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 °C, 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 by bottom -up design method, featuring five-layer casing and one-layer expandable casing. To enhance wellbore stability, advanced drilling fluids were formulated: strongly inhibitive and low-friction water-based drilling fluids for the upper combination, 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, a structured real-time optimization workflow was implemented for drill bit selection, BHA design, and drilling parameters, involved continuous analysis of rock mechanical properties, MSE, and stick-slip vibrations to navigate the critical trade-off between ROP and downhole tool integrity. Field validation across three appraisal wells demonstrated statistically significant performance improvements: 88% increase in average ROP, 32% reduction in drilling cycle time, and 81% decrease in NPT ratio. This integrated approach provides a replicable technical framework for economically developing ultra-deep resources in analogous fold-thrust belt settings globally.