Optimization of well drilling and completion engineering technologies for West Wenjisang ultra-low-pressure underground gas storage

The Wenjisang underground gas storage (UGS) group in the Tuha oilfield is the first UGS group in West China. In the West Wenjisang No.1 UGS, the first-phase project of the Wenjisang UGS group, the target Jurassic Xishanyao Formation gas reservoir exhibits the pressure coefficient of only 0.2–0.22 after years of recovery, the overlying caprocks of the Qigu Formation to Sanjianfang Formation contain thickshales which are extremely prone to hydration and spalling, and the Xishanyao Formation reservoir is seen with severe lost circulation, which bring great challenges to high-quality fast well drilling and completion operations. To deal with these challenges, three optimization schemes of intermediate casing depths were investigated from the persepctives of ROP enhancement by using large-size composite bits and high-buildup-rate drilling tools, ultra-low-pressure reservoir protection and safe drilling technology featuring polyamine drilling fluid + lost circulation control, and cementing/completion techniques such as slurry column structure optimization and casing centralization. Field applications show that, compared with that of pilot test wells, the average drilling period is shortened by 44 days, and the average single-well loss is reduced by 1094 m³, suggesting a significantly improved cementing quality of intermediate casing in horizontal wells. The presented technology and solutions provide technical references for well drilling and completion of analogous UGSs in China.