This article is a summary of the authors’ published article (Watanabe et al. 2019), which is an open access article distributed under the terms of the Creative Commons CC BY license. Superhot geothermal environments (above ca. 400 ℃) represent a new geothermal energy frontier. However, the networks of permeable fractures capable of storing and transmitting fluids are likely to be absent in the continental granitic crust. Here we report the first-ever experimental results for well stimulation involving the application of low-viscosity water to granite at temperatures ≧400 ℃ under true triaxial stress. This work demonstrates the formation of a network of permeable microfractures densely distributed throughout the entire rock body, representing a so-called cloud-fracture network. Fracturing was found to be initiated at a relatively low injection pressure between the intermediate and minimum principal stresses and propagated in accordance with the distribution of preexisting microfractures, independent of the directions of the principal stresses. This study confirms the possibility of well stimulation to create excellent fracture patterns that should allow the effective extraction of thermal energy.

View full abstract

This article is a summary of the author’s thesis (Ogata, 2019) which won the best doctoral thesis award from the Japanese Society for Rock Mechanics (JSRM). Coupled thermal-hydraulic-mechanical-chemical (THMC) models were developed for realistically estimating the long-term permeability evolution within rock enhanced by mineral reactions such as pressure dissolution. Two coupled THMC models that incorporate the pressure dissolution within the rock matrix and rock fractures were firstly developed, and the validity of the models was examined by replicating the experimental data. Subsequently, a coupled THMC model that can consider the mineral reactions within dual-porosity media was proposed and applied to predict the long-term evolution of the permeability of fractured rock in a geological repository of high-level radioactive waste (HLW). The results revealed that the pressure dissolution within rock fractures has a critical influence on the permeability evolution of fractured media. Finally, a THMC coupled model incorporating the damage theory to consider the influence of fracture generation was developed, and the consecutive processes in rock permeability evolution, from fracture generation to subsequent sealing during the period of the geological disposal of HLW, were shown.

View full abstract

This article provides a summary of part of a series of studies that received the Frontier Award of the Japanese Society for Rock Mechanics (JSRM) in the 2019 fiscal year. It is generally accepted that the physical and mechanical properties of rock materials change significantly based on clay mineral content; therefore, clay mineral content and type are considered to be significant factors in the mechanical properties of rock. In order to clarify the effect of clay mineral type, a series of experiments (one-dimensional swelling-pressure and constant-pressure permeability tests) have been carried out on clay mineral samples. In addition, uniaxial compressive test, using gypsum specimens mixed with clay minerals, were conducted to assess the effect of clay mineral type and content on the degree of strength reduction. Swelling-pressure and hydraulic conductivity of the clay mineral samples differed according to clay mineral type. Clay mineral content and type are also factors that affect uniaxial compressive strength values and the degree of strength reduction. Furthermore, the results suggested that the uniaxial compressive strength of clay mineral-bearing rock materials under dry conditions can be evaluated on the basis of the interlayer bonding force (i.e., the type of chemical bond) in clay minerals. Our results are a useful contribution to base data for evaluating the physical and mechanical properties of clay mineral-bearing rock materials.

View full abstract

A coupled numerical model was developed to study the fluid flow and mass transport behavior of rock fractures under coupled thermal-hydraulic-mechanical-chemical (THMC) conditions. In particular, the model was employed for the purpose of expressing the evolution of permeability and the reactive transport behavior within rock fractures by considering the geochemical processes (e.g., pressure dissolution). In order to validate the model, it was utilized to replicate the experimental measurements of the evolution of the hydraulic aperture and the element concentrations obtained from two flow-through experiments using single granite and mudstone fractures. The simulated results were seen to coincide with the experimental data for the evolution of the hydraulic aperture and the effluent element concentrations without adopting any fitting parameters that are often used in other THMC coupled models presented in literature. Overall, the developed model should be valid for evaluating the evolution of the fluid flow and the mass transport behavior within rock fractures induced by mineral dissolution under stress- and temperature-controlled conditions.

View full abstract