Surfactants have been considered to mobilize the residual oil left after water flooding of oil reservoirs. The focus has been on surfactant flooding of water-wet reservoirs utilizing mainly their ability to reduce oil-water interfacial tension, while the potential of surfactant flooding for non water-wet reservoirs has not been investigated too much degree.

The purpose of this study was to investigate the effect of reducing the interfacial tension between oil and water on the relative permeability curves at different wettability conditions. Several core flooding experiments were carried out at different wettability conditions including multiple rate water flooding followed by multiple rate surfactant floods. The wettability conditions were characterized before and after the core floods. Relative permeability functions and capillary pressure curves were estimated by history matching the recovery and the pressure drop data obtained from the core floods.

For the non water-wet systems the measured remaining oil saturation was found to decrease with increasing capillary number at low capillary number. The measured capillary desaturation curve (CDC) for the mixed-wet and oil-wet conditions are not true CDC and must be corrected for capillary end effects. The relative permeability was found to vary with capillary number and also with wettability. At low capillary numbers typical for water flooding, the relative permeability curves were constant. By increasing the capillary number the relative permeability for oil was found to increase for mixed-wet and oil-wet conditions.

Since most of the sandstone reservoirs are characterised as mixed-wet, the potential for classical surfactant flooding can be low because the real residual oil saturation is low. The oil recovery from mixed-wet reservoirs can be accelerated by alteration of the relative permeability curves by surfactants.

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