The thermal stability of Polyanionic Cellulose (PAC) in drilling applications depends primarily on its product grade and the specific downhole environment:
Temperature Resistance Range: Standard-grade PAC typically withstands temperatures between 120°C and 130°C; specially modified, high-temperature-resistant grades can tolerate temperatures ranging from 140°C to 150°C.
Influencing Factors:
pH Value: PAC exhibits maximum stability in a weakly alkaline environment with a pH range of 8 to 11.
Salinity: It retains its functional properties even in saturated brine solutions, although thermal degradation may accelerate slightly under such conditions.
Oxidation: Downhole oxygen acts as a catalyst for degradation; therefore, PAC is typically used in conjunction with oxygen scavengers.
Failure Manifestations: Once the thermal resistance limit is exceeded, the molecular chains of the PAC break down. This leads to a drastic surge in drilling fluid fluid loss and a significant drop in viscosity, which can subsequently trigger wellbore instability or result in stuck pipe incidents.
Selection Recommendations: For temperatures below 130°C, standard-grade PAC is suitable; for temperatures between 130°C and 150°C, a high-temperature-resistant grade of PAC should be selected; if temperatures exceed 160°C, it is recommended to switch to more thermally stable treatment agents, such as sulfonated polymers or other synthetic polymers.
