Nanocelluloses dispersed in water (0.8%, air atmosphere) start to degrade at 110 °C. CNC and Mech-CNF samples were stable after heat aging at 140 °C for 3 days. Nanocelluloses are alternatives to guar gum and xanthan in drilling fluids. All tested nanocellulose samples had better thermal stability than guar gum and xanthan. Formate ions improves thermal stability of nanocelluloses and carbohydrate polymers.