Numerical modeling of regional stress distributions for geothermal exploration

International audienceAny high-enthalpy unconventional geothermal projectcan be jeopardized by the uncertainty on the presence of the geothermal resource at depth. Indeed, for the majority of such projects the geothermal resource is deeply seated and, with the drilling costs increasing accordingly, must be located as precisely as possible to increase the chance of their economic viability. In order to reduce the " geological risk " , i.e. the chance to poorly locate the geothermal resource, a maximum amount of information must be gathered prior to any drilling of exploration and/or operational well. Cross-interpretation from multiple disciplines (e.g., geophysics, hydrology, geomechanics. . .) should improve locating the geothermal resource and so the position of exploration wells ; this is the objective of the Euro-pean project IMAGE (grant agreement No. 608553), under which the work presented here was carried out. As far as geomechanics is concerned, in situ stresses can have a great impact on the presence of a geothermal resource since they condition both the regime within the rock mass, and the state of the major fault zones (and hence, the possible flow paths). In this work, we propose a geomechanical model to assess the stress distribution at the regional scale (characteristic length of 100 kilometers). Since they have a substantial impact on the stress distributions and on the possible creation of regional flow paths, the major fault zones are explicitly taken into account. The Distinct Element Method is used, where the medium is modeled as fully deformable blocks representing the rock mass interacting through mechanically active joints depicting the fault zones. The first step of the study is to build the model geometry based on geological and geophysical evidences. Geophysical and structural geology results help positioning the major fault zones in the first place. Then, outcrop observations, structural models and site-specific geological knowledge give information on the fault zones family sets and their priority rule. In the second step, the physical model must be established, including constitutive equations for the rock mass and the fault zones, initial state and boundary conditions. At such large scales, physical laws and parameters are difficult to assess and must be constrained by sensitivity analysis. In the last step of the study, the results can be interpreted to highlight areas where the mechanical conditions favor the presence of a geothermal resource. The DEM enables accounting for the strong stress redistributions inherent to highly-segmented geometries, and to the dilational opening of fault zones under shearing. A 130x150 square-kilometers region within the Upper Rhine Graben is used as a case-study to illustrate the building and interpretation of a regional stress model

Adapted numerical modelling strategy developed to support EGS deployment.

International audienceThe exploitation of Enhanced/Engineered Geothermal Systems (EGS), for electricity and/or heat production, is a promising way to increase the amount of renewable energies contribution in the energetic mix in Europe. In regard to the required production characteristics (production temperature and flowrate) for the economical viability of EGS, the favourable targeted geological systems are deep and fractured. In order to reduce the risks and the prohibitive costs linked to the depth of such geothermal systems, numerical modelling is a useful tool to understand such deep fractured systems and to help in the construction and in the management of the deep infrastructures (wells architecture, stimulation of wells, implementation of adapted network of wells). Nevertheless, this forces to a change of paradigm in comparison to « classical » reservoir modelling based on mechanics of continuum media. Indeed 3D Discrete Fracture Network (DFN) approach looks fairly adapted to catch the mechanical and hydraulic phenomena in the fractured rock mass around wells and to understand the global systems in the network of wells. The conceptualisation of the fractured rock mass is a crucial step for such DFN models not only for the geometry but also to constrain the constitutive behaviour of singularities (fault zones, fractures etc.), depending on the tectonic context. We present some results illustrating how DFNs can be used to study the EGS behaviour at several scales

Impact of faults and their mechanical properties on the regional stress field

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Crustal Fault Zone: New geothermal reservoir? Structural dataset and preliminary 3D TH(M) modelling of the Pontgibaud fault zone (French Massif Central)

International audienceNumerous recent studies indicate that crustal-scale fault zones represent efficient conduits for meteoric fluids to flow down to mid-crustal depths (Haines et al., 2016). The present study aims to understand the potential of a new and novel type of geothermal system for high temperature and electricity production: Crustal Fault Zones (CFZ). One such example is the Pontgibaud fault zone (French Massif Central), a 30 km-long and 3 km-wide mineralized fault zone. The Pontgibaud fault zone is also characterized by numerous CO2-rich-thermo-mineral springs. Moreover, this area is also defined by local and regional surface heat flow values of 110 mW/m 2 (International Heat Flow Commission database) involving temperature gradients between 37 and 41 °C/km. The Pontgibaud fault zone has been well studied in the last few years (Bellanger et al., 2017), electrical conductivity anomalies have been identified (Ars et al., 2019) and the temperature anomaly has been estimated (Duwiquet et al., 2019). Estimation of the geothermal resource have been established by 2D geological numerical modelling, with Thermal-Hydraulic (TH) coupling. Here we propose to integrate structural field observation into 3D geological numerical modelling with Thermal-Hydraulic-(Mechanical) (TH(M)) coupling. We then evaluate the various applications for high-temperature geothermal exploitation

D4.6 Important physical processes and modelling schemes of induces microseismicity

This report contains a Synopsis and Two papers: Coupled continuum modelling of fracture reactivation and induced seismicity during ehnanced geothermal operation, by Wassing, van Wees and Fokker, draft version submitted to Geothermics Self-induced seismicity due to fluid circulation, by Aochi, Poisson, Trusiani, Rachez, and Schmittbul, draft version submitted to Geophysical Journal International. In the synopsis it is clarified that the main aim of the study is to understand the geomechanical causes and processes of induced seismicity in various contexts and at various scales

Low incidence of SARS-CoV-2, risk factors of mortality and the course of illness in the French national cohort of dialysis patients

International audienceThe aim of this study was to estimate the incidence of COVID-19 disease in the French national population of dialysis patients, their course of illness and to identify the risk factors associated with mortality. Our study included all patients on dialysis recorded in the French REIN Registry in April 2020. Clinical characteristics at last follow-up and the evolution of COVID-19 illness severity over time were recorded for diagnosed cases (either suspicious clinical symptoms, characteristic signs on the chest scan or a positive reverse transcription polymerase chain reaction) for SARS-CoV-2. A total of 1,621 infected patients were reported on the REIN registry from March 16th, 2020 to May 4th, 2020. Of these, 344 died. The prevalence of COVID-19 patients varied from less than 1% to 10% between regions. The probability of being a case was higher in males, patients with diabetes, those in need of assistance for transfer or treated at a self-care unit. Dialysis at home was associated with a lower probability of being infected as was being a smoker, a former smoker, having an active malignancy, or peripheral vascular disease. Mortality in diagnosed cases (21%) was associated with the same causes as in the general population. Higher age, hypoalbuminemia and the presence of an ischemic heart disease were statistically independently associated with a higher risk of death. Being treated at a selfcare unit was associated with a lower risk. Thus, our study showed a relatively low frequency of COVID-19 among dialysis patients contrary to what might have been assumed