Four-Parameter Modification of the Lawal-Lake-Silberberg Equation of State for Calculating Gas-Condensate Phase Equilibria

The rapid development of digital computers during the past several years has made the calculation of hydrocarbon phase behavior feasible by methods based on rigorous thermodynamic principles. A number of these methods report comparisons of cubic equations of state (EOS) and laboratory PVT results for a wide variety of reservoir fluids and conditions. Most of them emphasize the C7+ fraction splitting as the key element in attaining an agreement between EOS and laboratory results. The work reported in this paper reflects our experience with predictive EOS without splitting C7+ fraction to match laboratory data. The use of the Four-Parameter modification of Lawal-Lake-Silberberg (LLS) EOS has made the splitting of C7+ fractions and the adjustment of EOS parameters unnecessary. A modification of the LLS EOS has been formulated specifically for application to gas condensate reservoir fluids. The modification consists of establishing the heptanes-plus coefficients, αc7+, βc7+ in the attractive pressure term and new parameters for Ac7+ and Bc7+ which are related linearly to the characteristic properties of heptanes-plus fractions. These characteristic properties, molecular weight, API gravity, and Watson characterization factor are obtained from 4460 experimentally determined data points of condensate reservoir fluids which cover a range of temperatures from 40°F to 322°F and pressures up to 15,500 psia. The usefulness of the modified LLS EOS in improving phase equilibria predictions of complex systems without splitting C7+ fraction is illustrated by applying the EOS to several crude-oil and gas-condensate systems. The LLS EOS was also applied in predicting laboratory PVT data of published multicomponent systems which contain significant amounts of aromatic hydrocarbons. The prediction results of applying other equations of state (PR, SRK) without altering their parameters are also presented for comparison.