The basic performance of a novel high-efficiency spontaneous imbibition agent (SXJ-2) was investigated in this study. The mechanisms of spontaneous imbibition and wettability reversal of an oil-wet sandstone surface (OSS) by SXJ-2 were studied by infrared (IR) analysis, scanning electron microscopy (SEM), zeta potential determination, wetting angle experiment, and spontaneous imbibition determination in this work. The interfacial tension between oil and water for the SXJ-2 system is relatively low. SXJ-2 has a better performance than other systems in altering the surface wettability and has a higher imbibition recovery under the same time interval than the other systems. Spontaneous imbibition recoveries of cores with high permeability are greater than that of cores with low permeability for the same surfactant solution. Spontaneous imbibition recoveries decrease as the surfactant concentrations decrease, and the spontaneous imbibition recovery is the highest when the concentration is fixed at 0.5 wt %. The higher the initial water saturation the lower the spontaneous imbibition recovery is. The ultimate imbibition recoveries of cores increase with the decrease of core length. The spontaneous imbibition recovery at room temperature is higher than that at 80 °C. At the initial and later stages of imbibition, the experimental process is controlled by capillary force and gravity, respectively. While at the intermediate stage, the process of imbibition is jointly controlled by capillary force and gravity. The absorption peak at 1170 cm–1 is caused by the sulfone −SO2– stretching vibration, indicating that the surfactant molecule SXJ-2 is adsorbed on the surface of sandstone. Due to the adsorption of the surfactant SXJ-2 on the rock surface, the size of asphaltenes was reduced, rendering the solid surface more water-wet. Thus, the mechanism of the wettability reversal of the OSS by SXJ-2 was indirectly verified by IR analysis and SEM scanning.

In this study, the mechanism of migration/plugging and the main performance of polymer nano-microspheres (HQ) were investigated using environmental scanning electron microscopy (ESEM), swelling hydration experiments, sand-filling pipe flooding experiments, and microscopic displacement experiments. Results show that the swelling ratio of HQ in salt solution first increases and then reaches equilibrium with the increase in swelling time. The expansion ratio of HQ increases rapidly at the initial stage of swelling and reaches its maximum value after swelling for 7 days. The maximum value of the expansion ratio was 15. In addition, the higher the salinity, the smaller the particle size of the studied microspheres. The particle size of the microspheres increases with increasing temperature. According to the visualized microscopic experiments, the migration and blocking mechanism of microspheres in the pore throat were discovered. When the particle size is 1/3–1/7 times the pore throat diameter, the microsphere particles can enter the interior of the pores. The microspheres aggregate together, and bridge plugging occurs due to the mechanical trapping of pore throats. In this case, the pore throats are blocked by the produced inner filter cake. The pressure at each point fluctuates up and down in a zigzag pattern during the displacement experiment, revealing the “migrated-plugged-breakthrough-replugged” mechanism for HQ. It can be seen from the oil displacement experiment that the final cumulative recovery and the enhanced oil recovery were 65.71 and 17.31%, respectively. Thus, the studied nano-microspheres have excellent oil displacement performance.