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

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Watanabe et al.’s (2017a) work published in Nature Geoscience critically challenges the scientific community’s hypothesis of a significant reduction in the formation permeability below the Brittle Ductile Transition (BDT) depth and argues in favour of perspectives of exploitable supercritical geothermal resources in the ductile crust. A subsequent Thermo-Hydro-Mechanical (THM) experimental study (Watanabe et al. 2017b) shows a drastic change in the fracture network morphology from planar to dendritic in supercritical conditions, which again supports the earlier argument. However, despite the significant implications of these findings, an adequate conceptual framework that describes these phenomena is yet to be conceived. The formulation of such a model can in turn be based on previous work by the German team on the constitutive model of rocks spanning the brittle-ductile transition. The mentioned model has been successfully employed to explain previously contrasting precursory seismic and gravimetric signals of volcanic eruptions as related with high-temperature rheology of rocks. A successful extension to describe the above mentioned experimental phenomena requires addressing several open questions relating fluid and rock rheology to fracture morphology and permeability evolution. This study will unravel the complex mechanisms behind natural fracture formation beyond the brittle condition in the earth crust. To this purpose, further experiments will be conducted first at Tohoku University, decoupling each component thought to play a role in the dendritic fracture development, which are pore space, supercritical fluid rheology, and ductile rock rheology, in order to assess their individual impacts. Based on the experimental results, the constitutive model developed by the German partners will be further extended towards supercritical pore fluids and implemented into the open-source multi-physics code OpenGeoSys. Finally, the explanatory capabilities of the newly developed concepts towards intricate dynamical processes in the earth’s crust (e.g., supercritical hydro-thermal flows with evolving rock permeability, low-frequency earthquakes, dyke propagation, etc.) will be assessed in simulations of selected large-scale scenarios to be conducted by the German-Japanese team utilizing High Performance Computing (HPC) capabilities at the partner institutions.