Grafting lignite with sulformethal phenoldehy resin and their performance in controlling rheological and filtration properties of water–bentonite suspensions at high temperatures

• Lignite grafted with sulformethal phenoldehy resin can control the fluid loss and rheological properties of water–bentonite slurry effectively at aging temperatures as high as 200 °C.
• Grafting lignite with sulformethal phenoldehy resin by aqueous condensation polymerization method is simple and economical.
• The lignite grafted with side chain is more easily adsorbed on the bentonite surface, and result in stability of water–bentonite slurry under high temperature.

A lignite graft polycondensate with water retention and dispersion properties in a water–bentonite slurry was synthesized by grafting sulformethal phenoldehy (SMP) resin onto a scaffold of humic acid (main component of lignite) through aqueous condensation polymerization. Characteristic results of the chemical structure and molecular weight of the polycondensate showed a successful grafting of the SMP. The resulting lignite graft polycondensate used as drilling fluid additive effectively controlled the filtrate volume and exhibited rheological stability at aging temperatures as high as 200 °C. The behavior of the polycondensate distinguishes it from most common synthetic high-temperature fluid loss additives, such as SMP, which excessively viscosify slurries. Colloidal properties of the water–bentonite admixture were investigated by adsorption and zeta potential experiments, and a possible mechanism was proposed.