A novel approach to surfactant flooding under mixed-wet conditions

PhD thesis in Petroleum engineeringIn early days, a large number of research studies have been done based on the assumption that most of the sandstone reservoirs are strongly water-wet. Now it is widely accepted that most of sandstone reservoirs are at wettability conditions other than strongly water-wet. Surfactant flooding is one of the promising enhanced oil recovery methods that has been studied for many years. Traditional surfactant flooding studies have been reported utilizing mainly the mobilization of residual oil by increasing the capillary number and also assuming the water-wet formation. Capillary Desaturation Curve (CDC) represents the oil recovery potential by surfactants at water-wet conditions. Many investigations reported and assumed that this CDC concept is valid also for other wettability conditions. This thesis represents the results from several core flooding experiments carried out in sandstone rock at different wettability conditions. The research study is focused on analyzing and understanding the oil recovery mechanisms by surfactants at mixed-wet conditions. At mixed-wet I non water-wet conditions, it is found that the measured remaining oil and water saturation can be a function of the number of pore volume injected and can be also largely affected by capillary end effects. The residual oil saturation is difficult to obtain in core floods at mixed-wet I oil-wet conditions. Therefore the measured remaining saturation Vs capillary number in laboratory experiments does not represent the true CDC. From the results of unsteady core floods, no conclusion can be drawn about the effect of surfactant on residual oil saturation at mixed wet conditions. Interpretation of experimental results show that the main oil recovery mechanism by surfactant at mixed-wet condition is accelerated oil production due to increased oil relative permeability at high Nc; not necessarily the reduction of residual oil. When evaluating tertiary oil recovery at mixed-wet conditions, the focus should be directed towards relative permeability curves rather than residual oil saturation