Temperature Increase during the Depressurization of Partially Hydrate-Saturated Formations within the Stability Region

Depressurization experiments of methane-hydrate-bearing sediments were conducted to measure changes within the hydrate stability region before dissociation started. Pore-filling hydrate with a saturation of about 40% was formed in water-saturated silica sand samples with a porosity of 0.4. The initial pressure was kept constant at around 14.5 MPa, and the initial temperature was varied between 281.3 and 283.7 K. When the samples were depressurized at rates ranging from -1.2 to -3.9 MPa/min, temperature increases of 0.26-0.37 K on average were measured. These were caused by a decreasing aqueous methane solubility as well as the liberation of isolated free gas, leading to additional heat-releasing hydrate formation (Delta HMH-f = -51.86 kj mol(-1) of CH4 at 280 K). The results suggest that the solubility obeys Henry's law as long as free methane is present in the pore space. The temperature changes during depressurization shift the equilibrium pressure, leading to an anticipated dissociation during the process. The increase in hydrate saturation, in turn, reduces the effective permeability of the formation, reducing the extent to which a formation can be depressurized by a single vertical wellbore. Sensitivity studies for the above state sediment conditions show that the induced increase in the temperature raises the equilibrium pressure of up to 0.7 MPa and raises the hydrate saturation by 7%, for an initial in situ pressure of 35 MPa and varying temperatures.