Effect of surfactants on the interface characteristics and imbibition processes in shale oil reservoirs

Imbibition is a crucial method for shale reservoir development. However, the mechanism underlying the surfactant-enhanced imbibition in shale oil reservoirs remains unclear. The mainstream perspective holds that wettability is the determining factor, whereas alternative views suggest that ultra-low interfacial tension and emulsification ability are more conducive to improving imbibition recovery. In this study, we investigated the effects of three surfactant solutions (zwitterionic, anionic, and non-ionic) on the imbibition recovery and rate of shale oil reservoirs using spontaneous imbibition experiments. The ability of surfactants to alter the shale surface wettability, reduce the oil–water interfacial tension, and promote spontaneous emulsification was examined. The mechanism of surfactant-enhanced imbibition recovery in shale reservoirs was analysed using the dynamic inverse bond number (N_B⁻¹). The results show that surfactant solutions can improve the imbibition recovery of shale oil reservoirs. At a concentration of 1.167 mM, the imbibition recovery was in the order of cocoamidopropyl betaine (CAB, 25.6 %) > sodium dodecyl benzene sulfonate (SDBS,17.51 %) > isooctyl alcohol polyoxyethylene ether (JFC,11.39 %). In additional, the surfactants improved the imbibition rate during the late stage. Wettability is the primary factor in improving imbibition recovery. Spontaneous emulsification is another crucial factor affecting imbibition recovery in shale oil reservoirs. The transport of oil droplets/emulsion in the shale pore space occurs in a ‘laminar flow’ mode. It is necessary to control the timing of water films filling the pore space because high interfacial tension can lead to rapid filling, causing ‘jamming’ of the emulsion at the throat. The dynamic N_B⁻¹ value analysis shows that gravity dominates imbibition in the early stage, whereas both gravity and capillary pressure dominate in the late stage. Furthermore, the emulsifying effect of the surfactant should be fully utilized in the late stage of imbibition to promote emulsion transport in the pore throats via the ‘laminar flow’ mode. The results of this study suggest that future research on spontaneous imbibition in shale oil reservoirs should focus on emulsification, particularly its impact on the migration of emulsions and oil droplets within pores, to identify new strategies for shale oil reservoir development.