Facile synthesis of chromium chloride/poly(methyl methacrylate) core/shell nanocapsules by inverse miniemulsion evaporation method and application as delayed crosslinker in secondary oil recovery

Cr(III)–hydrolyzed polyacrylamide (HPAM) gels have been extensively studied as a promising strategy controlling waste water production for mature oilfields. However, the gelation time of the current technologies is not long enough for in-depth placement. In this study, we report a novel synthesis method to obtain chromium chloride/poly(methyl methacrylate) (PMMA) nanocapsules which can significantly delay the gelation of HPAM through encapsulating the chromium chloride crosslinker. The chromium chloride-loaded nanocapsules (Cr–NC) are prepared via a facile inverse miniemulsion evaporation method during which the hydrophobic PMMA polymers, pre-dispersed in an organic solvent, were carefully controlled to precipitate onto stable aqueous miniemulsion droplets. The stable aqueous nanodroplets (W) containing Cr(III) are dispersed in a mixture of organic solvent (O1) with PMMA and nonsolvent medium (O2) to prepare an inverse miniemulsion. With the evaporation of the O1, PMMA forms Cr–NCs around the aqueous droplets. The Cr–NCs are readily transferred into water from the organic nonsolvent phase. The Cr–NCs exhibit the tunable size (358–983 nm), Cr loading (7.1%–19.1%), and Cr entrapment efficiency (11.7%–80.2%), with tunable zeta potentials in different PVA solutions. The Cr–NCs can delay release of Cr(III) and prolong the gelation time of HPAM up to 27 days.