Estimating Al 2 O 3 -CO 2 nanofluid viscosity: A molecular dynamics approach

High-viscosity CO <inf>2</inf> is of interest to the oil and gas industry in enhanced oil recovery and well-fracturing applications. Dispersing nanoparticles in CO <inf>2</inf> is one way of achieving increased viscosity. However, parametric studies on viscosity estimation of CO <inf>2</inf> nanofluids is not found in the open literature. A comparison of various interatomic potentials for their accuracy in predicting viscosity is also missing. In this work, we studied Al <inf>2</inf> O <inf>3</inf> nanoparticles in CO <inf>2</inf> base fluid. We screened the inter-molecular interaction potential models available for CO <inf>2</inf> -CO <inf>2</inf> interactions and found that the TraPPE-flexible model (with MORSE potential) to be most suitable for conditions used in this work. We estimated the CO <inf>2</inf> -Al <inf>2</inf> O <inf>3</inf> interaction potential using quantum mechanical simulations. Using this combination for CO <inf>2</inf> -CO <inf>2</inf> and CO <inf>2</inf> -Al <inf>2</inf> O <inf>3</inf> interactions, we explored the effects of temperature and nanoparticle size on viscosity using molecular dynamics simulations (MD). We predicted that the viscosity would increase with increase in temperature and particle size. We also calculated the base fluid self-diffusion coefficient to investigate the effect of Brownian motion and its contribution to changes in viscosity. We found that it decreases with increase in particle size and temperature, thereby indicating that Brownian motion does not contribute to the increased viscosity. Further, the nanolayer formed at the Al <inf>2</inf> O <inf>3</inf> -CO <inf>2</inf> interface is studied through density distributions around the nanoparticle; the thickness of this nanolayer is found to increase with nanoparticle diameter. Finally, we examined the structures of CO <inf>2</inf> fluid in presence of nanoparticles at different thermodynamic states through radial distribution functions. The current work sheds light on the viscosity enhancement by the addition of nanoparticles; it is hoped that such studies will lead to tools that help tailor fluid properties to specific requirements.