Abstract: There are some issues in the upper strata of Qiongzhusi Formation of Jingyan-Jianwei Area, southern Sichuan Basin, such as complex pressure systems, wellbore instability, and poor drillability, etc, which lead to prolonged drilling cycles and high well construction costs. To deal with these issues, we established the tri-pressure profile and complex formation sections in the Jingyan-Jianwei Area by analyzing data of drilled wells. The necessary sealing points for the first and second were optimized, and a conventional wellbore structure scheme for this region was formulated. Based on the goal of cost reduction, a slimming scheme with a $\phi$190.5 mm wellbore for well completion was proposed. Through hydraulic and pipe string mechanical analyses, the adaptability evaluation of the slimming wellbore structure was conducted. For single-well implementation, the first and second spuds of the slimming structure well employ $\phi$127.0 mm drill pipes, and the third spud adopts $\phi$127.0 mm+$\phi$114.3 mm drill pipes. For platform well, the first and second spuds of the slimming structure well utilize $\phi$139.7 mm drill pipes, and the third spud adopts $\phi$127.0 mm+$\phi$114.3 mm drill pipes. When the well deviation angle is less than or equal to 90°, a horizontal well with a horizontal section length of 3 000 m can be implemented. By applying the conventional wellbore structure scheme, the actual drilling cycle was shortened by 21.68% compared to the previous stage. If the slimming wellbore structure is adopted, it is expected that the cost of a single well can be reduced by 1.8 million yuan. The research and technical achievements can offer significant optimization directions for the subsequent drilling engineering design of the Qiongzhusi Formation shale gas, and are anticipated to reduce the well construction costs, facilitating the efficient development and large-scale production of shale gas resources in the Qiongzhusi Formation of Jingyan-Jianwei Area.

Key words: optimization of sealing points, slimming of wellbore structure, drilling assembly, horizontal extension limit, shale gas in the Qiongzhusi Formation