Dynamic Coupling Mechanisms and Collapse Risk Mitigation in Gas Hydrate-Bearing Reservoirs: A THMC Framework for Safe Deepwater Drilling

Abstract Natural gas hydrate (NGH) has emerged as a strategic alternative energy resource for the 21st century, with the South China Sea containing substantial reserves estimated at 80 billion tons of oil equivalent, 2.5 times those found in terrestrial permafrost regions. However, its exploitation faces critical wellbore stability challenges due to shallow hydrate occurrence in unconsolidated sediments with a Safe Mud Weight Window (SMWW) narrower than 0.03 g/cm3. Existing research remains limited by single-physics models that fail to capture coupled feedback mechanisms between hydrate dissociation and formation deformation. Here, we develop a sequential thermo-hydro-mechanical-chemical (THMC) coupling framework with two-way data exchange between TOUGH + HYDRATE and FLAC3D to systematically investigate wellbore stability evolution under drilling fluid exposure. Numerical simulations demonstrate that drilling-fluid invasion promotes near-wellbore hydrate dissociation through coupled thermal, hydraulic, and chemical disturbances, with transient self-stabilization occurring as dissociation-generated gas accumulation elevates pore pressure. Time-dependent analysis reveals that larger wellbore diameters significantly accelerate collapse timing, while underlying gas layers fail faster than hydrate layers due to lower mechanical strength. Prolonged drilling fluid exposure progressively narrows safety windows, with 7-day soaking reducing SMWW by 41–57% across different formations through coupled collapse pressure increase and fracture pressure decrease. The THMC framework identifies formation weakening mechanisms through yield zone evolution and progressive stress migration from near-wellbore to far-field regions. These findings establish a quantitative framework for predicting time-dependent wellbore stability in hydrate formations, providing critical insights for safe drilling operation design in deepwater gas hydrate exploitation.