Effect of salinity and alcohol partitioning on phase behavior and oil displacement efficiency in surfactant‐polymer flooding

Abstract

AbstractThe equivalent alkane carbon number (EACN) of a crude oil, namely Ankleshwar crude, is successfully modeled by a mixture of pure alkanes. The EACN of the crude oil is found to be 9.3, and an appropriate mixture of nonane and decane exhibited phase behavior similar to that of the crude oil. A surfactant system for a water flooded reservoir at 80 C and having a salinity in the range of 2% to 3% NaCl is formulated by blending a phosphated ester with a petroleum sulfonate in the weight ratio of 2/5. The addition of phosphate ester not only increases the salt tolerance of the petroleum sulfonate system, it also broadens the IFT minimum. The oil displacement tests at 80 C in sandpacks and Berea cores showed that the surfactant formulation containing tertiary amyl alcohol (TAA) displaced 92% oil in sandpacks and 79% crude oil in Berea cores. The oil recovery efficiency was poor when formulations contained other alcohols. From the effluent surfactant concentration, it is shown that there is a correlation between the tertiary oil recovery, surfactant breakthrough and surfactant retention in porous media. It is proposed that, because alcohols such as isopropyl alcohol (IPA), isobutyl alcohol (IBA) and secondary butyl alcohol (SBA) partition significantly in the equilibrated excess brine phase, the alcohol‐depleted surfactant slug forms stable emulsions resulting in faster breakthrough of surfactant in the effluent and lower oil displacement efficiency. In the case of TAA‐containing formulation, there is a partitioning of TAA in the oil phase. Therefore, there is a mass transfer of alcohol from surfactant slug to the oil ganglia in porous media. This produces a transient ultralow IFT between residual oil and the surfactant solution which mobilizes oil, resulting in higher oil displacement efficiency.