[1] Knowledge of the mechanical properties of gas-hydrate-bearing sediments is essential for simulating the geomechanical response to gas extraction from a gas-hydrate reservoir. In this study, drained triaxial compression tests were conducted on artificial methane-hydrate-bearing sediment samples under hydrate-stable temperature-pressure conditions. Toyoura sand (average particle size: D 50 = 0.230 mm), number 7 silica sand (D 50 = 0.205 mm), and number 8 silica sand (D 50 = 0.130 mm) were used as the skeleton of each specimen. Axial loading was conducted at an axial strain rate of 0.1% min −1 at a constant temperature of 278 K. The cell and pore pressures were kept constant during axial loading. We found that the strength and stiffness of the hydrate-sand specimens increased with methane hydrate saturation and with the effective confining pressure, and the secant Poisson's ratio decreased with the effective confining pressure. The stiffness depends on the type of sand forming the skeleton of the specimens, although the strength has little dependence on the type of sand. According to an earlier work, hydrate-sand specimens are thought to contract in the early stage of axial loading before starting to expand owing to the dilatancy effect, as is the case for many other geological materials. The test results in this study are discussed in relation to the deformation mechanism proposed in an earlier work.

[1] This study used proton nuclear magnetic resonance (NMR) to measure pore size distribution of sediment in order to characterize methane hydrate-bearing sediment by pore size distribution and permeability. Sand sediment with different grain size distributions was measured as fundamental research and for application of natural methane hydratebearing sediment. Sediment pore size distribution and porosity were measured by NMR and mercury porosimetry. The absolute permeability of sediment measured by water flow based on the Darcy law was compared with the permeability calculated from NMR spectra based on the sediment delivery ratio equation. NMR spectra were also analyzed by a conversion technique in order to obtain pore size distribution with higher spatial resolution.Citation: Minagawa, H., Y. Nishikawa, I. Ikeda, K. Miyazaki, N. Takahara, Y. Sakamoto, T. Komai, and H. Narita (2008), Characterization of sand sediment by pore size distribution and permeability using proton nuclear magnetic resonance measurement,

An experimental study of the dissociation of methane hydrate (MH) by hot-water injection and depressurization was carried out at the National Institute of Advanced Industrial Science and Technology (AIST). These experiments helped us understand some important aspects of MH behavior such as how temperature, pressure, and permeability change during dissociation and gas production. In order to understand the experimental results, a model of MH dissociation in a porous media was designed and implemented in a numerical simulator. In the model, we treated the MH phase as a two-component system by representing the pore space occupied by MH as a separate component. Absolute permeability and relative permeability were formulated as a function of MH saturation, porosity, and sand grain diameter and introduced into the numerical model. Using the developed numerical simulator, we attempted history matching of laboratory-scale experiments of the MH dissociation process. It was found that numerical simulator was able to reproduce temperature change, permeability characteristics, and gas production behavior associated with both MH formation and dissociation.

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It is essential to consider the mechanical behavior of natural gas hydrate reservoir to have sustainable production of natural gas from the reservoir under the seafloor. Although the mechanical properties of marine sediments containing natural gas hydrates are essential to simulate the geo-mechanical response to gas production from the reservoir, they are not fully understood. In this study, effect of confining pressure on mechanical properties of artificial methane hydrate bearing sediment was experimentally examined and discussed. Drained triaxial compression tests were conducted for artificial sediment specimens containing Toyoura sand (average particle size D 50 = 230 × 10 -6 m and fine fraction content F c = 0 %) and synthetic methane hydrate (hydrate-sand specimen), following the testing methods of Masui et al. (2005). During axial loading process in the tests, the specimen was compressed axially at 0.1 %/min of strain-rate under hydrate-stable condition, with 278 K of temperature and 8 MPa of pore water pressure. The cell pressure, or total confining pressure, was kept constant at 8.5 MPa, 9 MPa, 10 MPa or 11 MPa. Axial displacement and lateral displacement were measured with 25 mm and 5 mm linear variable differential transformers. Axial load was measured with a 50 kN load cell. The findings can be summarized as follows; (1) the hydrate-sand specimen becomes ductile by increase of confining pressure like many other geological materials; (2) the hydrate-sand specimen appears to be restrained from expanding diametrically by confining pressure; (3) strength and stiffness of the hydrate-sand specimen increases with confining pressure; (4) strength of No.8 silica sand containing methane hydrate does not greatly differ from that of hydrate-sand specimen in this study; and (5) cohesion and internal friction angle of the hydrate-sand specimen, or the Mohr-Coulomb failure criterion, can be formulated as a function of methane hydrate saturation. These findings concerning the effect of confining pressure will be of considerable help not only in full understanding of the deformation mechanism of methane hydrate bearing sediments, but also in proposal of constitutive equation and numerical simulation in the future.