Impacts of crosslinker concentration on nanogel properties and enhanced oil recovery capability

The use of nanogels, or crosslinked polymeric nanoparticles, has been proposed as a means of improving oil recovery in low permeability reservoirs. Nanogels can transport deep into reservoirs and improve homogeneity due to their small size and deformability. Typically, nanogels are polymerized using monomers and crosslinkers, which transform polymers from linear structures to 3D structures. Nanogel properties - including its swelling ratio and strength - can be fine-tuned by the crosslinker concentration. In this study, we investigated the impacts of crosslinker concentration on partially hydrolyzed polyacrylamide (HPAM)-based nanogels. The effect of the degree of crosslinking on the physicochemical properties of nanogel, the corresponding adsorbing behavior on rock surfaces, and consequently, the oil recovery improvement was studied. The results show the nanogels maintained a lower swelling ratio and less negative charge when synthesized at a higher crosslinker concentration. Higher crosslinker concentrations also resulted in lower dispersion viscosity because of the lower volumetric fraction and weaker interparticle attraction. The relationship between viscosity and dispersion concentration is in an agreement with the Krieger-Dougherty model. Core flooding experiments were conducted under a water-only condition and an oil-water two-phase condition. Nanogels with a lower crosslinker concentration were better able to reduce core permeability. It was discovered that nanogel injection pressure continuously increased in water-saturated porous media, whereas it reached a stable state in a two-phase condition. Core flooding tests in water-saturated cores also indicated that nanogels with a higher degree of crosslinking adsorbed more onto rock surfaces. The adsorption over time of each test fits well with the pseudo-second order equation. In addition, despite similar interfacial reduction capability, nanogels crosslinked to a lesser degree were able to improve oil recovery to a greater extent.