Kazuo Aoki

Triaxial compressive properties of artificial methane‐hydrate‐bearing sediment

[1]xa0Knowledge 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: D50 = 0.230 mm), number 7 silica sand (D50 = 0.205 mm), and number 8 silica sand (D50 = 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 Poissons 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.

VARIABLE-COMPLIANCE-TYPE CONSTITUTIVE MODEL FOR METHANE HYDRATE BEARING SEDIMENT

In order to evaluate a methane gas productivity of methane hydrate reservoirs, it is necessary to develop a numeric simulator predicting gas production behavior. For precise assessment of longterm gas productivity, it is important to develop a mathematical model which describes mechanical behaviors of methane hydrate reservoirs in consideration of their time-dependent properties and to introduce it into the numeric simulator. In this study, based on previous experimental results of triaxial compression tests of Toyoura sand containing synthetic methane hydrate, stress-strain relationships were formulated by variable-compliance-type constitutive model. The suggested model takes into account the time-dependent property obtained from laboratory investigation that time dependency of methane hydrate bearing sediment is influenced by methane hydrate saturation and effective confining pressure. Validity of the suggested model should be verified by other laboratory experiments on time-dependent behaviors of methane hydrate bearing sediment.

Estimation of the dynamic fracture process of rock material utilizing high speed photography

The experimental study is conducted to estimate fracture process of the cylindrical rock specimen. In this experiment, an explosive is used as the explosion source, and a pipe filled with water is arranged between the explosive and the cylindrical rock specimen. The main purpose of this fracture test is to collect the experimental data on the behaviors of the dynamic fracture of the rock. In addition, one of the aims of this test is to estimate the dynamic tensile strength of the rock in wide range of strain rate utilizing Hopkinsons effect. Therefore, during the fracture process of the rock, the free surface velocity and the fracture part near the free surface were observed by a laser vibration meter and high speed camera. The precise detonator was used to control the initiation time of the explosive by using an accuratley controlled blasting machine. The results of the fracture test for Kimachi sandstone and the validity of this test are discussed. In order to understand the relationship above fracture condition and the incident underwater shock wave into the rock specimen, the numerical simulation is carried out. The 2D hydrodynamic code based on ALE finite difference scheme is employed. In the case of the fracture test with 50 mm water pipe, the incident underwater shock wave into the cylindrical rock specimen has irregular pressure distribution near the shock front.

Study on CO2 Hydrate Formation as Stockpiling in Marine Sediments

Publisher Summary Methane hydrate reservoirs are found in marine sediments, and various proposals have been reported to produce natural gas from such reservoirs. The original concept of a methane hydrate production system has been proposed, in which carbon dioxide hydrate is used. In this technique, the CO2 hydrate layer develops in the upper part of the methane hydrate reservoirs, and the artificial roof is constructed to prevent a landslide; it may also prevent the emission of decomposed methane gas into the marine environment. In addition, the system features the sequestration of CO2 by the formation of CO2 gas hydrate after the mining of methane gas hydrate. This makes it possible to maintain the stability of sea floor, and to prevent geo-hazards such as sudden landslides. If this technique is put into practice, the energy problem and global environmental problems will be solved at the same time. This chapter presents the result of experiments on the formation and the growth of CO2 hydrate using an apparatus that simulates marine sediments. Both of the landslides at the sea floor and the emission of methane into the sea can be prevented by using the technique. In addition, the geological structure can be reinforced by the formation of a solid CO2 hydrate layer where the methane hydrate was extracted. This also features the sequestration of CO2 gas into the sediment. The experimental data of gas consumption was compared with theoretical data.