Fracture-Controlled Gas Leakage through the Hydrate Stability Zone under Coupled THMC and Salinity Effects in Subsea Sediments

Natural gas hydrates are crystalline, ice-like compounds formed by water molecules arranging into cage-like lattices that encapsulate gas molecules under low-temperature and high-pressure conditions typical of continental margins. Within these environments, free gas migrating upwards is generally expected to be trapped upon entering the hydrate stability zone (HSZ) through hydrate formation. Nevertheless, extensive geological and geophysical observations indicate that free gas can traverse the HSZ and escape at the seafloor, suggesting the presence of dynamic leakage mechanisms that are not yet fully understood.

In this study, we develop a fully coupled thermal–hydro–mechanical–chemical (THMC) framework [1] that explicitly incorporates salt transport and hydrate generation and apply it to a three-dimensional subsea geological model. The model is used to investigate gas migration and leakage through the HSZ under realistic pressure–temperature conditions. Simulation results reveal that gas leakage is governed by a transient, fracture-controlled process. Initial hydrate formation locally reduces permeability, acting as a temporary barrier that traps migrating gas and promotes progressive pore pressure build-up beneath HSZ. Continued pressurization compromises sediment mechanical stability, triggering fracture initiation and propagation.

Following fracture development, gas preferentially migrates through these newly formed high-permeability pathways, bypassing the surrounding low-permeability hydrate-bearing sediments. Within the fractured zones, rapid gas invasion promotes local hydrate formation, which is inherently self-limiting. Hydrate growth results in a progressive reduction in local water saturation, while salt is excluded from the hydrate phase and accumulates in the remaining pore fluid. The combined effects of water depletion and salinity increase thermodynamically suppress further hydrate formation, even under favourable pressure–temperature conditions. At the margins of the fractured zones, hydrate saturation becomes locally elevated, forming low-permeability hydrate-rich barrier that effectively restrict lateral water supply and salt diffusion into the fractured zone. This spatial heterogeneity in hydrate distribution reinforces the persistence of gas-conductive pathways within fractures zone. In contrast, the central parts of fractured zone remain characterised by high gas saturation and limited hydrate accumulation, preserving high gas relative permeability and enabling sustained gas flow through the hydrate stability zone.

As gas continues to be supplied, pore pressure progressively increases within and beneath the existing fracture network. This renewed pressurisation promotes further mechanical weakening of the surrounding sediments, leading to the second and more fractured zones. Ultimately, the development of interconnected fracture networks allows free gas to breach the hydrate stability zone and reach the seafloor, resulting in gas leakage into the overlying water column. Once these fractures connect to the seafloor, natural gas is released, causing leakage into the overlying water column.

Therefore, the limited water availability and salinity effects on hydrate formation are fundamental controls on gas leakage through the HSZ, as they restrict further hydrate growth and accelerate more generation of fractures, thereby maintaining highly permeable pathways for gas migration. This highlights the importance of fully coupled THMC processes with considerating salt transport in assessing subsea gas escape and associated geohazards.

[1] L. Zhang, B. Wu, Q. Li, Q. Hao, H. Zhang, Y. Nie, A fully coupled thermal–hydro–mechanical–chemical model for simulating gas hydrate dissociation, Applied Mathematical Modelling, 129 (2024) 88-111.