Deep geothermal energy allows clean, non-intermittent, heat and/or power production regardless of weather conditions at any hour of the day or night. It will contribute to the decarbonization of our economy reaching its maximum mitigation potential by 2050 (ANCRE, 2015). However, exploitation of subsurface natural resources is faced with an uncertain environment. This is sometimes coined as the geological risk. Whatever the deep geothermal technology - conventional heat mining of deep aquifers, enhanced/engineered geothermal systems or power production in magmatic settings – and its maturity level, this feature makes geothermal operations high-risk projects with substantial initial investments (several M€) related to drilling costs. Even if insurance policies have recently been adapted to new targets, a single exploration failure may deter operators from a region with assumed good potential but complex geology for decades (e.g. the Hainaut aquifer in Northern France in the early 80’s). Better knowledge of the subsurface is then a key bottleneck for the deployment of deep geothermal technologies. It has been observed that the most efficient way to mitigate the geological risk is the collaborative integration of multidisciplinary data and interpretations into a geomodel of the subsurface. In a geothermal context, the first goal of such conceptual models is the prediction of the spatial distribution of temperature. Then, in order to reach economic profitability, deep geothermal projects need power levels that require convective exchanges with the reservoir at high flow rate through production and injection wells. Parallel to that, transient convective processes, which are ubiquitous in high temperature magmatic settings, also control the temperature distribution and the natural state of many sedimentary basins and basement type geothermal plays. Aforementioned conceptual models must consequently be dynamic by nature and integrate subsurface mass and energy transfers controlled by multiscale geological structures. Numerical simulation has become a powerful method for scientific inquiry on par with experimental and theoretical approaches, especially when data are as scarce and heterogeneous as subsurface data. Moreover, much progress has been made during the last decades in static geological modeling, dynamic geothermal reservoir modeling and performance computing with several contributions from CHARMS’ partners. Yet, many developments are still largely independent and confined to academic circles. There is no off-the-shelf software that integrates all of them in a consistent framework. The main objective of CHARMS is to take that step further and deliver the foundations components of an open framework so that integrated dynamic conceptual models of geothermal systems in complex geological settings can be produced from the early phases of exploration, to increase the probability of success, and evolve continuously through collaborative contributions into operational reservoir models to guarantee sustainable exploitation. The project is based on the three following pillars: • a consistent framework to link evolutionary complex geological models and the definition of the nonlinear physics of geothermal flows, • the improvement of the parallel ComPASS platform, which already has promising results, with numerical schemes tailored to accurately model multiphase multicomponent geothermal flows on unstructured meshes with discontinuities (fault, fractures…), • baseline validation tests and industrial cases, including complex well geometries, to assess the usefulness of the new tools. CHARMS gathers scientists who know each other and have a strong experience in subsurface modeling activities. BRGM (French Geological Survey) will lead the project leveraging the numerical expertise of the University of Nice and Paris 6, and the Maison de la Simulations as well as the industrial experience of Storengy (ENGIE Group).

The objective of GERESFAULT is to contribute to increase of the amount of energy extracted from the subsurface in France and Europe, by exploring new geothermal systems: crustal fault zones. Usually, geothermal exploration focus on areas that are well known for their elevated temperatures at shallow depths. If the subsurface is hot, but not sufficiently permeable, artificial techniques (with more or less success) can increase fluid circulation within the hot medium, thus creating so-called “Enhanced Geothermal Systems”. GERESFAULT, by contrast, will focus on the exploration of highly permeable fault zones (allowing for high flow rates) rooted at the depth of the brittle-ductile transition (350-400°C). Naturally, high permeability zones necessarily implies hot fluid ascent up to an economically depth level of about 2-3 km. The Pontgibaud fault zone (French Massif Central) is a relevant study case and is currently being explored by TLS-Geothermics, a geothermal exploration company that have acquired geological, geochemical and geophysical data for more than three years. In this framework, GERESFAULT propose a new way to explore potential geothermal crustal fault zones through a multi-scale combination of field studies, experimental petrophysics, geophysics and numerical modelling. The consistency and integration of the results from one scale to the other (upscaling) will be particularly addressed since the newly acquired petrophysical properties and geological models will help constrain the final 3D numerical hydrothermal system, from the fault to the crustal scale. The input of geophysical data into GERESFAULT should permit the construction of a 3D numerical, geological, static model, which will