CO₂-switchable nanohybrids for enhancing CO₂ flooding in tight reservoirs: From stable colloids to a relevant viscoelastic fluid

Conventional CO₂ EOR techniques based on microscale chemicals have limited efficiency in tight reservoirs because the micro-nanopores of these reservoirs impede their injectivity and propagation in porous media. This work elucidated a novel well-defined silica nanohybrids named DMA-NPs with the distinct CO₂ switchability for enhancing CO₂ flooding in tight reservoirs. DMA-NPs densely encapsulated with a CO₂-functional moiety of dimethylamine was synthesized by sequential surface modification and amidation reaction. The proof-to-concept for the CO₂-switchable nanohybrids was studied by TEM, TGA, SEM, ¹H NMR, FT-IR, DLS, rheological measurements and core flooding tests. The results indicated that DMA-NPs underwent reversibly physical transition, from stable colloidal particles with hydrodynamic diameter of 62 nm to a relevant viscoelastic fluid, by repeatedly bubbling CO₂ or introducing air to remove CO₂. Moreover, DMA-NPs dispersion whose viscosity was close to water, could preferentially flow through dominant porous media. When the dispersion met with the displacement front of CO₂, these stable colloidal particles self-assembled into a relevant viscoelastic fluid which reduced CO₂ mobility and diverted CO₂ into a lower permeable zone, and thus more than 30% of original oil in place bypassed by the initial CO₂ displacement was recovered.