OS3A-3:EFFECT OF FINES ON MECHANICAL PROPERTIES OF METHANE HYDRATE BEARING SEDIMENT

Masayuki HYODO, Shintaro KAJIYAMA, Akira KATO,, Yukio NAKATA ,  Norimasa YOSHIMOTO
Department of civil & Environmental Engineering, Yamaguchi University, JAPAN

    The temperature-controlled high pressure triaxial testing apparatus was developed such that the back pressure and confining pressure could be controlled under various temperature and high pressure conditions in order to examine the mechanical behaviour of MH-bearing sand specimens under deep seabed stress and temperature conditions. A series of triaxial tests has been carried out to determine the mechanical properties and dissociation characteristics of sands with fines containing methane hydrate, using an innovative high pressure apparatus which has been developed to reproduce the in-situ conditions expected during proposed methane extraction methods. It was found that the strength of MH sand increased with MH saturation due to particle bonding and that the bonding effect was particularly dependent on the grain size of the host sand. Then a high pressure and low temperature plane strain testing apparatus was also developed for visualizing the deformation of methane hydrate bearing sand due to methane hydrate production. This apparatus can control temperatures and pressures equivalent to an MH reservoir in deep seabed. Additionally, observation windows are installed in front of and behind the specimen in order to allow the local deformation of the specimen during shear tests to be measured. The specimen is a cuboid with 80mm width, 60mm thickness and 160mm height. A 5mmｘ5mm mesh was drawn on the membrane in front of the observation window. Using this testing apparatus, plane strain compression and methane hydrate dissociation by depressurization tests were performed with the measurement of localized deformation. Local deformation analysis was performed by observing the cross-points of the mesh during shear tests and using this data in PIV analysis. Thermocouples were installed at 60mm and 30mm from the bottom of the specimen in order to measure the variation of the temperature during the dissociation of MH. Silica sand mixed with fine material and Toyoura sand were the materials used for comparison. The following conclusions were drawn by these experimental works.

    1. In drained shear tests on host sands, initial stiffness and strength decreased with increasing fines content and there was a strong trend for contraction of volumetric strain.
    2. For all specimens, the initial stiffness and peak strength of those containing MH increased due to MH`s cementation force, and volumetric strain behaved in a more dilative manner. However, both initial stiffness and strength decreased with increasing fines content of host sands.
    3. In plane strain shear tests, Tc and Toyoura sand as host sands with and without MH were compared. Due to the existence of MH, initial stiffness and strength increased in both materials, however the tendency was more apparent in Toyoura sand compared with Tc.
    4. The local deformations occurred more clearly in Toyoura sand, compared with fine material. It also appeared more clearly when the specimen contained MH.
    5. During depressurization, marked deformation was not observed, because of an increase of effective stress. However, after depressurization, repressurization caused the specimen to fail in the case of high initial shear stress conditions.