Undrained triaxial creep tests of carbon dioxide hydrate-bearing sand

There is an increasing demand for Carbon Dioxide Capture and Storage (CCS) which is a technique to store emitted CO₂ underground to reduce CO₂concentration in the atmosphere. Recently a new technology for reacting liquid or supercritical CO² with pore water in the ground and storing it in the ground as CO² hydrate (CDH), which is a chemically stable solid, has been attracting attention. Compared with the conventional CCS, this method enables stable storage for long term even in geological structures without shield layer. In addition, expansion of storage area is expected by this method. However, there are a lot of unknowns in the mechanical properties of CDH bearing sediments. In this study, undrained triaxial creep tests are conducted and the mechanical characteristics of the CDH bearing ground are clarified through the experimental results.

In the experimental study, we use a specialized triaxial apparatus, which enables triaxial tests in low temperature and high-pressure conditions to simulate deep seabed ground. Artificially CDH bearing sand specimens are prepared by using the apparatus. After isotropic consolidation, triaxial creep tests are conducted by keeping the deviator stress constant under undrained condition.

From the experiments, the mechanical properties on CDH bearing sand specimen are obtained regarding the strength, the dilatancy behavior based on effective stress path, and the characteristics of creep deformation with elapsed time. The main findings from this research are as follows:

(1) The CDH bearing sand specimens show significant creep strain with elapsed time, whereas in the case of saturated Toyoura sand specimen, the progress of creep strain with passage of time is not observed.

(2) It was confirmed that after the strain rate decreased and reached its minimum value, the creep strain of the CDH bearing sand specimen accelerated and fractured.

(3) Creep behavior of sand specimen with CDH is characterized by the 'creep stress ratio' which is defined as the ratio of the creep stress to the maximum deviator stress of the specimen.

Conclusively, it is clarified that the sand specimen exhibits remarkable time dependent behavior by containing CDH.