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.