During the drilling process of Yingmaili's Paleogene strata,complex underground troubles, such as sticking,sloughing,collapse,loss of circulation,occurred frequently,dragging down drilling rate. In order to determine the mechanism of wellbore instability in Paleogene strata of Yingmaili, and provide a theoretical basis for the selection or optimization of performance and types of drilling fluids,systematic a-nalysis had been completed by experiments of clay mineral composition and their physical and chemical properties. Combined with the study of the drilling fluid technology for gypsum salt formation, the KCl-po-ly sulfonamide unsaturated brine drilling fluids was optimized, the technical requirements on the drilling fluids in maintaining wellbore stability was put forward. Field application results in the Well YM2-6 showed that the KCl-poly sulfonate unsaturated brine drilling fluids could meet the requirements of keeping wellbore stability in the Paleogene gypsum mudstone, and reduce the risk of downhole troubles; meanwhile , expansion of the clay minerals was not the main reason of borehole instability in mudstone. Through the research on instability mechanism of the Paleogene strata in this area, the risk of borehole instability had been reduced,and provided the theoretical basis for smooth drilling in this formation.%英买力地区古近系地层在钻井中频繁出现卡钻、掉块、坍塌、井漏等井下故障,严重影响了钻井速度.为了弄清英买力地区古近系地层的井壁失稳机理,为钻井液选型和性能优化提供理论依据,对该地区黏土矿物组成及理化性能进行了系统分析.结合该地区盐膏层钻井液技术研究,对KCl-聚磺欠饱和盐水钻井液体系性能进行了优化,提出了该钻井液体系在维护井壁稳定方面的技术要求.YM2-6井现场应用结果表明,KCl-聚磺欠饱和盐水钻井液体系能够满足古近系膏泥岩段井壁稳定的要求,减小了发生井下故障的风险；黏土矿物的膨胀并不是导致膏泥岩段井壁失稳的主要原因.通过研究该地区古近系地层的井壁失稳机理,减小了井壁失稳的风险,为古近系地层的顺利钻进提供了理论支持.
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