Abstract: During the drilling of a buried hill fractured limestone formation in an area of China Sea,a severe circulation lost occurred. The pressure-control drilling technology was used halfway through the drilling process,and the returned mud contained high levels of hydrogen sulfide gas,making it impossible to proceed according to the relevant well control requirements. Later,the seawater drilling process was used and annular liquid lever was measured 50 meters below the rotary table surface. However,during drilling,hydrogen sulfide from the formation continuously entered the wellbore and was replaced at the wellhead,posing a threat to the safety of on-site operators and drilling tools. According to calculations,the equivalent density of formation leakage pressure in this well section is less than the equivalent density of pore pressure,and there is no safe operating pressure window. In addition,during drilling,there was no return of rock debris due to severe leakage. In order to solve this technical problem,the mud cap drilling process was implemented on site. During the water drilling period,on the one hand,a thick stagnant gas plug containing a sulfur removal agent was intermittently pumped from the wellhead to the annular space to solve the risk of hydrogen sulfide overflow. On the other hand,the BHA was simplified,drilling parameters were optimized,and drilling speed was controlled to reduce the size of rock cuttings. Finally,the drilling operation was successfully completed for 940 m.

Key words: crack in buried hill, severe leakage, hydrogen sulfide, mud cap