Selecting an optimal surfactant for use in EOR: an experimental study

Abstract

The objective of this thesis is to choose an optimum surfactant for enhanced oil recovery (EOR) in carbonate formations by studying the phase behavior, interfacial tension (IFT) and wettability alteration. The cost of surfactant is critical; thus, I have focused on low-cost commercial surfactants. An estimated cost of a typical one percent surfactant solution in one barrel of injected water is $8.75. Thus, when a project involves injecting 100,000 barrels per day of one percent surfactant solution, the daily cost of surfactant is $875,000. Thus, in one year, the surfactant cost is about $320,000,000. I chose three types of surfactants: two anionic surfactants, Enordet 0332 and 0342 olefin sulfonates, from Shell; one non-ionic surfactant, Neodol 91-8 ethoxylated alcohol, also from Shell and a bio-surfactant, Rhamnolipid, from AGAE technologies. In the first part of my study, I studied the phase behavior of the above-mentioned surfactants to determine the optimum salinity for each surfactant. Specifically, I prepared ten one-percent surfactant solutions with salinities ranging from zero to 90,000 NaCl. I mixed each surfactant solution with oil in 1:1 ratio and allowed the mixture to equilibrate for three weeks. The majority of the surfactant-oil systems exhibited Winsor type III microemulsion at around 80,000-ppm salinity. Therefore, I carried out all tests at this salinity. In the second part of my study, I evaluated the effect of each surfactant on IFT reduction and wettability alteration. I mixed four one-percent surfactant in 80,000-ppm NaCl brine with the same volume of oil, and allowed three weeks for the mixture to reach equilibrium. IFTs were measured for the equilibrated mixtures at 25°C, 50°C, and 75°C using a spinning drop tensiometer. The anionic surfactant Enordet 0342 reached the lowest IFT compared with the other surfactants studied. In the third part of my study, I conducted four wettability studies on several carbonate core discs. I aged the cores in oil for five weeks at 2000 psi and 195°F. Then, I used the equilibrated surfactant-brine solution and oil to measure contact angle using the drop shape analyzer (DSA) at the same temperatures used for the IFT measurement. Three of the four surfactants caused wettability alterations, except the bio-surfactant that demonstrated no wettability alterations. I have concluded the following results from this study: 1. The Winsor type III microemulsion (that is, existence of a three-phase microemulsion) formed in all of the four mixtures; however, the non-ionic surfactant yielded the largest Winsor type III microemulsion volume. 2. The anionic surfactant, Enordet 0342, exhibited the lowest IFT of 0.000048 dyne/cm while the non-ionic surfactant exhibited an IFT of 0.0014 dyne/cm. The anionic surfactant Enordet 0332 and the biosurfactant exhibited IFT about 0.05 dyne/cm. 3. Contact angle measurements indicated that both anionic surfactants altered the rock wettability to water-wet conditions the most, while the non-ionic surfactant altered wettability to the water-wet condition to a lesser level. 4. The most optimal surfactant for the tested carbonate samples, at 80,000-ppm NaCl and 75°C, was the non-ionic surfactant.