Drilling technology for the first unconventional ultra-deep well in West Longgang Region

Well Longgang-61 is the first unconventional ultradeep well in West Longgang Region in Sichuan Basin with a total depth of 6 618 m. The Jurassic wellbore wall is poor in stability; the vertical pressure system is complex; the H2S content in deep marine formation is over 30 g/m3; there also exists CO2 zone and brine zone with abnormally high pressure; the bottom hole pressure is over 110 MPa and the temperature is over 150 ℃. For this reason, Well Longgang-61 adopted the following drilling techniques: using 6-spud system and unconventional casing program to isolate various complex intervals, effectively preventing and controlling downhole geologic risks; using air drilling in leakage-prone formations in Penglai Town-Shaxi Temple Formation to prevent fluid loss and prevent deviation and ensure fast drilling; for formations with strong abrasivity in Ziliujing Formation and Xujiahe Formation, the drilling bit desing was optimized and accelerating test was performed for PDC drilling bit in slim hole section; and optimizing high temperature poly-suphonated water-based drilling fluid so as to provide it with strong capability in resistance to high temperature, salt and calcium. Practice shows that the drilling technology for unconventional ultra-deep well Longgang-61 not only addressed the drilling difficulty caused by formation complexity and realized safe and fast drilling to the target zone, but it was innovated in drilling of ultra-deep wells in Sichuan and Qiongchong areas, creating a set of complete drilling technologies for ultra-deep wells.