Nanoparticles have shown great potential in many sectors of the oil and gas industry, including enhanced oil recovery (EOR). They can be used to improve water flooding by altering the wettability of the porous medium, reducing the interfacial tension, blocking pores, or preventing asphaltene precipitation. Ensuring the stability of nanofluids injected into reservoirs is essential and a great challenge. However, high temperature favors particle collisions and high salinity (ionic strength) decreases electrostatic repulsion between particles. Therefore, nanofluids are extremely unstable at reservoir conditions. In this paper, we investigated the effects of electrolytes (brine and seawater) and temperature (up to 80 °C) on the stability of silica nanofluids. The nanofluids are characterized by dynamic light scattering (size), turbidity (stability), and zeta potential (electrostatic repulsions). One solution to increase the stability is to compensate for the loss of repulsive forces due to salts in the solution through increased electrostatic and/or steric repulsions by changing the pH of the base fluid. At high ionic strength (42 g/L NaCl and seawater), the stability of 0.1 and 0.5 wt% silica nanofluids at basic pH is about one day, regardless of temperature. In contrast, at pH 1.5, the nanofluids have a stability of at least three weeks at 80 °C. The results obtained with base fluids containing divalent cations confirmed their more destabilizing effect. This study confirmed that it is possible to stabilize silica nanofluids beyond one month at reservoir conditions just by lowering the pH near the isoelectric point.

Wettability has a dramatic impact on fluid displacement in porous media. The pore level physics of one liquid being displaced by another is a strong function of the wetting characteristics of the channel walls. However, the quantification of the effect is still not clear. Conflicting data have shown that in some oil displacement experiments in rocks, the volume of trapped oil falls as the porous media becomes less water-wet, while in some microfluidic experiments the volume of residual oil is higher in oil-wet media. The reasons for this discrepancy are not fully understood. In this study, we analyzed oil displacement by water injection in two microfluidic porous media with different wettability characteristics that had capillaries with constrictions. The resulting oil ganglia size distribution at the end of water injection was quantified by image processing. The results show that in the oil-wet porous media, the displacement front was more uniform and the final volume of remaining oil was smaller, with a much smaller number of large oil ganglia and a larger number of small oil ganglia, when compared to the water-wet media.