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

渡边等人在《自然地球科学》上发表的S(2007a)的工作对科学界关于脆性韧性转变(BDT)深度以下地层渗透率显著降低的假说提出了批判性挑战，并支持在韧性地壳中开发超临界地热资源的前景。随后的热-水力-机械(THM)实验研究(Watanabe等人)。2007b)显示了超临界条件下断裂网络形态由平面转变为树枝状的剧烈变化，这再次支持了前面的观点。然而，尽管这些发现具有重大意义，但描述这些现象的适当概念框架尚未构思出来。反过来，这种模型的制定可以基于德国团队之前在跨越脆性-韧性转变的岩石本构模型方面的工作。该模型已成功地用于解释火山喷发与岩石高温流变性有关的前兆地震和重力信号。为了成功地描述上述实验现象，需要解决几个悬而未决的问题，将流体和岩石流变学与裂缝形态和渗透率演化联系起来。这项研究将揭开地壳脆性条件以外的天然裂缝形成背后的复杂机制。为此，将首先在东北大学进行进一步的实验，将被认为在树枝状裂缝发育中发挥作用的各个成分--孔隙空间、超临界流体流变学和韧性岩石流变学--解耦，以评估它们各自的影响。基于实验结果，德国合作伙伴开发的本构模型将进一步扩展到超临界孔隙流体，并在开源的多物理程序OpenGeoSys中实现。最后，新发展的概念对地壳中复杂的动力学过程的解释能力(例如，随着岩石渗透率的变化而变化的超临界水热流、低频地震、堤坝传播等)。将在德国-日本团队利用伙伴机构的高性能计算(HPC)能力进行的选定大规模情景的模拟中进行评估。

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.