An internally consistent procedure to characterize single carbon number fractions for phase equilibrium of petroleum mixtures: Application to Brazilian pre-salt reservoir fluids

Abstract

Each natural accumulation of hydrocarbons has unique chemical and molecular characteristics. A complete experimental assessment of every substance involved is impossible and, in most cases, gaps left by laboratory tests must be filled by empirical correlations to provide the minimum set of parameters necessary for the proper functioning of a thermodynamic model. In the present work, a new procedure is developed for calculating the critical properties and acentric factor of single carbon number (SCN) fractions, valid for the C7-C22 interval (Mw≤300g/mol) and successfully tested for mixtures containing heavy fractions with average molecular weights up to 520g/mol. The proposed method is based on a refit of existing correlations that fully respects the internal consistency of the equations. These, in turn, were selected by observing the trends described by the critical compressibility factor in the Soave–Redlich–Kwong (SRK) and Peng–Robinson (PR) cubic equations of state, resulting from the introduction of a volume translation parameter. After applying the new procedure to characterize ten oil samples collected from High-Pressure High-Temperature (HPHT) reservoirs in the Brazilian pre-salt cluster, thermodynamic parameters derived from flash and differential liberation tests were successfully predicted. With average absolute deviations of 8.38% and 8.32% for the SRK and PR equations of state, respectively, the proposed method is superior (or at least equivalent) to other characterization schemes considered in the study, with the advantage of being thermodynamically consistent.