Cr(III)-Entrapped in Nanocapsules Prepared By Double Emulsion Method for Delayed Gelation of Hydrolyzed Polyacryamide in Secondary Fooding

The main objective of a gel treatment in most mature oil fields is to overcome challenges in the heterogeneity of the reservoir by blocking highly permeable channels or fractures without damaging productive zones. In recent decades, Cr(III)-polyacrylamide (HPAM) gels have been extensively applied as blocking agents for sweep efficiency improvement. However, the gelation time of the current gels is not long enough for in-depth placement. This study systematically describes a novel approach of using nanocapsules to entrap and delay the release of Cr(III) to extend the gelation time. The Cr(III)-entrapped nanocapsules are successfully prepared by a controlled water-in-oil-in-water double emulsion method. Hydrophobic polymers and surfactant are pre-dissolved in oil medium for the shell formation during evaporation of the oil and stabilization of the emulsion, respectively. The oil evaporation occurs at room temperature which leads to the precipitation of the polymer into the interface of the aqueous droplets. The size of the nanocapsules ranged from 175-690 nm depending on the surfactant concentration. A Cr(III) loading ratio of 0.5-6% and entrapment efficiency of 5-20% are obtained by optimizing different process variables such as Cr(III) content in the aqueous droplet, polymer content in the oil medium, and surfactant concentration. The nanocapsules coated with stabilizers can be easily dispersed in water and mixed with the HPAM solution. Gelation test results show that encapsuled Cr(III) significant delays the formation of the HPAM gel for more than 20 days which is dependent on pH and temperature. The nanocapsule formation process is general and may be applied to a wide range of polymers and encapsulated materials in subsurface applications.