A wider exploitation of deep geothermal reservoir requires the development of Enhanced Geothermal System technology. In this context, drilling and stimulation of high-enthalpy geothermal wells raise technical challenges. Understanding and predicting the rock behavior near a deep geothermal wellbore are decisive to implement stimulation strategies to reach the couple temperature/flowrate target. Numerical modeling can contribute to enhanced stimulation processes thanks to a better understanding of impact of stress release, pressure changes and rock cooling in the near-wellbore area. In this paper, we use Discrete Element Method (code PFC2D, © Itasca Consulting Group), and more specifically bonded-particle model to capture the thermo-mechanical processes at metric scale. The application case corresponds to the beginning of thermal stimulation at Reykjanes in well RN-15/IDDP-2 (Iceland, IDDP-2 project and H2020 project DEEPEGS). A cold fluid is injected at a depth of 4.5 km where the rock temperature is above 430 °C and the well pressure is around 34 MPa. Since we have site-specific data and logging images after drilling, we attempt to link the simulations with the reality. The numerical results are confronted with incipient interpretation of logging images and with analytical solution to go towards validation of the modeling approach. Numerical results show breakouts and thermally and/or mechanically induced fractures consistent with the analytical solutions. Moreover, the sensitivity analysis on uncertain parameters yields important clues regarding some logging features as, for example, asymmetric damaging or caving.

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It is proposed to complement the numerous geometrical constructions of fault-related folds relevant to fold-and-thrust belts by the introduction of mechanical equilibrium and of the rock limited strength to discriminate between various deformation scenarios. The theory used to support this statement is the maximum strength theorem that is related to the kinematic approach of limit analysis known in soil mechanics. The classical geometrical construction of the fault-propagation fold (FPF) is proposed for illustration of our claim. The FPF is composed of a kink fold with migrating axial surfaces ahead of the region where the ramp propagates. These surfaces are assigned frictional properties and their friction angle is found to be small compared with the usual bulk friction angle to ensure the full development of the FPF, a first scenario. For larger values of the axial surface friction angle, this development during overall shortening is arrested by the onset of fault breaking through the front limb, a second scenario. The amount of shortening at the transition from folding to break-through faulting is established.

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The internal deformation of the Appalachian Plateau décollement sheet has a distinctive style involving kink bands and thrusts. In areas where the décollement sheet is underlain by thin salt, the dominant structures are thrusts developed at shallow levels, underlain by a series of steep kink bands that terminate downwards at the Silurian salt décollement. Where the salt is thick, large asymmetrical anticlines developed with hinterland-verging kinks on their back-limbs that deformed the entire supra-salt sequence. In order to understand the constraints on deformation, we have used analytical mechanical modelling based on the maximum strength theorem. The simplified model consists of three layers: two are fluids and the third, intervening layer is a stratified competent material. The model is compressed horizontally and the predictions made are based on the kinematic approach of classical limit analysis. Two modes of deformation are investigated: the thrust and the kink band. The modelling shows that kink bands dominate deformation at large burial depth. At shallower depth and small regional bedding dip, the dominant mode is thrusting. In areas of open folding it is predicted that through-going hinterland-verging kink bands will form at a critical limb dip angle of about 10°.Supplementary material: Technical details of the mechanical theory behind this article are available at https://doi.org/10.6084/m9.figshare.c.3799492