Abstract:

The recovery of crude oil from shale oil reservoirs remains a significant challenge due to the ultra-low permeability and complex pore structures. Surfactants have been increasingly investigated as a potential method to enhance oil recovery in unconventional reservoirs by reducing interfacial tension and altering wettability. In this work, the performance of surfactants for potential application in shale oil reservoirs was evaluated by investigating spontaneous imbibition behavior. The results showed that the interfacial tension of 0.15 wt% CA-90+0.15 wt% SDS was reduced to 1.03×10^-2 mN/m under formation conditions. The wetting contact angle of the oil-wet core sheet was reduced from 135.5° to 49.5°, i.e., the wettability of the rock was changed from oil-wet to water-wet, and the capillary force was thus changed from resistance for oil imbibition to driving force. When the water content was 70%, the viscosity of the emulsion decreased to 23.4 mPa·s which was close to the viscosity of crude oil, but the fluidity was higher, which was more conducive to improving oil recovery. The dynamic adsorption capacities of 0.15 wt% CA-90 and 0.15 wt% SDS were 0.85 mg/g and 0.97 mg/g, respectively, which had good anti-adsorption ability, and the damage rate of shale permeability was less than 8%. The imbibition recovery of slick water+compounded system could reach 31.27%, which was much higher than that of spontaneous imbibition recovery in slick water system （19.43%）, and the imbibition oil displacement efficiency was thus increased by 60.94%, which greatly improved the oil recovery for shale reservoirs.

Key words: shale reservoir, surfactant, interface tension, spontaneous imbibition, recovery