Nanoemulsion with ultra-small droplets and its imbibition in tight reservoirs

Nanoemulsions have been shown to significantly enhance imbibition-driven oil recovery in tight oil reservoirs, owing to their nanoscale droplets, low adsorption, and superior wettability alteration capabilities. However, they are susceptible to droplet growth and phase separation, resulting in destabilization and failure. Meanwhile, the relatively large droplets (typically ranging from 50 to 200 nm) restrict their access to smaller pores that contain significant hydrocarbon reserves, further decreasing the imbibition efficiency. In this study, we prepared an ultra-small droplet nanoemulsion (10–30 nm) with excellent stability by optimizing the types of nonionic surfactant and organic solvent employed. Cryo-transmission electron microscopy was used to elaborate the formation of ultra-small droplets, and the NMR imbibition tests were conducted to examine its enhanced imbibition effect. Experimental results showed that the ultra-small droplet nanoemulsion remained stable (with an average droplet diameter of 15 nm) for up to six months, far surpassing the stability of conventional large droplet nanoemulsions at a similar condition. This new nanoemulsion demonstrated a higher oil displacement contribution across various pores, achieving a 42.9% imbibition oil recovery, which was 27.1% and 6.5% higher than deionized water and conventional nanoemulsions, respectively, significantly enhancing the imbibition effect.