Research on Drilling Fluid Pollution Mechanism and Treatment in Deep Blocks of Western Sichuan

In the Chuanxi block of the Sichuan Basin, drilling frequently encounters polyhalite, resulting in poor rheological properties of drilling fluid contamination. The viscosity of the drilling fluid funnel has increased from 60 to 120 s, and the final cut pressure has increased from 10 to 45 Pa. In order to find the source of pollution in the drilling fluid and its treatment methods, a series of studies have been conducted. Research has found that the mixed halite in western Sichuan is mainly gray with a small amount of dark gray, appearing as clumps, brittle, and relatively soft. A small amount of transparent plate-like crystals can be seen under the microscope, with poor water absorption and a salty and bitter taste. The main components of polyhalite are hard gypsum, a small amount of unhydrated carnallite, and quartz. No special element residues are found in the analysis of the drilling fluid filter cake. The analysis of the drilling fluid filtrate shows contamination by Ca2+/Mg2+, which is the main cause of thickening and increased water loss in the drilling fluid. By partially replacing the drilling fluid and adding 0.2% sodium pyrophosphate to remove excess Ca2+/Mg2+ from the drilling fluid, the final cut-off pressure is reduced to 7 Pa and normal conditions are restored. By analyzing the mechanism of drilling fluid pollution caused by deep-seated halite in the western Sichuan Basin, it can provide guidance for on-site maintenance of drilling fluid systems, pollution prevention, and research and development of drilling fluid materials.