Effects of alkaline/surfactant/polymer on stability of oil droplets in produced water from ASP flooding

Alkaline/surfactant/polymer (ASP) flooding technology has been used successfully in the Daqing oilfield, but the produced water is more difficult to treat than that from water flooding and polymer flooding. Simulation experiments were conducted to investigate the effects of ASP on the stability of oil droplets in the produced water. Settling experimental results showed that polymer favored oil–water separation when its concentration was less than 1000 mg l−1. Oil concentrations in the water after settling separation increased with the increase of surfactant concentration, and increased with the increase of NaOH concentration up to 200 mg l−1 and then decreased with the further increase of alkaline concentrations. The interfacial properties of oil–water were investigated to find the mechanisms for the stability of oil droplets in produced water from ASP flooding. The synergetic action of surfactant ORS-41 and NaOH was found to greatly influence the interfacial tension of oil and water. ORS-41 and polymer can increase the density of negative electrical charges on the surface of oil droplets and the interfacial elasticity of oil and water. Alkaline however can increase the density of negative electrical charges of oil droplets and the interfacial elasticity of oil and water in lower alkaline concentrations, but the effect is reversed in higher alkaline concentrations. On the other hand, the flocculating ability of the polymer also helps small oil droplets coalesce and form big ones. The flocs of oil droplets were found in produced water from ASP flooding with a microscope.