A New Equilibrium Coefficient Correlation Method for Compositional Simulators

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 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 pseudobinary mixture, and the functional form of this pseudobinary 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 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 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 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. Introduction Reservoir simulation is a versatile tool in analyzing the performance of reservoir oil recovery processes. In the case where significant interphase mass transfer occurs during a recovery process, it has become a general practice to use a process, it has become a general practice to use a compositional simulator to depict the variations of the equilibrium phase compositions and properties with spatial position and time in the properties with spatial position and time in the reservoir. The reliability of a compositional simulation depends strongly on the phase equilibrium calculation tool used. Cubic equations of state, though convenient and reliable in the calculation of phase equilibrium, suffer slow numerical convergence in performing flash calculations near the critical region and are generally not suitable for multi-dimensional field-scale recovery simulations. The K-table look up, though computationally more efficient, cannot cover all the composition ranges encountered in a typical simulation study and can result in unreliable predictions of the compositional effect for a even process. The purpose of this paper is to develop an efficient purpose of this paper is to develop an efficient and fully compositionally dependent K-value correlation which can be used to calculate phase equilibrium compositions without invoking the phase equilibrium criteria and the equation of phase equilibrium criteria and the equation of state. The K-value correlation developed is based on the convergence pressure concept. The correlation of K-value via convergence pressure is based on the premise that "the K-value for one component in a complex system is the same as the K-value of the same component in other systems at the same temperature and pressure, providing that the convergence pressures of the two systems are the same."