Dissolved Methane Diffusion Drives Hydrate-Bearing Pressure Core Degradation during Long-Term Storage in Water

The UT-GOM2–1 Hydrate Pressure Coring Expedition collected methane hydrate-bearing sediments at in situ pressure from the deepwater Gulf of Mexico, Green Canyon Block 955 (GC 955), in 2017. These pressure cores were stored in methane-free fresh water at ∼24 MPa and 6 °C, well within the methane hydrate stability condition. X-ray images of these pressure cores showed varying degrees of reduction in the core diameter in 2019. In the storage water, dissolved methane concentration increased from fresh water to 78–95% of the methane solubility over 5 years. We show by comparing observations with 2D numerical simulations that pressure core degradation is driven by dissolved methane diffusion and methane hydrate dissolution. The degradation is controlled by two time scales. Initially, radial diffusion dominates as methane diffuses into the annulus. This process is fairly rapid and is on the time scale of tens of days. Thereafter, continued vertical diffusion occurs as methane diffuses upward into the overlying large volume of storage water. While methane hydrate is lost from the edge of the core, the inner core still contains methane hydrate at the original saturation. To improve future pressure core preservation, we suggest reducing the inner radius of the storage chamber, sealing the open end of the core liner, using methane-saturated storage water, and reducing the storage temperature.