Physical property changes in hydrate‐bearing sediment due to depressurization and subsequent repressurization

Physical property measurements of sediment cores containing natural gas hydrate are typically performed on material exposed, at least briefly, to non‐in situ conditions during recovery. To examine the effects of a brief excursion from the gas‐hydrate stability field, as can occur when pressure cores are transferred to pressurized storage vessels, we measured physical properties on laboratory‐formed sand packs containing methane hydrate and methane pore gas. After depressurizing samples to atmospheric pressure, we repressurized them into the methane‐hydrate stability field and remeasured their physical properties. Thermal conductivity, shear strength, acoustic compressional and shear wave amplitudes, and speeds of the original and depressurized/repressurized samples are compared. X–ray computed tomography images track how the gas‐hydrate distribution changes in the hydrate‐cemented sands owing to the depressurizaton/repressurization process. Because depressurization‐induced property changes can be substantial and are not easily predicted, particularly in water‐saturated, hydrate‐bearing sediment, maintaining pressure and temperature conditions throughout the core recovery and measurement process is critical for using laboratory measurements to estimate in situ properties.