A critical review of the application of nanomaterials in frac fluids: The state of the art and challenges

Nanomaterials have drawn considerable attention of the oil and gas industry due to their peculiar properties and interesting behaviors. Many experiments, trials and practices were conducted by petroleum scientists and engineers in the last two decades to use various novel nanomaterials to improve exploration and production. Based on the published literature, this article comprehensively reviews the strategies and experience of nanomaterial application in frac fluids, the current problems, and relevant challenges. Based on elaborated design, the nanomaterials such as nano-sized metal, metal oxide, silica, carbonate, carbon, polymer, fiber, organic-inorganic hybrid and other composites can be incorporated in frac fluids to greatly enhance or precisely tune the properties and performances. Consequently, nanomaterial-assisted frac fluids perform well in different aspects including density, rheology, stability, heat conductivity, specific heat capacity, fluid loss, breaking, clean up, proppant suspendability and frictional drag. To optimize the performance and cost-effectiveness of nano-frac fluids, advanced principles and theories in physical chemistry, heat and mass transfer, mechanics and rheology along with industrial design philosophy have been considered and applied. According to the investigation of the literature, nanomaterials have successfully fulfilled the following functions in frac fluids: (1) Improving the rheological behavior by intermolecular interactions (e.g., pseudo-crosslinking in frac fluids, or changing the aggregation pattern of surface-active molecules in surfactant based fluids); (2) Increasing the stability of fluids by enhancing the interfacial strength and toughness, especially in foams and emulsions; (3) Forming a low-permeability pseudo-filter cake to lower the fluid loss; (4) Increasing the viscosifying effect of polymers, which dramatically decreases the required loading of polymer in the fluid; (5) Boosting the thermal stability of frac fluids; (6) Improving the regained fracture conductivity; (7) Reducing the frictional drag of frac fluids; (8) Helping self-suspended proppants achieve better performance and (9) Reducing the required displacing pressure for the residual frac fluid by decreasing interfacial tension to help clean up. These achievements, along with the related design ideas, are reviewed. This paper also discusses the major difficulties and challenges for nano-frac fluids including compatibility, cost and HSE issues. Comprehensive laboratory work should be performed before field application to ensure the reliability of nano-assisted fluid formulations. Large-scale industrial production and a steady supply of nanomaterials will promote the application of nano-frac fluids. Exposure risk, eco-toxicity and biodegradability of nanomateials should be paid more attention. Incorporating the attractive, cutting-edged achievements in chemical and material sciences, nano-frac fluid is predicted to be fully accepted by the petroleum industry due to its great potential and the increasingly declining price of nanomaterials.