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High-temperature and high-pressure rheology of rock fractures: impacts on hydrothermal fluids circulation in magmatic systems.

Applicant Professor Dr. Keita Yoshioka, since 9/2021
Subject Area Palaeontology
Term from 2019 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 416615483
 
Final Report Year 2023

Final Report Abstract

Supercritical geothermal resources where the water is at or above the supercritical condition (373.95 C◦ and 22.064 MPa) are considered to be 2 to 5 times more productive in terms of power generation because of their high enthalpy. Practical challenges to exploit such supercritical geothermal resources are two holds. One is its depth that meets the supercritical condition for water and associated drilling difficulties in both technical and economical points. The other challenge is its perceived low permeability in rocks below the brittle ductile transition. However, the study conducted by Watanabe et al. (2017) challenges this notion of low permeability below the brittle ductile condition. Their experimental study under high temperature and high pressure demonstrated that the morphology of hydraulically induced fractures changes from planar to dendritic in supercritical conditions. Their study indicates that supercritical geothermal reservoirs can be hydraulically stimulated to form a dendritic fracture network, which may be suitable for economical development of supercritical geothermal resources. Our main objective of this project is to understand and formulate the experimental observations in Watanabe et al. (2017) from experimental and numerical standpoints. Under the high temperature and high pressure conditions of their experiments, both fluid and rock rheologies change and we do not know how the fracture morphology is impacted by these changes. To separate the effects of fluid rheology from those of rock rheology, we conducted hydraulic fracturing experiments on nonporous material, polymethyl methacrylate (PMMA) under the conditions where PMMA samples transition from brittle to ductile while the water remained below the supercritical condition. From these experiments and numerical simulations, we observed that the transition of rock rheology does not contribute to the fracture morphology complexity. With PMMA being non-porous materials, these findings provides us with key information to narrow down the complex mechanisms of dendritic fracture formation mechanism and the complex fracture must be induced by fluid rheology and associated fluid infiltration into the porous material. To confirm the findings from the PMMA experiments, we performed hydraulic fracturing experiments on granite with true-triaxial setting under supercritical conditions and monitored acoustic emissions. The monitored acoustic emission indicate that the fracture pattern becomes more complex when the experiments were conducted under supercritical conditions, which supports possible supercritical geothermal resource exploitation. Apart from these two main experiments, we have conducted many other studies ranging from a new experimental procedure to test failure envelops under supercritical conditions to numerical studies concerning with seismic risk to hydraulic stimulation for supercritical geothermal resources. Project results are published in 11 peer-reviewed papers in high impact journals such as Nature Communications, Geophysical Research Letters, and Journal of Geophysical Research.

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