The Well Shengke-1 is one of ultra-deep wells with total depth of 7026m, bottom hole temperature of 235 ℃ and more than pressure of 100MPa. Ultra-deep well drilling fluid faces many serious challenges. Severe engineering problems would arise if the drilling fluid is not treated properly and even the success of the well is determined by the drilling fluid. The difficulties including complex formation, long barefoot borehole, and ultra-high temperature were confronted. The stratums of φ215.9mm hole included section KongDian 1 and section KongDian 2, in which the clay minerals in upper shale stratum consist of mainly mixed layer of inordinate illite and montmorillonite. Therefore, the troublesome conditions could probably occur during drilling. The open-hole interval was as long as about 3000m, and pressure indices of formations varied enormously. In upper stratum, after high pressure brine formation had been drilled at 4598m, the density of drilling fluid increased to 1.83g/cm 3 . Meanwhile, the pressure index of under-stratum was about 1.30g/cm 3 , so lost circulations had happened many times in high differential pressure. The high temperature of the drilling would induce complicated reaction between the clay and drilling fluid so as to slough of the clay. During drilling yield point and gel strength increased quickly with the effect of high temperature. The Poly-sulfonate plugging and anti-sloughing (5155~4598m), high density poly-sulfonate non-invasive (4598~5800m), and ultra-high temperature and super high density drilling fluid( 5800~7026m) had been used in 215.9 mm interval of the well, respectively. The drilling fluid kept stationary for a long time during completion which caused drilling fluid thickening, dehydration and mud cake thickening. The solid-free anti-high temperature completion fluid was used to guarantee the success of logging and casing. It was proved in the field application of Shengke1 that the drilling fluids exhibited a series of advantages, including excellent rheology behavior, good thermal stability and low filtration rate. It was easy to treat, formulate and maintain, and could deal with the salt and brine contamination. It had been stable under 235℃ of the BHT in Shengke1, the highest temperature encountered in China at that time, and guaranteed the well to be completed successfully.The drilling fluid formulation prepared through lab selection and compatibility experiment includes: 3% bentonite + 0.2% KPAM + 0.1% PAC-141 + 1% emulsifier + 2% SR-1 + 3% SD-102 + 4% KFT + 3% DYWD + 6% lubricant + 2% glycol.High density poly-sulfonate noninvasive drilling fluid(4598~5800m). High temperature (180~200 ℃, at the bottom) andsloughing characterized this interval. The drilling fluid provided glycol anti-sloughing additives, silicon-fluorine stabilizer to enhance the inhibity, humic acid and sulfonate resin filtrate reducers to control the filtrate, noninvasive additives to plug shale microcrack effectively and reduce the filtrate into shale. The drilling fluid system, with strong inhibity, good ...