High pressure triaxial compression test in soft sedimentary rocks

The release of CO2 into the atmosphere is thought to be a major factor contributing to global warming, and technology for separating and recovering CO2 from the gases emitted from large-scale sources and storing it in deep underground aquifers (hereafter referred to as CCS: Carbon dioxide Capture and Storage) is already being used commercially in other countries as a measure to combat global warming. When starting a CO2 geological storage project, as part of risk management, it is necessary to consider whether there is a potential threat of causing seismic activity or ground deformation that could have a negative impact, and to plan and implement countermeasures. 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 considered to be one of the important issues in terms of gaining social acceptance at the project planning stage. The authors are currently 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 one of the issues is the physical properties of the ground to be input into the analysis model. It is well known that brittle rocks under atmospheric pressure show plasticity under confining pressures of tens to hundreds of MPa, and it is easy to imagine that soft sedimentary rocks also show similar mechanical behaviour. However, there are only a few cases of published high-pressure triaxial compression tests using drilling core samples collected from deep underground, for example. In this study, triaxial compression tests were conducted using sandstone and mudstone block samples (comprising the middle Pleistocene to the upper Pliocene) collected from outcrops and shaped into specimens (height 100 mm, diameter 50 mm) in the laboratory under confining pressures equivalent to CO2 storage sites. Regardless of the age of the sediment, the principal stress difference in mudstone increased to 1-2% axial strain, after which it remained almost constant. There was no clear yield point in the ‘stress-strain curve’, and the mudstone showed strain-hardening behaviour. The pore water pressure increased as the axial strain increased. In the sandstone, no clear shear surface was formed even at an axial strain of around 5%. The specimens did not become barrel-shaped after testing, but instead showed a shape of overall shrinkage. The volume change continued to decrease as the axial strain increased. This is thought to be because the difference in the principal stress did not reach its maximum strength. In the future, we plan to conduct experiments that take into account the pressure history (depositional depth and overburden) that the specimens have been subjected to in the past, before loading tests.