Mobility of Sub-50 nm Iron Oxide Nanoparticles with Ultrahigh Initial Magnetic Susceptibility in Intact Berea Sandstone at High Salinity

Superparamagnetic iron oxide nanoparticles (IONPs), which have been investigated extensively as contrast-enhancing agents in biology, are being explored for subsurface applications such as electromagnetic tomography, fracture mapping, and enhanced oil recovery. However, two key challenges must be addressed: (a) high magnetic susceptibility and (b) colloidal stability and mobility under harsh reservoir conditions of high salinity and temperature. Herein, we synthesize IONPs grafted with poly(2-acrylamido-3-propanesulfonate-co-acrylic acid) poly(AMPS-co-AA) to achieve a high surface grafting density of polymer (49%) with minimal aggregation to yield sub-50 nm IONPs. The IONPs were found to be colloidally stable at 120 °C for a period of one month at pH 8. In crushed Berea sandstone, polymer-grafted IONPs exhibited significantly high mass breakthrough (84%) and low retention (149 μg/g) when used with a sacrificial polymer preflood (0.1% v/v). Intact Berea core experiments showed an 8-fold improvement in mass breakthrough (65%) and a two-thirds reduction in retention (from 433 μg/g to 160 μg/g) when compared to previous studies with IONPs synthesized via coprecipitation. The high grafting density of polymeric stabilizer and small nanoparticle size contribute to the improved mobility in consolidated porous media at high salinity.