Chemical flooding of oil reservoirs .6. Evaluation of the mechanism for oil expulsion by spontaneous imbibition of brine with and without surfactant in water-wet, low-permeable, chalk material

Experiments are performed in order to study the effect of interfacial tension, IFT, and core length on the imbibition of a surfactant solution into an oil-saturated chalk material. The wetting phases are synthetic brine and a 1.0 wt.% solution of an alkyl propoxyethoxysulfate dissolved in the brine. The non-wetting phase is n-heptane. The IFT between the surfactant solution and the oil is 0.02 mN m(-1), The chalk material is low permeable, about 3 mo, and it has a high porosity, about 40%. Imbibition profiles, the amount of oil produced as a function of time, are determined at room temperature for small cores (5.0 cm) and long cores (55 cm) with and without surfactant present. Without surfactant present, the imbibition mechanism is a countercurrent flow governed by capillary forces. In the presence of surfactant, different mechanisms are observed at different time intervals. At the start, a countercurrent flow is observed, but later, the oil is produced mainly from the horizontal surface by a cocurrent flow based on gravity forces. The imbibition rate at low IFT is very slow, and within the time frame of the experiments, about 200 days, the oil recovery does not exceed the fast oil recovery without surfactant present.