The well Jiantan-1 is a key exploratory well of PetroChina located in the Jianshan structure of the central rhinoplasty belt in the Chaidamu Basin. The main purpose of this 6343 m well is to explore the gas reserves in the base rocks of the Jianshan structure. The highest bottom hole temperature measured is 235℃, one of the highest bottom hole temperatures encountered in onshore drilling. The formations drilled in this area have fractures that are fully developed, long sections of gypsum and mudstones, high pressure saltwater zones, high CO2
concentrations and high geothermal gradient. Borehole wall collapse, mud losses, overflow, creeping and tight hole in the gypsum/mudstone formations are downhole troubles encountered and sometime intertwined with each other during drilling. To deal with these problems, laboratory study was conducted on several aspects of the drilling fluid used, which are high temperature stability, rheology and high temperature filtration control. An organic salt drilling fluid was formulated with additives specially selected, such as ultra-high-temperature filter loss reducers, anti-collapse additives with plugging capacity, lubricants and thermal stabilizers etc. This drilling fluid is able to resist high temperatures up to 240℃. After aging for 72 hours, the HTHP filtration rate was maintained with 10 mL, the coefficient of friction was less than 0.1, and the invasion depth on a sand-bed less than 12 cm. In addition, the fourth and fifth intervals of the well penetrated formations with micro-fissures and a basal rock weathered crust, and mud losses occurred many times into the fissures. A lost circulation material slurry for the well Jiantan-1 was then developed with choice rigid, flexible and gel-like plugging additives which are suitable for use at elevated temperatures. Using this mud loss control slurry, the pressure bearing capacity of the formations into which mud losses have occurred was finally enhanced. The organic salt drilling fluid and the mud loss control slurry played a major role in preventing and controlling borehole wall collapse and mud losses, ensuring the success of the drilling of the first ultra-high temperature Jiantan-1 well in Chaidamu Basin.