Petroleum Science and Technology, 26:1522–1544, 2008 Copyright © Taylor & Francis Group, LLC ISSN: 1091-6466 print/1532-2459 online DOI: 10.1080/10916460701776823 Deterioration of Performance of Mixing Rules in Phase Behavior Modeling of High-Density Reservoir Fluids F. U. Babalola 1 and A. A. Susu 1 1 Department of Chemical Engineering, University of Lagos, Lagos, Nigeria Abstract: The mixing rules are used in the cubic equations of state to determine the values of the attractive force parameter, a, and the repulsive force parameter, b , mixtures. The mixing rules are applied here to reservoir fluids. It was discovered that parameter a should not be treated as a constant since it varied significantly with pressure. It was therefore regressed by two straight lines, and the resulting equation of state gave a very good fit to PVT data of reservoir fluids. Keywords: mixing rules, mixtures, parameter 1. INTRODUCTION The behavior of hydrocarbon mixtures at reservoir and surface conditions is determined by its chemical composition and the prevailing temperature and pressure. This knowledge of phase behavior is vital to the development and management of reservoirs and affects all aspects of petroleum exploitation and production. Although a reservoir fluid may be composed of many thousands of compounds, the phase behavior fundamentals can be explained with some degree of accuracy by examining the behavior of pure species. The complexity of the defining model, however, increases as the number of species increase. Since reservoir fluids are basically made up of hydrocarbons, they follow similar patterns of phase behavior. Studies of various hypotheses have led to recent deductions (Danesh, 1998) that the composition of a reservoir fluid depends on the depositional environment of the formation, its geological maturity, and the migration path from the source to the trap rocks. Fluids advancing into a trapping reservoir are usually of different compositions. Reservoir fluids are generally Address correspondence to Alfred Susu, Department of Chemical Engineering, University of Lagos, Lagos, Nigeria. E-mail: alfredasusu22@hotmail.com 1522