Polyelectrolyte coated individual silica nanoparticles dispersed in concentrated divalent brine at elevated temperatures for subsurface energy applications

Individual sub-60 nm polyelectrolyte coated silica nanoparticles (SiNPs) were stabilized in concentrated brine at temperatures up to 120 °C. The particles have potential applications in enhanced oil recovery, CO₂ sequestration, and underground delivery of encapsulated payloads. The polyelectrolyte copolymer was grafted to modified silica particles with a general thermal method in DMSO that is broadly applicable to many types of nanoparticles and polymers with less significant hydrolysis than in aqueous based approaches. SiNPs were first coated with amine functional groups and then with poly(2-acrylamido-2-methyl-1-propanesulfonic acid-co-acrylic acid) (poly(AMPS-co-AA)) through a thermally driven amidation of the acrylic acid units in a dimethyl sulfoxide (DMSO) rich medium. Permanent aggregation and bridging were limited by controlling reaction time for temperatures up to 130 °C. The amine coated SiNP were dispersed as individual particles and the polyelectrolyte formed small flocs in DMSO. Upon transfer of the grafted SiNP from DMSO to a fully aqueous medium the hydrodynamic diameters (D_h) ranged from 30–60 nm. The particles were colloidally stable in API brine (2 wt% CaCl₂ and 8 wt% NaCl) at 90 °C and pHs of 5.5–9.5 for 19 days. At 120 °C, minor aggregation to 130 nm was observed after 19 days at pH 5.5, whereas the stability decreased to 1 day at neutral pH. The particles were highly mobile in a column packed with 40–50 mesh Ottawa sand with a recovery of 82.2 % of the injected NPs. The exceptional colloidal stability and low retention in packed columns can be attributed to the steric protection of 2-acrylamido-2- methyl-1-propanesulfonic acid (AMPS) segments from particle-particle and particle-sand attractive forces, and multipoint grafting through the AA anchor groups.