Recently, nano-EOR has emerged as a new frontier for improved and enhanced oil recovery (IOR & EOR). Despite their benefits, the nanoparticles tend to agglomerate at reservoir conditions which cause their detachment from the oil/water interface, and are consequently retained rather than transported through a porous medium. Dielectric nanoparticles including ZnO have been proposed to be a good replacement for EOR due to their high melting point and thermal properties. But more importantly, these particles can be polarized under electromagnetic (EM) irradiation, which provides an innovative smart Nano-EOR process denoted as EM-Assisted Nano-EOR. In this study, parameters involved in the oil recovery mechanism under EM waves, such as reducing mobility ratio, lowering interfacial tensions (IFT) and altering wettability were investigated. Two-phase displacement experiments were performed in sandpacks under the water-wet condition at 95°C, with permeability in the range of 265–300 mD. A crude oil from Tapis oil field was employed; while ZnO nanofluids of two different particle sizes (55.7 and 117.1 nm) were prepared using 0.1 wt. % nanoparticles that dispersed into brine (3 wt. % NaCl) along with SDBS as a dispersant. In each flooding scheme, three injection sequential scenarios have been conducted: (i) brine flooding as a secondary process, (ii) surfactant/nano/EM-assisted nano flooding, and (iii) second brine flooding to flush nanoparticles. Compare with surfactant flooding (2% original oil in place/OOIP) as tertiary recovery, nano flooding almost reaches 8.5–10.2% of OOIP. On the other hand, EM-assisted nano flooding provides an incremental oil recovery of approximately 9–10.4% of OOIP. By evaluating the contact angle and interfacial tension, it was established that the degree of IFT reduction plays a governing role in the oil displacement mechanism via nano-EOR, compare to mobility ratio. These results reveal a promising way to employ water-based ZnO nanofluid for enhanced oil recovery purposes at a relatively high reservoir temperature.

Untreated nanoparticles possess huge surface areas compared to their mass, resulting in strong inter-particle interactions in saline water. This induces a strong tendency of particles' agglomeration, rapid sedimentation and consequently reduced the mobility of nanoparticles in the aquatic environment, which ultimately lowering the effective viscosity of the nano system. This study aimed to investigate the effect of stabilizers on the stability of dielectric nanofluid, to provide a better electrorheological characteristic for Nano-EOR purposes. In this research, zinc oxide (ZnO) was employed as dielectric nanoparticles under various nanoparticles concentration (0.1, 0.05, 0.01 wt. %). Anionic surfactants (SDS, SDBS, and Oleic acid) were compared in an attempt to prepare the homogeneous dispersions with long-term stability at high temperature (~ 95°C). The laboratory experiments were designed to evaluate the sedimentation behavior of nanoparticles using visualization method; whereas UV-vis spectrophotometry was employed to quantitatively characterize the stability of the nanoparticle dispersions. Further, dynamic light scattering (DLS) were also used to determine the size distribution of dispersed nanoparticles. The stabilized nanofluids were then subjected for measuring of electrorheological behavior using a rotating viscometer attached to a custom-built solenoid coil. From the experimental results, it is concluded that the most stable aqueous dispersion of ZnO nanoparticles is obtained at 0.1 wt. % with the aid of 0.025 wt. % SDBS under the conditions of 60 min of ultrasonication, adjusted at the pH value of 2. The ZnO/SDBS dispersion having a hydrodynamic size of 240.9 nm exhibits extreme stability at high temperature of 95°C, with the supernatant ZnO concentration decreasing only 19% compared with a decrease of 100% for the bare ZnO/Brine system. The rheological measurements indicated that all the nanofluids exhibit pseudoplastic (shear thinning) behavior. While the 0.1 wt. % ZnO/SDBS dispersion provide an enhancement in the relative viscosity of nanofluid up to 11% compared to brine as a basefluid, indicating the role of stability to achieve an electrorheological effect by activating dielectric ZnO nanoparticles. Additionally, the viscosity ZnO nanofluid increased with the increase of particle concentration under an applied field, which shows the strong dependence of viscosity on particle loading. The combined treatment with the surfactant, pH and ultrasonication is recommended to enhance the electrorheological characteristics of ZnO nanofluid. Hence, the mobility of a stabilized nanofluid can be efficiently controlled by regulating the applied field for EOR purposes.

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