Abstract: For the issue of frequent and severe oil-based drilling fluid losses in shale formations and the ineffectiveness of conventional plugging methods, an oil-based high filtration loss pressure-bearing plugging agent has been developed. Tests on its suspension stability, filtration time, initial strength, pressure-bearing capacity, and compatibility with drilling fluids were conducted. The properties and plugging mechanism of the agent were analyzed, along with its application process and field performance. The study shows that the oil-based high filtration loss plugging agent is a uniform mixture of fibers, porous inert materials, infiltrating materials, and filter-aids in specific proportions. The formulated slurry has good suspension stability, a total filtration time of less than 50 seconds, high initial strength of the solidified body, stable rapid filtration properties at 150 ℃, a pressure-bearing range of 10.0 to 19.0 MPa, and good compatibility with drilling fluids. Field application results show that this technology is simple and safe in operation, with strong adaptability of the oil-based drilling fluid and high pressure-bearing capacity. It is suitable for plugging small, medium, and large-scale losses in oil-based drilling fluids, as well as for addressing leakage in water-sensitive shale formations using water-based drilling fluids. This study offers a new solution for plugging the oil-based drilling fluid loss in shale formations.

Key words: oil-based drilling fluid, shale, filtration loss, lost circulation, pressure-bearing plugging