Selection Criteria and Formation of a Surfactant Slug for High-Temperature, Moderate Salinity Reservoir Conditions

Abstract

ABSTRACT The main objective of this investigation was to formulate a surfactant system for a water-flooded reservoir at 80°C and having a salinity in the range of 20-30 gms/liter. The equivalent alkane carbon number (EACN) of the crude oil at 80°C was found to be 9.3 as determined by a comparison of its optimal salinity with those of pure hydrocarbon oils. Isopropyl alcohol, isobutyl alcohol, secondary butyl alcohol and tertiary amyl alcohol (TAA) were employed as cosolvents in this study. The optimal salinity at 80°C of a petroleum sulfonate (TRS10-80), crude oil and any of the above alcohol containing system was found to be less than 1.3%. A phosphated ester (KF AA-270) was used as a cosurfactant with TRS10-80 in the weight ratio of 2/5 which increased the optimal salinity in the range of 25-35 gms/liter. However, the addition of this cosurfactant decreased the solubilization parameters drastically. In order to improve the solubilization of brine and oil in the middle phase microemulsion, inorganic salts such as Na2CO3 and sodium tripolyphosphate were added to the formulation. The partitioning of alcohol in various phases was determined by gas chromatography. It was found that only TAA partitioned in the excess oil phase and middle phase whereas other alcohols partitioned in the middle and excess brine phase. Displacement tests were carried out at 80°C in sandpacks and Berea cores. The tertiary oil recovery efficiency was 92% when 0.2 PV of TAA containing surfactant slug was injected while the tertiary oil recovery efficiency was poor in systems containing other alcohols. It is concluded that the choice of alcohol and its partitioning behavior are important criteria for designing formulations for high temperature reservoir conditions. In the present study, the partitioning of alcohol in the oil phase seems to correlate with greater oil displacement efficiency for the surfactant system studied. It is proposed that the mass transfer of alcohol (TAA) from the aqueous surfactant slug to the oil phase in porous media promotes the oil displacement presumably due to the ultra-low IFT associated with this mass transfer process. The tertiary oil recovery for surfactant slugs having different alcohols correlated with surfactant breakthrough in the produced fluids. It was observed that the higher surfactant recovery in the produced fluids and a delayed surfactant breakthrough corresponded to a greater oil recovery.