A New Simplified Compositional Simulator

Abstract

Abstract A new simplified fully implicit compositional simulator has been developed. This simulator is different from existing models because it combines the conventional black oil equations with a compositional injection component equation. The simulator runs at speeds that far exceed the speeds in current compositional simulators, primarily because the speed is independent of the number of components. Data preparation is both simplified and improved because component pseudoization is not needed. The simulator has been tested successfully for gas cycling projects including the Third SPE Comparative Solution Project and CO2 flooding projects. Introduction Compositional simulation is one of the most important tools for hydrocarbon recovery predictions. The fully compositional simulator with molar flow equations coupled to a cubic equation of state (EOS) package is the only popular approach for simulating compositional behavior in oil reservoirs. In the past 20 years a simple accurate equivalent to the fully compositional simulator has been researched. Coats added a term for the dissolved oil in the gas phase (Rvo) to a black oil simulator, and successfully modeled primary depletion and waterflooding in gas condensate reservoirs. Cook modeled lean gas cycling projects by correlating black oil PVT properties to pressure and a second parameter, cumulative pore volumes of the injected component that have passed through a grid cell (CGI). Henry showed that the Cook correlation was not accurate for some mixtures. Mott improved the Cook approach by correlating the PVT properties to mole fraction of the injected component in the gas phase. He used empirical solubility parameters to partition the injection component between the gas and liquid phases, and assumed that the K value for the injection component was constant. For some lean gas cycling problems and CO2 cycling the solubility and K value of the injection component vary in a complex fashion, and the Mott approach is not accurate. In summary, a straightforward but accurate simplified compositional/pseudo- compositional formulation for the general problem has not been found to date. A further improvement is the focus of the work presented here. The parameters in the new model include the conventional black oil PVT parameters, and new compositional parameters - oil and gas molar densities, and injection component K value. The parameters are correlated as a function of pressure and injection component mole fraction in the gas phase using a generalized cubic equation of state package. The quality of the correlations far exceeds the quality of correlations used in earlier pseudo compositional models. DESCRIPTION OF PVT PACKAGE TO GENERATE THE COMPOSITIONAL DATA The first step in creating an accurate modified compositional formulation is to develop a simple and accurate correlation for the compositional dependency of black oil PVT properties. The procedure used here is basically that of Cook and Jacoby but with a different correlating parameter, as discussed below. A linear sequence of between one and thirty cells of equal volumes is chosen for the calculation. All are taken to be initially at specified values of temperature and pressure. P. 177^