Mechanism and Countermeasures of Wellbore Instability in the Longmaxi Formation of the Western Chongqing Block

Abstract The Longmaxi Formation in the Western Chongqing Block of the Sichuan Basin holds significant potential for shale gas resources and has gradually become an important area for exploration and development. However, the challenges posed by high temperature, high pressure, and wellbore instability during the drilling of deep shale gas wells significantly reduce the efficiency of drilling operations and present considerable challenges for oil-based drilling fluid technology. This article investigates the reservoir characteristics of the Longmaxi Formation in the Western Chongqing Block to uncover the mechanisms behind wellbore instability. To address the development of structural fractures in formations, gel microspheres capable of plastic deformation under stress were selected as the primary sealing agent. These microspheres synergistically interact with rigid and flexible nano–micron materials to form an integrated “scaffold-filler” barrier, achieving complete sealing of nano–micron fractures in the formation. Through experimental optimization, a high-temperature-resistant, strong-sealing oil-based drilling fluid system was developed. This system exhibits resistance to temperatures up to 200 °C and effectively seals pore-fracture sizes ranging from 0.034 μm to 5.376 μm, demonstrating superior comprehensive performance compared to conventional drilling fluids. Field application confirmed stable drilling fluid properties, with a borehole enlargement rate below 5%. The mechanical drilling rate increased by 80.35% relative to neighboring wells, and no complex wellbore incidents occurred. This study establishes novel approaches for achieving robust sealing of micro-fractures and developing high-temperature-resistant, oil-based drilling fluids with strong-sealing capabilities. It provides a theoretical foundation and technical support for addressing wellbore instability challenges in the West Chongqing Block.