Abstract:
The northwestern part of the piedmont zone in the northern Longmenshan in western Sichuan features complex geological conditions, with multiple large thrust faults. The development of faults and high steep structures has led to severe wellbore instability and high risk of pipe sticking, posing significant challenges to safe and efficient drilling. Well A, one risk exploration well in this region, was taken as an example. To address these issues such as repeated wellbore collapse and pipe sticking occuring in drilling through bauxitic mudstone and carbonaceous shale of Wujiaping Formation, key technologies for safe drilling with controlling collapse and pipe sticking were developed. These include investigating the collapse mechanisms of unstable formations, optimizing strongly sealing and anti-collapse drilling fluid systems, employing anti-sticking eccentric bits, improving drilling methods, and conducting real-time risk warning of pipe sticking caused by falling debris based on mud logging data. The research results indicate that the non-uniform expansion of illite-montmorillonite mixed layers after water absorption in the bauxitic mudstone and carbonaceous shale of Wujiaping Formation is the main cause of wellbore rock disintegration and spalling. Falling debris from the wellbore can cause abnormal torque fluctuations, and the risk of pipe sticking can be identified by monitoring torque trends. Oil-base drilling fluids can effectively reduce the risk of wellbore instability caused by rock expansion after water absorption in bauxitic mudstone and carbonaceous shale, creating conditions for lowering drilling fluid density to prevent lost circulation. Using an eccentric bit to drill a pilot hole followed by reaming can significantly reduce the risk of pipe sticking due to falling debris. After field application in Well A, the folded and inverted formations were safely drilled out, ensuring the successful achievement of geological objectives and providing strong support for the exploration and development of ultra-deep oil and gas resources in Longmenshan nappe structure.