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Dissociation of methane hydrate crystals in pore space caused by temperature increase or depressurization, which is one of the main concerns in developing techniques to exploit natural gases in clathrate hydrate in sediments, was observed by using a glass mimic of sediments. Hydrate crystals were formed at a system temperature of T=274.0 K and system pressure of p=12.5 MPa or T=279.6 K and p=10.1 MPa and then decomposed by increasing the system temperature to 289 or 290 K, respectively. Two experimental runs of depressurization were also conducted. Hydrate crystals formed at T=282.9 K and p=10.1 MPa or T=274.0 K and p=12.5 MPa were decomposed by depressurization to 1.7 or 0.5 MPa at a system temperature of 285.5 or 274.3 K. The glass mimic of sediments (glass micromodel) was composed of a quartz glass plate on which straight microchannels were carved and another quartz glass plate covering the microchannels. The microchannels were approximately rectangular in cross section with a top width of 1.0×102 μm and were arranged in a grid pattern at regular intervals of 2.0×102 μm. Methane gas released from dissociating methane hydrate crystals was commonly observed to form slugs that completely occupy the pores in all experiment runs. The mechanism of the dissociation of the methane hydrate crystals was discussed considering the heat and mass transfer of methane around the dissociating methane hydrate crystals.

An experimental study was performed to visually observe the driving force dependence of hydrate growth in a porous medium filled with either liquid water and dissolved CO 2 or liquid water and gaseous CO 2 . The given system subcooling, ÁT sub , i.e. the deficiency of the system temperature from the triple CO 2 ÀhydrateÀwater equilibrium temperature under a given pressure, ranged from 1.7 K to 7.3 K. The fine dendrites initially formed at ÁT sub ¼ 7.3 K changed quickly into particulate crystals. For ÁT sub ¼ 1.7 K, faceted hydrate crystals grew and the subsequent morphological change was hardly identified for an eight-day observation period. These results indicate that the physical bonding between hydrate crystals and skeletal materials becomes stronger with decreasing driving force, suggesting that the fluid dynamic and mechanical properties of hydratebearing sediments vary depending on the hydrate crystal growth process.

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SUMMARY A model for the stress‐dependent elastic wave velocity response of fractured rock mass is proposed based on experimental evidence of stress‐dependent fracture normal and shear stiffness. Previously proposed models and previous experimental studies on stress‐dependent fracture stiffness have been reviewed to provide a basis for the new model. Most of the existing stress‐dependent elastic wave velocity models are empirical, with model parameters that do not have clear physical meanings. To propose the new model, the rock mass is assumed to have randomly oriented microscopic fractures. In addition, the characteristic length of microfractures is assumed to be sufficiently short compared to the rock mass dimensions. The macroscopic stress‐dependent elastic wave velocity response is assumed to be attributed to the stress dependency of fracture stiffness. The stress‐dependent fracture normal stiffness is defined as a generalized power law function of effective normal stress, which is a modification of the Goodman's model. On the other hand, the stress dependency of fracture shear stiffness is modeled as a linear function of normal stress based on experimental data. Ultrasonic wave velocity responses of a dry core sample of Berea sandstone were tested at effective stresses ranging from 2 to 55 MPa. Visual observation of thin sections obtained from the Berea sandstone confirms that the assumptions made for microstructure of rock mass model are appropriate. It is shown that the model can describe the stress‐dependent ultrasonic wave velocity responses of dry Berea sandstone with a set of reasonable material parameter values. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.