In this paper, effects of temperature, initial oil and injection gas compositions, and feed gas–oil ratio on oil–CO ₂ interfacial tensions, interfacial thicknesses, and minimum miscibility pressures (MMPs) in nanopores are specifically studied.

When water imbibes into tight rocks, the disconnection of bulk gas phase will significantly impact the performance of gas recovery. In this work, we have conducted a systematic micromodel study on the disconnection of gas phase during water imbibition. Seven types of micromodels were designed that can geometrically mimic basic pore structures of a tight sandstone. Then, we conducted capillary-dominant imbibition experiments and analyzed the effects of pore geometry and pore-throat ratio on the transient evolutions of gas–water interfaces. Our pore-scale results reveal that snap-off and bypassing flows are the two main mechanisms that determine disconnection and entrapment of the gas phase. Moreover, we qualitatively linked the pore-scale two-phase displacements to the core-scale “permeability jail” phenomenon (i.e., nearly immobile of non-wetting and wetting phases across a wide range of saturation values). Our study will enrich the knowledge of entrapment behaviors of the gas phase during water imbibition into tight formations.

To improve oil recovery significantly in low-mid permeability reservoirs, a novel modified nano-SiO₂ hyperbranched copolymer (HPBS), consisting of polyacrylamide as hydrophilic branched chains and modified nano-SiO₂ as the core, was synthesized via an in situ free radical polymerization reaction. The structure and properties of the hyperbranched copolymer were characterized through a range of experiments, which showed that HBPS copolymers have better stability and enhanced oil recovery (EOR) capacity and also smaller hydrodynamic radius in comparison with hydrolyzed polyacrylamide (HPAM). The flooding experiments indicated that when a 1000 mg/L HPBS solution was injected, the resistance factor (RF) and residual resistance factor (RRF) increased after the injection. Following a 98% water cut after preliminary water flooding, 0.3 pore volume (PV) and 1000 mg/L HPBS solution flooding and extended water flooding (EWF) can further increase the oil recovery by 18.74% in comparison with 8.12% oil recovery when using HPAM. In this study, one can recognize that polymer flooding would be applicable in low-mid permeability reservoirs.