Nanofluid of Graphene-Based Amphiphilic Janus Nanosheets for Oil Recovery
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Since our primary energy transition still remains uncertain, fossil fuel energy would still be our dominant energy resource for decades to come. One of the main components of fossil fuel is crude oil. Chemical flooding is one of the most common used methods for enhancing oil production. However, large amounts of chemicals injection are challenges with costs, reservoir damage, and environmental issues. Recently, nanotechnology has shown its great potential for bringing revolutionary changes in petroleum industry. One of them is to use nanofluids for enhanced oil recovery (EOR). However, simple nanofluid containing only nanoparticle for EOR is inefficient, especially when used with low concentrations. In this thesis, we have designed and produced a novel nanofluid of graphene-based amphiphilic Janus nanosheets that was very effective for both secondary and tertiary oil recovery at low concentrations. The nanosheets were synthesized by single surface hydrophobization of graphene oxide, and checked by AFM, Raman, UV-Vis, FTIR, TGA, and XPS. The stability of fresh water dispersed nanosheets was also evaluated. When injected, the nanosheets displayed a unique interfacial behavior, which corresponded to some new oil displacement mechanisms. Furthermore, three-phase contact angle measurements demonstrated the ability of surface wettability alteration. A method was further proposed to calculate interfacial tension between nanofluid and oil phase. To achieve practical applications, we discovered a facile and scalable method to prepare the nanosheets in large quantities with much higher efficiency than wax masking method by manipulation of hydrogen bonding. To realize a stable dispersion in salt water at surface, a model based on modified DLVO theory was firstly developed to investigate colloidal stability of the nanosheets at different salt and temperature conditions by understanding chemical and physical properties of the nanofluid. The model predictions were in good agreement with experimental stability evaluations. By using low concentrations of poly(sodium 4-styrenesulfonate) (PSS), graphene-based amphiphilic Janus nanosheets were stabilized in high-salt brine. The stabilization mechanisms were elucidated both by Molecular Dynamics simulations and experiments. The retained unique interfacial behavior was also demonstrated by simulations, experiments, and thermodynamic analysis. Application of the fluid system in EOR showed very high performance and synergistic effort.