Experimental study on the mechanism of adsorption-improved imbibition in oil-wet tight sandstone by a nonionic surfactant for enhanced oil recovery

In recent years, production from tight oil reservoirs has increasingly supplemented production from conventional oil resources. Oil-wet formations account for a considerable proportion of tight oil reservoirs. Surfactant can change wettability and reduce interfacial tension, thus resulting in a better oil recovery. In this manuscript, a nonionic surfactant was introduced for tight oil-wet reservoirs. The oil recovery in the oil-wet sandstone due to spontaneous imbibition was 8.59% lower than that of the waterwet sandstone due to surfactant. The 0.1% surfactant solution corresponded to the highest imbibition recovery rate of 27.02% from the oil-wet sample. With the surfactant treatment, the treated core quickly changed from weakly oil-wet to weakly water-wet. The capillary force acted as the driving force and promoted imbibition. The optimal surfactant adsorption quantity in the oil-wet sandstone was observed in the sample at concentrations ranging from 0.1% to 0.3%, which also corresponded to the highest oil recovery. Analysis of the inverse Bond number N;suggested that the driving force was gravity for brine imbibition in the oil-wet cores and that it was capillary force for surfactant imbibition in the oil-wet cores. When the surfactant concentration was lower than the critical micelle concentration, the surfactant concentration was negatively correlated with the inverse Bond number and positively correlated with the oil recovery rate. When the surfactant concentration was higher than the critical micelle concentration, the oil recovery increased with a smaller interfacial tension. Nuclear magnetic resonance suggested that the movable pore and pore throat size in the oil-wet sample decreased from 0.363 mm in the untreated rock to 0.326 mm with the surfactant treatment, which indicated that the surfactant improved the flow capacity of the oil. The findings of this study can help to better understand the adsorption impact of surfactants on the characteristics of the oil/water and solid/liquid interfaces. The imbibition mechanism in oil-wet tight sandstone reservoirs was further revealed. These systematic approaches help to select appropriate surfactants for better recovery in oil-wet tight sandstone reservoirs through imbibition.