Mechanical and dissociation properties of methane hydrate bearing sands with various fines

Abstract:

A high pressure and low temperature plane strain testing apparatus was also developed for visualizing the deformation of methane hydrate (MH) 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. Tc and Toyoura sand were the materials used for comparison. 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. The local deformations occurred more clearly in Toyoura sand, compared with fine material. It also appeared more clearly when the specimen contained MH. 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.