Construction of coal pitch-based HA-K grafted poly condensates and their excellent anti-temperature and viscosity-reducing properties

Humic acids (HAs) are widely used as filtrate and viscosity reducers in drilling fluids. However, their practical utility is limited due to poor stability in salt resistance and high-temperature resistance. High-temperature coal pitch (CP) is a by-product from coal pyrolysis above 650 °C. The substance's molecular structure is characterized by a dense arrangement of aromatic hydrocarbon and alkyl substituents. This unique structure gives it unique chemical properties and excellent drilling performance, surpassing traditional humic acids in drilling operations. Potassium humate is prepared from CP (CP-HA-K) by thermal catalysis. A new type of high-quality humic acid temperature-resistant viscosity-reducer (Graft CP-HA-K polymer) is synthesized with CP-HA-K, hydrolyzed polyacrylonitrile sodium salt (Na-HPAN), urea, formaldehyde, phenol and acrylamide (AAM) as raw materials. The experimental results demonstrate that the most favorable conditions for the catalytic preparation of CP-HA-K are 1 wt% catalyst dosage, 30 wt% KOH dosage, a reaction temperature of 250 °C, and a reaction time of 2 h, resulting in a maximum yield of CP-HA-K of 39.58%. The temperature resistance of the Graft CP-HA-K polymer is measured to be 177.39 °C, which is 55.39 °C higher than that of commercial HA-K. This is due to the abundant presence of amide, hydroxyl, and amine functional groups in the Graft CP-HA-K polymer, which increase the length of the carbon chains, enhance the electrostatic repulsion on the surface of solid particles. After being aged to 120 °C for a specified duration, the Graft CP-HA-K polymer demonstrates significantly higher viscosity reduction (42.12%) compared to commercial HA-K (C-HA-K). Furthermore, the Graft CP-HA-K polymer can tolerate a high salt concentration of 8000 mg·L−1, measured after the addition of optimum amount of 3 wt% Graft CP-HA-K polymer. The action mechanism of Graft CP-HA-K polymer on high-temperature drilling fluid is that the Graft CP-HA-K polymer can increase the repulsive force between solid particles and disrupt bentonite's reticulation structure. Overall, this research provides novelty insights into the synthesis of artificial humic acid materials and the development of temperature-resistant viscosity reducers, offering a new avenue for the utilization of CP resources.