Microemulsion phase behavior of live crude oil and revisiting the EACN framework for crude oils

Determination of the EACN of dead, surrogate, and live crude oils are revisited. The main objective of the study was to better understand the EACN of crude oil systems and to find more desirable practices in terms of approximation of crude oils. Microemulsion phase behaviors were measured in a wide range of oil compositions while keeping all formulation variables fixed except the oil. Pure hydrocarbons were used as oil phases or diluents to investigate the effect of nonideal mixing between crude oil and diluents. A synthetic live crude oil was constituted by adding 39.5 mol% of methane to a dead crude oil and then microemulsion phase behavior was investigated to determine the optimum salinity and solubilization ratios at 172 bar (2500 psi) and 90 °C. The results were analyzed based on two existing approaches in EACN framework, i.e., pure n-alkane trends against EACN versus dilution curves against EACN. The method using crude oil dilution curve results in a greater EACN of the crude oil by 2 units than by the method utilizing the pure n-alkane trend as a basis. The apparent EACN of methane was found to be much greater than its alkane carbon number of 9.3 based on the pure n-alkane trend and 11.6 based on the crude oil dilution method, respectively. Through systematic construction and investigation of optimum salinity vs. EACN curves, desirable practices to determine EACN of crude oils are presented. The results could be useful to approximate live crude oil EACN by designing more representative surrogate crude oils. [Display omitted] •The two most commonly used EACN determination methods are critically evaluated.•Apparent EACN of methane is 9.3 by Approach I (by pure n-alkanes) and 11.6 by Approach II (by dilution curve).•Due to nonideal mixing, ideal mixing rules may not work well for crude oil systems.•In binary mixtures, ‘apparent’ EACN of diluents remains constant only below ~50 mol%.•Desirable practices are discussed for EACN determination and representative surrogate oil preparation.