EOS. The average deviation is less than 3%; at the same time, the computational cost is reduced by 4 – 14 times.
