Drilling-fluid technologies for borehole stability in ultra-deep wells,Yongjin oilfield,central Junggar Basin
The reservoirs of the Jurassic Xishanyao Formation in Yongjin oilfield are characterized by deep buried depth and com-plex geological structure.A casing program of the forth spud-in was adopted initially and later regulated to the third spud-in.Drilling complexities frequently happened in the second spud-in and below.Additionally,poor borehole stability,low drilling and completion efficiency,and severely contaminated reservoirs might constrain the advancement of field exploitation.Based on the analysis of chal-lenges in drilling-fluid technologies for ultra-deep wells in this block,some countermeasures were made for the borehole stability.To be specific,the polyamine drilling fluid with high inhibition was used in the second spud-in(with long openhole section)while the synthetic-based drilling fluid with strong sealing for the third(complex)one.Both of them achieved better effects after their eval-uation experiments and application.The experiments demonstrate these two kinds of drilling fluid with high inhibition and sealing,and the latter also for reservoir protection.And their application to 9 wells shows that(ⅰ)these drilling-fluid technologies suitable for ultra-deep wells ensure the stability in the second and third spud-in,as well as smooth tripping and mid completion or completion;(ⅱ)compared with prior drilled 8 wells,these 9 wells using the proposed technologies exhibit remarkable improvement in not only wellbore quality but drilling and completion efficiency,with the average drilling and completion cycle individually reduced by 69.61% and 65.29%,and the ROP enhanced by 80.88% on average,getting a positive effect in fast and optimal drilling;and(ⅲ)with little contamination,these technologies allow for satisfactory reservoir protection and attain highly commercial oil and gas flow from the 9 wells.And reserves in Yongjin oilfield have been drawn on for the first time.In conclusion,their triumphant application to ultra-deep wells in this field provides a powerful support to the subsequent exploitation and is also referential on such technologies for ultra-deep wells or certain wells with borehole instability in other blocks.
Yongjin oilfieldUltra-deep wellBorehole stabilityPolyamine drilling fluid with high inhibitionSynthetic-based drill-ing fluid with strong sealing
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