Analysis and Research on Borehole Shrinkage Mechanisms and Their Control in Directional Wells in Deep Composite Salt Formations

The composite salt formations in West China are characterized by the deep burial, complex tectonic stress, and alternating development of salt rock and mudstone, and salt rock creep during drilling poses great challenges to the safety of drilling. This research is aimed at the serious blockage and sticking in build-up sections in deep composite salt formations. A 2D mechanical model for inclined wells penetrating multiple vertical layers was established by combining the analysis of creep mechanisms of composite salt formations with the high-temperature creep experiment results so as to investigate the wellbore stability of directional wells in composite salt formations in a certain block from the perspective of rock mechanics and probe the influence of inclination angles on borehole shrinkage under different in situ stress and interlayer conditions. Moreover, a directional well in the block was taken as an example to analyze the creep and borehole shrinkage in composite salt formations. The method for wellbore stability control was proposed, and the drilling fluid density chart was established. This research reveals the relationship among the wellbore trajectory, the degree of borehole shrinkage, and drilling fluid density in build-up sections of directional wells in composite salt formations and provides a theoretical reference for the establishment of the method for controlling blockage and sticking accidents in directional wells.