A proper definition of sustainability should consider two key elements: space and time. These two elements have not been considered in commonly recognized definitions for this term. The present work examines in details sustainability of underground mineral and fuel resources for the current century in our world. Under these boundary conditions, availability of underground resources in this century is considered to be secured. In addition, the electricity output potentials in Japan, estimated by considering scenarios relating to not only social restrictions but also business profits for solar photovoltaics power, wind power, hydropower, geothermal power and bio-power, were comprehensively reviewed and examined. From the above mentioned energy sources, only solar photovoltaics power and wind power are expected to grow quantitatively in the years to come. However, from the viewpoint of level land area and primary energy consumption per unit level land area, power cost, area required to install power facilities, power of solar energy, quality of electric power, etc. it is essentially difficult to consider both of solar photovoltaics power and wind power as the key power source at the present stage . The best way may be to utilize them as a locally generated, locally used electricity supply systems that is combined with rechargeable batteries.
Underground sequestration of carbon dioxide (CO2) is a promising method for mitigating its greenhouse gas effect. This method will become much more feasible by compensating for its cost, if it can be combined with economically viable processes such as shale gas recovery and geothermal energy extraction from hot dry rock. When CO2 is injected deeper than 1000 m, the temperature and pressure usually satisfy the condition to make CO2 to be supercritical state, which has much lower viscosity than that of water. To examine behavior of the supercritical state CO2 (SC-CO2) in rock, we conducted hydraulic fracturing experiments using SC-CO2, water, and viscous oil in granite specimens under a tri-axial deviatoric stress condition. Source distributions of acoustic emission events showed that hydraulic fracturing with SC-CO2 can induce more intense cracks branching and spreading wider than those with water. The result suggests that SC-CO2 fracturing forms more effective pathways for shale gas recovery and hot dry rock geothermal extraction while enabling economically feasible underground sequestration of CO2.
Strain gauge and mechanical extensometers are commonly used to measure strain in rock samples. In recent years, diffraction techniques with X-rays and neutrons for investigating strain in engineering materials have been developed. Strain measurements using diffraction technique are based on Bragg's law. Lattice spacing changes with strain, which induces peak shift of Bragg peak. Strain value can be estimated from this peak shift value. Strain measurements using the world's highest intensity neutron beam can be performed at J-PARC (Japan Proton Accelerator Research Complex) constructed at Tokai in Ibaraki. Neutron powder diffractometer dedicating to investigate strain state in engineering materials has been constructed at BL19 in J-PARC, which is named as The Engineering Materials Diffractometer“ TAKUMI”. In order to examine applicability of the diffractometer to rock materials, in situ neutron diffraction experiments on rock samples under uni-axial compression have been performed. Higher resolution strain data has been obtained in shorter time compared to other diffractometers. In addition, neutron diffraction peaks of not only major component, such as quartz, but also minor components, such as feldspars, could be observed. Anisotropy of strain with respect to the quartz crystal orientation and discrepancy between macroscopic strain (measured by strain gauge) and lattice strain (measured by neutron diffraction) were also recognized. Change in peak width with respect to stress magnitude showed a different behavior depending on rock type. Strain measurements using neutron diffraction technique give us new insight in rock deformation which cannot be obtained by common technique.