Application and breakthrough of large-size hole drilling technology in some exploration well in Bohai Oilfield

3D pressure study and the data of drilled wells in vicinity prove that there is high temperature and high pressure in the middle and deep formations in some block of Bohai Oilfield. While drilling some exploration well in this block, the formation pressure coefficient was 1.3, max. 1.65, and bottom temperature was up to 178 ℃. The wellbore configuration was optimized based on formation pressure and geologic features. When the ?444.5 mm large hole was drilled to the top of Guantao Formation with a vertical depth of2 700 m, the risk in drilling of HTHP formation in the middle and deep formations could be mitigated. In Bohai area, it was the first time that ?444.5 mm hole was drilled to 2 700 m. Through technologies like BHA optimization and trajectory control, drilling with seawater/bentonite fluid, optimized cement slurry system and fluid column structure, etc., breakthroughs were made in deviation control and fast drilling, drilling fluid technology and cementing technology while drilling this large size hole, which provides guarantee for drilling into the HTHP section and for conducting geologic assessment.