Mechanism study of spontaneous imbibition with lower-phase nano-emulsion in tight reservoirs

Spontaneous imbibition has been proved to be a promising enhanced oil recovery method for tight reservoirs, but the mechanisms of spontaneous imbibition oil recovery (SIOR) are still unclear. Here, we use the self-prepared novel lower-phase nano-emulsion (LWPNE) to reveal the mechanisms. In LWPNE solution, nano-scale oil drops (NODPs) in 6 nm diameter are formed and dispersed with co-presence of micelle solubilizing N-hexane. The effects of LWPNE on both interfacial tension (IFT) and contact angle were studied at 0–10,000 mg/L salinity condition, and spontaneous imbibition efficiency was determined from imbibition experiments in Amott cells at 60 °C. Electron Microscope and Transmission Electron Microscope were used to analyze the emulsification and solubilization mechanisms of LWPNE. Experimental results show that the original 135° (oil-wet) contact angle can be reduced to 32.7° (water-wet); the IFT is decreased to 0.0038 mN/m, and electrolyte has no contribution to either IFT reduction or wettability alteration. The average oil recovery from spontaneous imbibition testing using 0.3 wt% LWPNE is 44.1%, which is 20.5% higher than that from systems at similar conditions but using brine only. The LWPNE can solubilize oil by increasing NODPs’ sizes to average 36 nm in diameter, which is the dominant mechanism to improve SIOR for LWPNE. The solubility of LWPNE is evaluated by the diameter growth rate of NODPs, and once it reaches 350%, a self-driving force drives oil drops to move forward. Moreover, the transformation mechanisms of spontaneous imbibition modes from imbibition to drainage are revealed by using the imbibition discriminant parameter (NBm−1).