Safe and Efficient Drilling in Presalt High-Sulfur Reservoirs in the Eastern Gas Fields of Ordos Basin

The main presalt high-sulfur reservoirs are located in the Ordovician Majiagou Formation, eastern gas field of the Ordos Basin, interbedded and developed with carbonate zone and gypsum zone. In addition, broken clastic rock, large interval of gypsum, and formations with abnormal pressure are observed. Further, wellbore collapse, tight hole, high-pressure natural gas containing H₂S invasion, lost circulation, stuck drilling, and other downhole failures and complex situations are easily encountered while drilling, leading to a low rate of penetration (ROP) and difficulties in safe and efficient drilling. Therefore, the casing program optimization design of “special drilling for particular reservoir” was carried out, which integrated high H₂S well control equipment, blowout prevention equipment, and equipment standards. Combination drilling, anti-deviation and fast drilling technologies, i.e. “high-efficiency polycrystalline diamond compact (PDC)bit + high-torque single-curved screw +measurement while drilling(MWD) + drilling jar” were studied and applied. A pilot test used precisely managed pressure control drilling technology was carried out. The high-density saturated brine drilling fluid system with strong anti-salt, anti-calcium, and anti-mud was optimized and compounded, forming the technology for safe and efficient drilling of presalt high-sulfur reservoirs in eastern gas fields of Ordos Basin. The technology was applied in five wells, with an average drilling duration of 54.3 d and an average ROP of 8.35 m/h. Specifically, there was an average drilling duration of 11.67 d in the third section, 52.03% shorter than that of the early drilled well, MT1, an average ROP of 6.85 m/h for the reservoir interval, 69.69% higher than that of MT1, and an average hole enlargement ratio of 6.16%. The electrical logging had a success rate of 100% for a first run. There is no downhole failure or complex situation. It provides a technical support for safe and efficient exploration and development of presalt high-sulfur gas reservoirs in the eastern gas fields of Ordos Basin.