The mountainous high steep structure in Tarim Basin, as a result of orogenesis, owes presses solid, structure extrusion factor and so on, caused the geology conditions, which the drilling operation must cope with, are very complex: The reservoir buried so deeply (the maximum depth presented 7600m), And there developed an very thick evaporate bed as coverage in Tertiary, which buried from 1526m to 7070m. So the formation pressure coefficient to reach to as high as 2.47, the density of drilling fluid used during operation reached as high as 2.55 sg the most, and BHT exhibited 186°C (Tests on site). Another problem is that there exhibited extensive interconnected fractures in the reservoir, which caused the serious loss circulation and overflow occurred frequently during drilling. Faced so many extreme formidable drilling conditions, This paper identifies and discusses the major HP/HT/HS drilling fluid challenges, the ultra high-density water-based drilling fluid designs, the key laboratory studies about the fluid's properties, and other considerations in HP/HT drilling.

Up to now, about 23 ultra-deep wells (>6000m) and 3 ultra-deep wells (>7000m) have been completed in Tarim, which are achieved by the application of a series of advanced techniques, especially the High-performance drilling fluids, which are briefly described in this paper, especially the results and observations are presented in YS-1(total depth is 7258m), DB-3 (total depth is 7090m) and KS-2(total depth is 6780m) wells. Finally the paper identified some of the drilling fluid challenges that still lie ahead during the ultra-deep drilling process.

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