Electrokinetics of Limestone Particles and Crude-Oil Droplets in Saline Solutions

Abstract

Summary Salinity adjustment of waterflooding has been applied recently as an enhanced-oil-recovery (EOR) technique in sandstone and carbonate reservoirs. Reaction mechanisms were different because of the variation in rock mineralogy and reservoir characteristics. Interactions between injection water, crude oil, and limestone particles are still ambiguous. Anions in seawater are believed to have altered carbonate surface potential to negative and, thus, created repulsion forces between crude-oil droplets (negatively charged) and a connate-water layer. As a result, rock wettability was altered toward water-wet. In this paper, the surface potential of crude oil and limestone particles was studied at 50°C. Ionic strength was varied using formation brine (230K ppm), seawater (54K ppm), shallow aquifer water (5K ppm), and fresh water. Cation (Na+, Ca2+, Mg2+) and anion (SO42−) concentrations were tuned individually in seawater also. The influence of H+ and OH− ions on the suspensions’ surface potential was investigated by diluting seawater and aquifer water with deionized water at different volume ratios. Two-phase (crude oil in water, limestone particles in water) and three-phase (crude oil, and limestone particles in water) tests were performed at pH 8. The surface potential of oil droplets was strongly affected by 10 vol% diluted seawater, seawater without divalent ions (Ca2+ and Mg2+), and deionized water because of the adsorption of OH− ions at the oil/water (O/W) interface. Sodium sulfate solutions (7,120 ppm) also increased the zeta potential absolute value of oil droplets. The effect of ionic strength on zeta potential was more pronounced in the oil-wet limestone particles than in the intermediate-wet samples. An aqueous layer around crude-oil droplets played a key role in determining droplet charges. Results from this study provide some insights on electrokinetics of limestone particles and oil droplets in different saline solutions. Wettability of the rock and oil recovery are affected directly by the zeta potential of oil droplets and suspensions.