Rate effect on the stress-strain relations of synthetic carbon dioxide hydrate-bearing sand and dissociation tests by thermal stimulation

Methane hydrates (MHs) have been recognized as an important material for use as a new energy resource. Recently, not only MHs, but also carbon dioxide hydrates (CO 2 -hydrates), have been attracting attention from the viewpoint of CO 2 storage in the form of CO 2 -hydrates. It is essential, therefore, to investigate the mechanical behaviour of gas hydrate-bearing sediments in order to achieve safe MH extraction and to ensure the long-term stability of CO 2 -hydrate-storaged submarine sediments. In order to gain further knowledge of CO 2 -hydrate-bearing sediments, we carried out three kinds of laboratory experiments on synthetic CO 2 -hydrate-bearing sand specimens: (1) undrained triaxial compression tests with a constant strain rate, (2) undrained triaxial compression tests with a step-changed strain rate, and (3) dissociation tests on CO 2 -hydrate-bearing sand specimens using the thermal stimulation method. The main findings obtained from these three experiments are as follows: First, it was found that CO 2 -hydrate-bearing sand shows larger strength and larger positive dilatancy than water-saturated sand without CO 2 -hydrates under undrained conditions. Second, CO 2 -hydrate-bearing sand clearly exhibits strain rate dependency with an increase in hydrate saturation. It is interesting to note that hydrate saturation’s dependency on strength does not appear in cases where the strain rate is quite slow. Third, a drastic increase in pore pressure and extensive tensile strain are observed simultaneously during the dissociation of CO 2 -hydrates. The pore gas pressure of CO 2 , produced by the hydrate dissociation, can exceed the liquefied boundary of CO 2 -gas, and the increase in pore gas pressure can be limited by the liquefaction of CO 2 -gas.