Induced seismicity has become a widespread issue as a result of the proliferation of geo-energy projects (Foulger et al., 2018). On one hand, geothermal energy production and geologic carbon storage are essential technologies to reach zero or negative net carbon emissions. On the other hand, the increased energy demand is boosting other operations, such as seasonal natural gas storage, subsurface energy storage, and disposal of wastewater from conventional and non-conventional oil and gas production. Injecting or pumping fluids at depth-a widespread practice in geo-energy operations-alters the in-situ stress field and may lead to fault rupture and induced seismicity (Buijze et al., 2017; Ellsworth, 2013; Grigoli et al., 2018). In several cases, the authorities have decided to cancel projects believed to be associated with large induced earthquakes and a non-exhaustive list includes the

Supercritical geothermal systems are appealing sources of sustainable and carbon-free energy located in volcanic areas. Recent successes in drilling and exploration have opened new possibilities and spiked interest in this technology. Experimental and numerical studies have also confirmed the feasibility of creating fluid conducting fractures in sedimentary and crystalline rocks at high temperature, paving the road towards Enhanced Supercritical Geothermal Systems. Despite their attractiveness, several important questions regarding safe exploitation remain open. We dedicate this manuscript to the first thermo-hydro-mechanical numerical study of a doublet geothermal system in supercritical conditions. Here we show that thermally-induced stress and strain effects dominate the geomechanical response of supercritical systems compared to pore pressure-related instabilities, and greatly enhance seismicity during cold water re-injection. This finding has important consequences in the design of Supercritical Geothermal Systems.

Contrasting deformation mechanisms precede volcanic eruptions and control precursory signals. Density increase and high uplifts consistent with magma intrusion and pressurization are in contrast with dilatant responses and reduced surface uplifts observed before eruptions. We investigate the impact that the rheology of rocks constituting the volcanic edifice has on the deformation mechanisms preceding eruptions. We propose a model for the pressure and temperature dependent brittle-ductile transition through which we build a strength profile of the shallow crust in two idealized volcanic settings (igneous and sedimentary basement). We have performed finite element analyses in coupled thermo-hydro-mechanical conditions to investigate the influence of static diking on the local brittle-ductile transition. Our results show that in active volcanoes: (i) dilatancy is an appropriate indicator for the brittle-ductile transition; (ii) the predicted depth of the brittle-ductile transition agrees with the observed attenuated seismicity; (iii) seismicity associated with diking is likely to be affected by ductile deformation mode caused by the local temperature increase; (iv) if failure occurs within the edifice, it is likely to be brittle-dilatant with strength and stiffness reduction that blocks stress transfers within the volcanic edifice, ultimately damping surface uplifts.

Scientific visualization developed successful methods for scalar and vector fields. For tensor fields, however, effective, interactive visualizations are still missing despite progress over the last decades. We present a general approach for the generation of separating surfaces in symmetric, second-order, three-dimensional tensor fields. These surfaces are defined as fiber surfaces of the invariant space, i.e. as pre-images of surfaces in the range of a complete set of invariants. This approach leads to a generalization of the fiber surface algorithm by Klacansky et al. [16] to three dimensions in the range. This is due to the fact that the invariant space is three-dimensional for symmetric second-order tensors over a spatial domain. We present an algorithm for surface construction for simplicial grids in the domain and simplicial surfaces in the invariant space. We demonstrate our approach by applying it to stress fields from component design in mechanical engineering.

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a b s t r a c tShales have become increasingly important because they play key roles in modern energy and environmental geomechanics applications, such as nuclear waste storage, non-conventional oil and gas operations and CO 2 geological storage. Shale behaves in a quasi-brittle manner, often exhibiting linear elasticity before reaching its peak stress. Furthermore, softening of the material leads to a residual state in which pure plastic flow is observed under constant values of deviatoric stress. Degradation of the elastic moduli and the accumulation of irreversible strains are believed to be primarily caused by the debonding and decohesion mechanisms in the structure, as well as the growth of microcracks. To capture these features, a constitutive model that couples elastic, plastic and damage theories is developed. The isotropic damage model is based on the Weibull probabilistic theory and describes the failure of brittle materials. This model is coupled with a modified version of the Lade-Duncan criterion to account for non-linear dependency of shear resistance with a mean stress typical of geomaterials. The two surfaces of damage and plasticity and the rate equations for the internal variables are postulated and thermodynamic consistency is subsequently investigated. The coupled plastic-damage constitutive model is integrated with an implicit stress return algorithm for the plastic part, while the damage part can be integrated implicitly. Numerical back-calculations of experimental results from two quasi-brittle shales demonstrate the ability of the model to reproduce the primary features of their mechanical behavior.

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