Drilling Fluid Research for Paleogene Strata in Yingmaili Area

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 analysis 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-poly 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.