Microemulsion phase behavior of active live oil

Abstract

Microemulsion phase behavior of live crude oil has been investigated at 1700 psi and 55 °C using an active crude oil with a total acid number of 1.18 mg-KOH/g-oil. The live crude oil was reconstituted by adding 36.0 mol% methane to dead crude oil. Sodium carbonate was used for a salinity scan, and also as an alkali to convert the acid components in crude oil to natural soap. The equivalent alkane carbon number (EACN) of dead crude oil was determined by measuring optimum salinity of dead crude oil and several surrogate crude oils diluted with pure n-alkanes. The optimum salinity of live crude oil was compared to those of dead and surrogate crude oils to determine the EACN of the live crude oil. To keep the surfactant pseudo-phase composition constant in the ternary phase diagram, the molar ratio of crude oil to synthetic surfactant was controlled. The natural soap and synthetic surfactant were kept the same for each volume ratio of crude oil to surfactant solution. The EACN of the crude oil was found to be independent of the soap-to-surfactant molar ratio, which means EACN is an intrinsic property of oil under the experimental conditions investigated. Based on the optimum salinity shift from dead crude oil to live crude oil, the compositional effect of methane with active crude oil was measured to be about the same as for inactive crude oil. The apparent EACN of methane was found to be equal to about 10.2. The solubilization ratio of the live oil at optimum salinity was greater than the value for the dead crude oil. The results indicate that the microemulsion phase behavior of live crude oil with acidic components can be interpreted using classical EACN concepts by the same criteria as that of inactive crude oil. Thus, the effect of the soap generation reaction can be decoupled from the physical interactions.