Blending vs. Grafting: Thermal Stability Optimization of Xanthan Gum With Carbon Microspheres as Fluid‐Loss Additives

Xanthan gum (XG) is a shear‐thinning fluid loss agent widely used in drilling fluids, yet its poor thermal stability limits its application in high‐temperature deep wells. In this work, carbon microspheres (CMS) were introduced into the XG matrix through covalent grafting and physical blending, respectively. The grafted product XG@CMS showed a tripled molecular weight (Mn = 8.11 × 10 5 g/mol, Mw = 8.72 × 10 5 g/mol) and improved solubility (WS = 93.4% ± 0.5%), along with a slightly increased maximum degradation temperature. However, its viscosity and shear‐thinning behavior decreased remarkably ( τ ₀ = 0.04 Pa, K = 0.01 Pa sⁿ). By contrast, physically blended XG/CMS maintained excellent viscoelastic properties ( τ ₀ = 1.34 Pa, K = 5.01 Pa sⁿ) and presented higher storage modulus, loss modulus, and conformational transition temperature. High‐temperature high‐pressure filtration tests showed that XG/CMS reduced fluid loss by 20.8%, attributed to electrostatic repulsion and plugging effects from degradation products. XG/CMS exhibited a zeta potential of −41.7 mV and micrometer‐scale particles, supporting the synergistic mechanism. This study provides a strategy for designing high‐performance fluid loss reducers for drilling fluids.