Experimental Investigation of Amphoteric and Microbial Surfactants for Enhanced Oil Recovery in Shaly Sandstones

Abstract

To meet the increasing demand for oil and gas, surfactants have been used to increase hydrocarbon recovery. Use of surfactants reduces the Interfacial Tension (IFT) at fluid/fluid interface and wettability at rock/fluid interface and mobilizes trapped oil out of the pores. However, there are two main limitations of the surfactant flooding process—first, high reservoir temperature & salinity, and second, adsorption of surfactants on the rock surface. Surfactant adsorption alters wettability of reservoir rock from oil-wet to water-wet. However, it may not increase oil recovery, especially in conventional reservoirs with high Total Dissolved Solids (TDS) and temperature due to excess surfactant adsorption. This study tested two synthetic amphoteric surfactants, one nonionic biosurfactant, and a base case with produced brine to understand wettability, IFT, surfactant adsorption, and their effect on oil recovery in shaly sandstone formation. Produced brine has a TDS of 238,000 ppm. First, surfactant stability tests were performed on the three surfactants. Then, IFT measurements were performed between crude oil and surfactant solutions along with produced brine. Next, wettability alteration was studied by measuring contact angle on oil saturated rock samples before and after being exposed with surfactants and produced brine. Then, surfactant adsorption experiments were performed using UV-Vis spectrophotometer to calculate the amount of surfactant adsorbed on the rock sample. Next, surfactants and produced brine imbibition experiments were performed on plug samples at 145°F and 500 psi pressure, and oil recovery was quantified using 12MHz Nuclear Magnetic Resonance (NMR) spectrometer. Results showed that all three surfactants reduced IFT and altered wettability, but biosurfactant showed most reduction of IFT, much lower surfactant adsorption, and made the sample most water wet as compared to amphoteric surfactants. Imbibition experiments showed that biosurfactant have the highest oil recovery, while amphoteric surfactants have oil recovery even lower than produced brine. This study shows that surfactant adsorption effects oil recovery, which can lead to loss of surfactants from solution to the rock surface. This study suggests that biosurfactants with glycolipids can be effectively used in shaly sandstone at high TDS and temperature.