Drilling fluid for ultra-deep wells with non-standard large borehole:Optimization and its application to FT-1 well
Drilling data on Gaofengchang structure in eastern Sichuan Basin indicates some phenomena frequently in drilling,such as contaminated drilling fluid as well as downhole complexities and sticking after gypsums creepage.In addition,there is no available reference data due to only few wells completed in Xiaoheba Formation and its below,and complex geological conditions with poor predictability.Being the first exploration one with non-standard large borehole in this area,FT1 well was also confronted with well-bore instability,large section of mud-making and gypsum-salt pollution,low annular velocity and heavy drilling-fluid volume,making its maintenance difficult and more prominent challenges in preventing leakage,blowout,and high-pressure differential sticking from the Carboniferous goaf and its below with abnormal high pressure.Thus,both drilling-fluid formula and maintenance process were analyzed through some indoor experiments in order for safe and fast drilling together with exploration and development of ultra-deep reservoirs as soon as possible.Moreover,combined with on-site maintenance,the optimized drilling fluid and its maintenance process were established for non-standard large-borehole section.Results show that this optimization is superior to traditional drilling fluid in such performances as collapse prevention,sand-carrying capacity,inhibition,temperature resistance,pollution resistance,sealing and lubrication.As a result,many problems can be solved with effect,including gypsum-salt pollution,wellbore instability,lost circula-tion,and sticking.ROP is improved evidently after its application,and complexities no longer occur.
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