In subsurface imaging and oil recovery where temperatures and salinities are high, it is challenging to design polymer-coated nanoparticles with low retention (high mobility) in porous rock. Herein, the grafting of poly­(2-acrylamido-2-methyl-1-propanesulfonic acid-co-acrylic acid) (poly­(AMPS-co-AA)) on magnetic iron oxide nanoparticles was sufficiently uniform to achieve low adsorption on model colloidal silica and crushed Berea sandstone in highly concentrated API brine (8% NaCl and 2% CaCl2 by weight). The polymer shell was grafted via amide bonds to an aminosilica layer, which was grown on silica-coated magnetite nanoparticles. The particles were found to be stable against aggregation in American Petroleum Institute (API) brine at 90 °C for 24 h. For IO nanoparticles with ∼23% polymer content, Langmuir adsorption capacities on colloidal silica and crushed Berea Sandstone in batch experiments were extremely low at only 0.07 and 0.09 mg of IO/m2, respectively. Furthermore, upon injection of a 2.5 mg/mL IO suspension in API brine in a column packed with crushed Berea sandstone, the dynamic adsorption of IO nanoparticles was only 0.05 ± 0.01 mg/m2, which is consistent with the batch experiment results. The uniformity and high concentration of solvated poly­(AMPS-co-AA) chains on the IO surfaces provided electrosteric stabilization of the nanoparticle dispersions and also weakened the interactions of the nanoparticles with negatively charged silica and sandstone surfaces despite the very large salinities.

The synthesis of polymer grafted nanoparticles that are stable at high salinities and high temperature with low retention in porous media is of paramount importance for subsurface applications including electromagnetic imaging, enhanced oil recovery and environmental remediation. Herein, we present an improved approach to synthesize and purify sub-100 nm IONPs grafted with a random copolymer poly(AMPS-co-AA) (poly(2-acrylamido-3methylpropanesulfonate-co-acrylic acid)) by means of catalyzed amide bond formation at room temperature. The improved and uniform polymer grafting of magnetic nanoparticles led to colloidal stability of IONPs at high temperature (120 °C) in API for a month. The transport behavior of the polymer grafted IONPs was investigated in crushed and in consolidated Berea sandstone. The high poly (AMPS-co-AA) polymer level on the surface (~34%) provided electrosteric stabilization between the NPs and weak interactions of the NPs with anionic silica and sandstone surfaces. This behavior was enabled by low affinity of Ca 2+ towards the highly acidic AMPS monomers thus enabling strong solvation in API brine. In crushed Berea sandstone, the retention was reduced by three fold and nine fold relative to our earlier studies, given the improvements in the grafted polymer layer. For intact core flood experiments in Berea sandstone carried out at elevated temperature (65 o C) and pressure (1000 psi net confining stress), the retention was 519 µg/g, comparable to the value for crushed Berea sandstone. Furthermore, the addition of a relatively small amount (0.1% v/v) of commercially available sacrificial polymer (e.g., HEC-10) further reduced IONP retention to 252 µg/g or 0.17 mg/m 2 by blocking retentive sites.

The colloidal stability of nanoparticles (NPs) stabilized by grafted polyelectrolyte (PE) brushes in concentrated divalent ion solutions, at either ambient or high temperature, is of interest in a wide variety of applications including medicine, personal care products, oil and gas recovery, reservoir imaging, and environmental remediation. Previous attempts to determine the length of PE brushes at these conditions have been limited by lack of colloidal stability particularly when divalent ions form complexes with the charges on the brushes. We find that brushes of highly acidic strong PE poly­(2-acrylamido-2-methyl­propane­sulfonate, AMPS) end-grafted to silica NPs provide colloidal stability at salinities up to 4.5 M CaCl2 or NaCl. Thus, the brush behavior could be studied with dynamic light scattering (DLS) and the electrophoretic mobility by phase analysis light scattering (PALS) from the salt-free condition to the extreme salinities of 4.5 M. In monovalent NaCl solutions, the highly extended poly­(AMPS) brushes at low salt concentration (C s) collapse monotonically with increasing C s. On the other hand, in divalent CaCl2 solutions the brushes underwent four distinct regimes of (i) a low C s collapse regime, (ii) a relatively broad plateau regime (0.1 M ≤ C s < 1 M), (iii) a weak reswelling regime, and (iv) a high C s collapse regime. The novel behavior in regimes ii–iv may be attributed to weak interactions of the poly­(AMPS) brushes with Ca2+. We also find that the brushes are more extended at 90 °C as thermal energy weakens interchain bridging, which is consistent with the behavior of free polymer chains dissolved in CaCl2 solutions at extreme salinities.

The sweep efficiency of CO 2 enhanced oil recovery can be improved by forming viscous CO 2 -in-water (C/W) foams that increase the viscosity of CO 2 . The goal of this study is to identify CO 2 -soluble ionic surfactants that stabilize C/W foams at elevated temperatures up to 120 °C in the presence of a high salinity brine using aqueous phase stability, static and dynamic adsorption, CO 2 solubility, interfacial tension, foam bubble size, and foam viscosity measurements. An anionic sulfonate surfactant and an amphoteric acetate surfactant were selected to achieve good thermal and chemical stability, and to minimize adsorption to sandstone reservoirs in the harsh high-salinity high-temperature brine. The strong solvation of the surfactant head by the brine phase and surfactant tail by CO 2 allows efficient reduction of the C/W interfacial tension, and the formation of viscous C/W foams at high salinity and high temperature. Furthermore, the effect of temperature and methane dilution of CO 2 on foam viscosity was evaluated systematically in both bulk and porous media. High temperature reduces the stability of foam lamella, which leads to lower lamella density and, therefore, lower foam viscosity. Methane dilution of CO 2 reduces the solvation of surfactant tails and makes the surfactant less CO 2 -philic at the interface. The consequent increase of the interfacial tension decreases the stability of foam lamella, as seen by the increase in foam bubble size, thereby reducing foam viscosity.

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