| In recent years, with the exploration and exploitation of deep oil and gas fields, drilling technique is becoming more and more difficult. One of the problems is the high temperature complicated formations are usually encountered in deep well. In order to meet the needs of high temperature complex formation drilling, it's urgent to enhance ultra-high temperature drilling fluid technique.Well Bishen1 is a main ultra-temperature and ultra-deep exploration well in Henan oilfield of CPCC in 2008. The actual bottom-hole temperature is 241℃(the domestic highest record so far), and the total depth is 6005 meters. The well drilling process faced with the problem of ultra-high temperature water base drilling fluid technology, the wellbore instability problem in deep zone also can not be neglected.In view of the well Bishen1 condition, this paper discussed ultra-high temperature drilling fluid and the wellbore instability mechanism of deep zone. Then the ultra-high temperature drilling fluid formula with good rheology, high inhibition, low filtration and strong anti-pollution were optimized, and run successful field test in well Bishen1.Firstly, the rock sample fabric of Bi Yang sag was analysed through X-ray diffraction and SEM, the relation between the deep reddish-brown shale surface hydration water volumes and compaction strength was investigated and the mechanism of wellbore instability mechanism of deep zone was generalized. Then the influence of temperature, pH and bentonite content to the slurry rheology was studied, discussed the relation between the HTHP filtration and cake. Based on single additives optimising and compatibility experiments, two ultra-high temperature water based drilling fluid systems were optimized. The comprehensive performance evaluation results of indicated that two systems have good rheology, anti-temperature,high inhibition and anti-pollution, which met the needs of the well Bishen1 high temperature formation drilling.
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