Chemical flooding of oil reservoirs .7. Oil expulsion by spontaneous imbibition of brine with and without surfactant in mixed-wet, low permeability chalk material

Experiments are performed in four cores in order to study the effect of interfacial tension (IFT), wettability, and core length on the imbibition of water in an oil-saturated chalk material. Different wettability conditions are obtained by mixing different fractions of n-heptane with crude oil and by aging the chalk material in the oil mixture over different time intervals. In the imbibition experiments, the principal wetting phase is synthetic brine and 1.0 wt.% solution of an alkyl-propoxy-ethoxy-sulfate dissolved in the brine. The IFT between the surfactant solution and the oil phase is between 0.004 and 0.02 mN m(-1). The chalk material is of low permeability, approximate to 3 mo, with a rather high porosity, approximate to 40%. Imbibition studies were performed under water-wet conditions, using n-heptane as oil, and under mixed-wet conditions, using a mixture of n-heptane and crude oil, in small cores of 5.0 cm length and in long cores of 55 cm length. The imbibition mechanism is discussed in relation to the imbibition rate, amount of oil produced, and visual observation of the oil expulsion from the vertically mounted cores. Two important observations are made. For the long-core experiments at high IFT, the final oil production under mixed-wet conditions is significantly higher than the final oil production under water-wet conditions. The opposite outcome is observed for the small-core experiments. At low IFT, the surfactant appears to prevent the imbibition of water into the small core under mixed-wet conditions. Only approximate to 5% of the oil was produced. For the long-core, a break in the oil production was observed at approximate to 10% recovery, which indicates a change in the oil displacement mechanism.