Foamed fluids with carbon dioxide in the gas phase have been recently studied as fracturing fluids to develop unconventional resources. This type of fracturing fluid is superior to water- or oil-based fracturing fluids for unconventional reservoirs, which are prone to damage by clay swelling and blocking of pore throats in water- or oil-rich environments. Conventional CO2 foams with surfactants have low durability under high temperature and high pressure, which limit their application. Nanoparticles provide a new technique to stabilize CO2 foams under harsh reservoir conditions. As CO2 foams will be applied as carrier fluids for proppant transport, it is essential to determine the in situ rheology of CO2 foams stabilized by nanoparticles under reservoir conditions in order to predict proppant transport and effective microchannels in reservoir fractures for improving oil production. This work studied the in situ shear viscosity and foam stability of supercritical CO2 foams stabilized by nanosilica (SiO2) in the flow loop apparatus with shear rates of 5950-17850 s(-1) at a pressure of 1140 +/- 20 psig and a temperature of 40 degrees C. Supercritical CO2 with density of 0.2-0.4 g/mL and viscosity of 0.02-0.04 cP under typical reservoir conditions was applied to generate foams. The foams were tested with high foam qualities up to 80% to minimize the usage of water. The effects of shear rates, surfactant, foam quality, salinity, and nanoparticle size on the rheology of gas foams were experimentally investigated. The foam texture and stability were observed through an in-line sapphire tube after generation under reservoir conditions. Finely textured and stable foams with high foam quality were generated. CO2 foams generated by different systems and gas qualities showed complex rheology and stability. The rheology of the foams demonstrated both shear-thinning and shear-thickening behaviors. The salinity significantly affects the foam behaviors by greatly decreasing foam stability, resulting in foam rheology in two ways depending on components, foam quality, and shear rates. While the viscosities and interfacial affinity of CO2 foams stabilized by nanoparticles under atmospheric pressure have been widely studied recently, no work has been reported to study the dynamic rheological behaviors of CO2 foams stabilized by nanoparticles and their stability/morphology after shearing under high pressure and elevated temperature. This research provides a pioneering insight into the rheology of viscous supercritical CO2 foams stabilized by nanoparticles.