Research on Sensitivity Factors of Wellbore Stability During Directional Well Construction in Deepwater Shallow Soft Hydrate Reservoirs

To investigate wellbore stability evolution during directional drilling in deepwater shallow soft hydrate reservoirs, a coupled thermo–hydro–mechanical–chemical (THMC) model incorporating hydrate phase change was established. The effects of drilling time, drilling fluid temperature, density, and well inclination were analyzed. The results indicate that wellbore collapse alters heat transfer paths, thereby expanding hydrate dissociation and plastic zones. With increasing drilling time, accumulated plastic deformation significantly intensifies wellbore instability. Maintaining drilling fluid density within an optimal range is the most critical factor for stabilizing the wellbore, and lowering the drilling fluid temperature can effectively reduce the hydrate dissociation extent and the borehole enlargement rate. Notably, well inclination exhibits a strong nonlinear influence: as inclination increases from 0° to 75°, the maximum plastic strain increases from 0.0047 to 0.0262, and the borehole enlargement rate rises from 38.1% to 152.6%; stability deteriorates dramatically beyond 30°. Higher initial hydrate saturation and greater water depth are beneficial to wellbore stability, whereas increased overburden thickness intensifies stress concentration. These results provide a reference for the directional drilling design of hydrate formations in deepwater shallow soft reservoirs.