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

Abstract

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 (CO2-hydrates), have been attracting attention from the viewpoint of CO2 storage in the form of CO2-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 CO2-hydrate-storaged submarine sediments. In order to gain further knowledge of CO2-hydrate-bearing sediments, we carried out three kinds of laboratory experiments on synthetic CO2-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 CO2-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 CO2-hydrate-bearing sand shows larger strength and larger positive dilatancy than water-saturated sand without CO2-hydrates under undrained conditions. Second, CO2-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 CO2-hydrates. The pore gas pressure of CO2, produced by the hydrate dissociation, can exceed the liquefied boundary of CO2-gas, and the increase in pore gas pressure can be limited by the liquefaction of CO2-gas.