High Pressure Triaxial Compression Test in Soft Sedimentary Rocks　—Relationship Between Loading Rate and Strength-Deformation properties—

CO2 geological storage is key to combat global warming. During the site screening process, impact of seismic activity on the storage site is required [1]. As the Japan islands located in the convergent zone of four tectonic plates and are known as one of the most earthquake-prone countries in the world, evaluating and predicting the impact of great earthquakes on reservoirs and cap rocks and disseminating this information to society is especially important issues in terms of gaining social acceptance at the project planning stage. The authors are developing an earthquake response analysis method for evaluating the stability of CO2 geological storage sites in advance in the event of a great earthquake, but the input physical parameters of the cap and storage rocks under in-situ stress condition are not enough obtained. As the most of Japanese candidate storage sites are at young sedimentary formations, we prepare specimens (height 100mm, diameter 50mm) from Early Pleistocene sandstone and mudstone block sampled from outcrops. Triaxial compression tests were conducted under confining pressures corresponding to the depths of CO2 storage sites. The loading rate was performed at strain rates of 180%/min, 100%/min, and 10%/min. For instance, when a mudstone specimen was loaded at an effective confining pressure of 14 MPa, a back pressure of 9 MPa, and a strain rate of 180%/min, the maximum strength (approximately 6 MPa) appeared near an axial strain of 1%, after which a gradual softening trend was observed. Distinct strain-softening characteristics were not observed in this experiment. The pore pressure reached its peak slightly earlier than the maximum strength. Although it decreased thereafter, a slight upward trend was observed despite the decrease in axial deviator stress. In this presentation, we will report the strength and pore pressure characteristics of rocks based on rock types and loading rates, and propose strength parameters effective for dynamic analysis. [1] International Organization for Standardization,2026, ISO standard 27914; Carbon Dioxide Capture, Transportation and Geological Storage – Geological Storage.