Experimental study of hybrid nanofluid-alternating-CO2 microbubble injection as a novel method for enhancing heavy oil recovery

Water-alternating-CO2 injection (CO2 WAG) is a successful method for enhancing heavy oil recovery and geologic CO2 storage. However, CO2 WAG still has some setbacks, such as water blocking, gravity overriding, and viscous fingering of injected CO2. Therefore, a new method, named hybrid nanofluid-alternating-CO2 microbubble injection, is proposed in this study to improve the effect of CO2 WAG. In this method, the hybrid nanofluid is used to reduce the interfacial tension (IFT) between the heavy oil and brine and alter the sandstone surface wettability. The CO2 microbubbles improve mobility control by minimizing gravity overriding and viscous fingering. In this study, for the first time, hybrid nanofluid and CO2 microbubbles are simultaneously used in WAG processes to enhance heavy oil recovery. Five types of nanoparticles are screened systematically by contact angle and IFT tests to find the best candidate for the hybrid nanofluid in hybrid nanofluid-alternatingCO2 microbubble injection and clarify the enhanced oil recovery mechanisms. Then, oil displacement tests are carried out by using a patented large-scale sandpack to assess the viability of hybrid nanofluid-alternating-CO2 microbubble injection and to analyze the effects of key parameters on the performance of hybrid nanofluidalternating-CO2 microbubble injection. Our experimental results indicate that Al2O3-SiO2 nanofluid has more influence than TiO2-SiO2 and TiO2-Al2O3 nanofluids on wettability alteration to the water-wet state and IFT reduction. Hybrid nanofluid-alternating-CO2 microbubble injection is a potentially good choice to enhance heavy oil recovery because of the synergistic effect of injecting hybrid nanofluid and CO2 microbubbles. Hybrid nanofluid-alternating-CO2 microbubble injection is suggested to be initiated as early as possible. A smaller slug size is preferred, and the water and gas slugs are suggested to be separately injected from the upper and lower parts of a heavy oil reservoir. The optimum WGR and slug size (1:1 and 0.05PV in this study) should be prudently considered when designing hybrid nanofluid-alternating-CO2 microbubble injection.