Research and Application of Drilling Technologies for Complex Ultra-Deep Well in Southern Margin Fold and Thrust Belt of Junggar Basin

The southern margin fold and thrust belt, a pivotal structural element within the multi-stage superimposed oil-bearing sedimentary basin of China's Junggar Basin, represents one of the most challenging geological structures for ultra-deep hydrocarbon exploration in China. This region is is distinguished by steeply dipping formations, burial depths exceeding 7000 m, temperatures surpassing 170 ℃, and pressures greater than 140 MPa. The geological complexity is further amplified by intricate formation pressure systems and significant lithological heterogeneity. This study concentrates on the Hutubi Anticline within this belt to address significant drilling challenges, including inaccurate formation pressure prediction, wellbore design difficulties, low drilling speed, high incidents rates of lost circulation and wellbore instability. A novel formation pressure prediction methodology was developed based on a three-dimensional velocity model and abnormal high-pressure formation mechanisms, utilizing successive constraint calculations of logging sound velocities, seismic layer velocities, interval velocity, and the principles of original sedimentary loading and unloading mechanics. By scrutinizing the sealing position of the wellbore structure, an unconventional wellbore structure was designed, featuring five-layer casing and one-layer expandable casing, employing a bottom-up design approach. To enhance wellbore stability, advanced drilling fluids were formulated: strongly inhibitive and low-friction water-based drilling fluids for the upper combination, and highly sealant and temperature-resistant oil-based drilling fluids for the middle and lower combinations. Meanwhile, deformation-filling leakage prevention technology was implemented in these combinations. Additionally, it was optimized drill bit selection, drill string configurations, and drilling parameters through rock mechanical properties analysis, mechanical specific energy, and stick-slip vibrations analysis, effectively mitigating well deflection in the upper sections and improving slow drilling rates in the lower sections. Field tests demonstrated substantial improvements, reducing the drilling time for ultra-deep wells in the Hutubi Anticline from 556 days to 342 days and decreasing the complex incident time rate from 10.41% to 1.79%. This research provides valuable technical insights for ultra-deep well drilling in analogous geological formations.