Study on wellbore instability mechanism and drilling fluid optimization of Yingsha block in southwest Tarim Basin

The southwest area of Tarim has good potential for exploration and development, but the geological conditions of Yingsha block are complicated and complex conditions occurred frequently in drilling operations. There were 11 times lost circulation, which ranged from N2 a to E1 a and the lithology ranged from mudstone and sand-mudstone interbeds to gypsum rock, and the drilling cycle is up to 719 days, which seriously restricts the exploration and development of oil and gas resources. In this paper, the rock mineral composition and physical and chemical characteristic profiles of the whole well have been established for the well drilled in this area. The characteristic profiles show that quartz (21.4%~57.4%) and clay minerals (17%~44.9%) are the main layers in the lower strata (5069 m-7015 m) drilled in Yingsha. The clay minerals are dominated by illite (58%~85.75%) without smectite. The highest proportion of illite/smectite formation is 25%, the highest expansion rate is 23.87%, and the lowest recovery rate is 3%. Therefore, the stratum has strong hydration and dispersion characteristics. The average porosity of 4064 m-6666.5 m is 1.86%-6%, and the formation cracks are well developed. The wellbore instability mechanism in Yingsha is the good physical property of the sandstone, the development of cracks and micro-cracks in the lower strata, the broken strata, and the strong hydration and dispersion ability. The performance evaluation of KCL-Polysulfonate drilling fluid used in Yingsha shows that the recovery rate increases to more than 80% when the amount of KCL is 7–10%. With the introduction of paraffin nano-emulsion, the filtration loss is reduced by 30%, and the inhibition and plugging performance of drilling fluid is further improved.