This article is summary of the paper published in “International Journal of Rock Mechanics and Mining Sciences” (Ishibashi et al., 2023a). To provide key parameters and constitutive laws essential for field-scale multiphysics simulations that accurately predict fracture network structures in enhanced geothermal systems (EGS) and resultant energy extraction, we investigate the comprehensive spectrum of hydraulic shear processes in granite fractures and reassess the connection between hydraulic and mechanical properties during shear slips. Key results from our novel laboratory experiments include the following: (1) Fracture permeability of granite increases due to hydraulic shear slip even at an effective normal stress exceeding 50 MPa, (2) Shear slip and stress drop are proportional, and the increase in fracture permeability correlates with the total shear slip displacement, and (c) although hydraulic shear slip tends to make fracture surfaces slightly smoother, the factual characteristics of surface are maintained after slip. By integrating our experimental results with seismological analysis, we first examine the energy balance during the hydraulic shearing of preexisting rock fractures and highlight the critical role of the elastic potential energy stored in the surrounding bulk rock masses. Subsequently, we derive a constitutive model that relates to the permeability change of granite fractures during hydraulic shearing under typical crustal stress conditions of EGS, estimating that the maximum change in fracture permeability due to shear dilation is approximately 20-fold, though scale effects are not considered. In summary, we successfully demonstrate novel and advanced insights into hydro-mechanical coupled processes during hydraulic shearing, aiming to improve the accuracy of fracture network designs in EGS technology.

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Excavation damaged zones are created when excavating underground caverns and generate AE at micro-amplitude. This research is a challenge to apply DAS with C-OTDR technology to AE measurement. The results of laboratory and in-situ experiments showed that differences in optical fiber specifications did not have a significant effect on AE measurements. In addition, it was found that source location can be identified in the same way as with conventional in-situ AE measurements. Furthermore, the monitoring range of conventional AE sensors is generally 5 m to 10 m, but the new method can extend the monitoring range to 35 m.

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This article summarizes the results of development based on the recent studies (Kojima et al., 2024a, b). On-site observation of excavated tunnel faces is difficult due to safety regulations and a shortage of geological experts. Although several tools have been proposed for remotely observing the face with photographs or videos, they do not provide three-dimensional (3-D) realistic observations and measurements. We developed a new method to solve this problem, using a 3-D point cloud and a virtual reality (VR) system. This method allows to examine a tunnel face in detail remotely. We validated the effectiveness of this new method by comparing it to a conventional on-site method by visual inspection in a tunnel where talus deposits are distributed. The VR method revealed the detailed distribution and geological structures of the talus at the tunnel crown, whereas the conventional method could not. These results show that the VR method could be an effective solution to this problem.

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This article summarizes a PhD dissertation titled Creating granitic geothermal reservoirs by carbon dioxide injection, submitted to the Graduate School of Environmental Studies, Tohoku University. Carbon dioxide (CO₂) has been proposed as an alternative fracturing fluid to create geothermal reservoirs, because it is less reactive to rock-forming minerals, capable to reduce water footprint, and easier to handle compared to the competing alternative gasses. CO₂ has also low viscosity (‹ 100 μPa⋅s) across wide range of conditions, potentially allowing its injection to induce complex, cloud-fracture network (CFN) at conventional (c.a. 150–300 °C) and superhot (› c.a. 400 °C) geothermal conditions. Experiments on intact granite samples clarified that CO₂ injection achieves CFN at conventional and superhot geothermal conditions, through the stimulation of pre-existing microfractures by the low-viscosity CO₂. The aperture of fractures in the CFN increases with temperature and differential stress, and the fracturing pressure can be predicted using the Griffith failure criterion. Then, experiments on cylindrical granite samples with sawcut, serving as an analogue of a natural fracture, elucidated that CO₂ injection achieves CFN in naturally-fractured granite, along with the shearing of the natural fractures. Finally, experiments into granite samples with CFN revealed that chelating agent solution injections improves the permeability of the CFN without inducing excessive rock deformation and acoustic emission, both at slightly acidic and alkaline conditions, and under varying stress state. At large scale, and in radial flow condition, chelating agent solution injection under slightly acidic condition induces higher degree of mineral dissolution around injection borehole; thus, the injection should utilize a lower chelating agent concentration to allow for a lower solution-viscosity, and more uniform degree of mineral dissolution over a greater distance from borehole.

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