Nanomaterial-Assisted Heavy Oil Viscosity Reduction: A Review of Mechanisms, Applicability, and Future Perspectives

Abstract Against the backdrop of declining conventional light-crude production, tighter low-carbon constraints, and growing demand for unconventional-resource mobilization, the efficient development of heavy, extraheavy, and ultraheavy oil requires viscosity-control strategies that are effective, stable, and condition-specific. Heavy-oil viscosity arises not only from molecular weight but also from resin/asphaltene supramolecular aggregation, wax-crystal networks, heteroatom/metal coordination, and oil–water interfacial resistance, limiting conventional thermal recovery, dilution, and chemical viscosity reduction. This review critically examines nanomaterial-assisted heavy-oil viscosity reduction using a framework that links material structure, mechanisms of action, operating conditions, and field implementation. This review identifies the primary molecular and structural targets of nanomaterial action, ranging from saturate, aromatic, resin, and asphaltene (SARA) fractions to resin/asphaltene supramolecular networks, and systematically compares major nanomaterial-based viscosity-reduction systems, including inorganic oxide nanoparticles, metal and metal oxide nanocatalysts, magnetic nanomaterials, carbon-based materials, surface-functionalized nanoparticles, nanoemulsions, and composite nanofluids. The underlying viscosity-reduction mechanisms are critically analyzed, including adsorption–dispersion, steric and electrostatic stabilization, interfacial emulsification, Pickering emulsion stabilization, wettability alteration, catalytic aquathermolysis, wax-crystal regulation, and physical-field-assisted synergistic effects. By integrating representative performance data with relevant operating conditions, this review demonstrates that interfacial- and structure-regulating nanomaterials are particularly suitable for medium- and low-temperature flow enhancement and pipeline transportation, whereas metal-based nanocatalysts exhibit greater potential for thermal-recovery processes and aquathermolytic upgrading of heavy oil. Surface-functionalized and composite nanosystems provide enhanced adaptability under challenging conditions, including high salinity, high water cut, and complex produced-fluid environments. Furthermore, key engineering challenges are critically evaluated, including nanoparticle dispersion stability, pore-scale migration and retention, quantitative elucidation of multiscale mechanisms, produced-fluid treatment, economic feasibility, and the limited availability of field-scale validation. Future advancement toward practical deployment requires the establishment of standardized evaluation protocols, validation under reservoir-relevant conditions, data-driven material screening strategies, and comprehensive technology-readiness assessments.