Mechanical characteristics of natural and artificial gas hydrate bearing sediments

Methane gas hydrate deposits have been discovered in the eastern Nankai trough near Japan. It is hoped that the hydrates will be extracted using a depressurization method. However, it is not yet known if the dissociation of gas hydrates will deform sediment strata and cause well-bore instability. This paper presented the results of tri-axial tests conducted using drill cores of gas hydrate-bearing sediments from the Nankai trough. The mechanical characteristics and differences of natural gas hydrates and artificially-formed hydrates were also investigated. The tri-axial testing apparatus had a capacity of 200KN with a maximum confining pressure and back pressure of 20MPa. Data acquisition was performed to measure the displacements in vertical and lateral directions, as well as to determine load, confining pressure, back pressure, and temperatures. Tri-axial compressive strength, elastic modulus, and Poisson's ratio were determined, and the pore saturation of the gas hydrate was calculated by measuring the volume of methane released during gas hydrate dissociation. The study showed that tri-axial compression tests on natural hydrate core specimens showed good agreement with the artificial methane hydrate sediments. However, volumetric changes caused by a dilatancy effect were more prominent in the artificial hydrate and sediments.4 refs., 3 tabs., 13 figs.