be constrained by additional field data. Petrophysical properties (porosity, permeability, density, electrical conductivuty, heat production rate, thermal conductivity) will be measured on core samples and on newly sampled rocks in order to better constrain the hydrothermal system within the fault zone. The associated scale transfer problems between different approaches will be addressed through geophysical modelling and percolation theory. The numerical modelling of the hydrothermal system will be performed at the scale of the fault zone but also at the crustal scale, for which a specific numerical code (ComPASS) will be used. Finally, a large-scale geodynamic approach will include the last 40 million years of trench retreat and should lead to the prediction of anomalously hot and permeable zones, at the scale of Europe. This innovative geodynamic approach has recently demonstrated that, in the case of a slab retreat, some thermal undulations that develop in the middle ductile crust also localize the damage zones in the upper brittle crust, and thus the associated geothermal system. A European view of these thermo-mechanical processes should help to assess European geothermal potential. During this 4-year GERESFAULT project, a 3 km deep borehole is planned - independently of GERESFAULT. Obviously, the first results from the project will be used to refine the location of the geothermal target, and similarly, the use of borehole data will provide additional data and key parameters of the hydrothermal system (productive zones and temperature distribution). However, GERESFAULT does not depend on the implementation of the drilling project. To reach the objectives of the project, the GERESFAULT team is made of 9 partners: 6 academic partners and 3 industrial partners. Further, project involves 26 scientists, and 4 master students, 4 post-docs and one research engineer will be hired in the framework of the project. A PhD thesis, co-funded by an industrial-academic partnership between TLS-Geothermicsn BRGM and ISTO has begun in March 2019 and is focused on one part of a subtask of GERESFAULT.

Scenarios for achieving carbon neutrality by 2050 predict a sharp increase in the use of hydrogen (H2). As with natural gas, large volumes of H2 will need to be stored underground, raising the question of storage integrity and the risk of leakage of this highly diffusive gas, which has significant flammability and explosion limits in air. At the same time, numerous projects are emerging around the world, focusing on the potential of natural H2, produced naturally by various processes in the earth's crust or deeper down, to become a resource. Natural H2 is often detected by its leakage at the surface, when it reaches the critical zone, the Earth's most superficial layer and the site of air-water-rock interactions. Studying how natural H2 migrates or is trapped is therefore a source of information for the monitoring strategies we need to implement to ensure the integrity of future underground H2 storages. Through the DENHyMS project, we propose to set up an in-situ laboratory to study the migration of natural H2 in the critical zone (0-100 m depth), using a documented case of natural leakage to provide information both on the integrity of storage sites and for natural H2 prospecting. The special feature of this project will be to study migration, reactivity and trapping in two different compartments of the critical zone: its deep part, or saturated zone, and its superficial part, or unsaturated zone, enabling us to conduct our research in a multiphase environment. This multidisciplinary project, involving seven partners, proposes to drill at least two dedicated wells to monitor the behavior of multiple gaseous species (H2, CO2, O2, CH4...) in saturated and unsaturated zones. Geological, geophysical and geochemical field studies will initially be used to select the best site for drilling. These studies will be conducted in line with best practice in terms of constructive dialogue with local stakeholders. During drilling, rock samples will be taken and characterized mineralogically and microbiologically. These samples will be monitored for degassing in the laboratory over a two-year period, in order to study degassing and productivity kinetics. Once the boreholes are equipped, geochemical and microbiological monitoring of the saturated and unsaturated zones will be carried out over two years, to describe gas migration, emission variability, the influence of redox reactions, and the role of the aquifer and bacterial consortia on H2 production or consumption. Complementary geological and geophysical work will be carried out to understand how certain seasonal variables (e.g. degree of water saturation of soil/rock formations) influence gas migration, and to help develop a conceptual fluid migration model. These data will be integrated into a reactive transport model describing gas/water/rock/biota reactivity in the critical zone. This model will simulate water and gas flows in a medium with variable water saturation, taking into account the reactive processes that can affect gas concentration. As the geochemical calculations can be coupled to the advection and diffusion equations, the model will ultimately enable us to assess the H2 flux existing in the study area. A PhD student and a post-doc will contribute to the field studies, interpretation and modeling of geochemical and microbiological data. By questioning our understanding of the role of aquifers, bacteria, geology, diffusion and reactivity, we will provide key elements for understanding how the subsurface promotes or limits H2 migration. This information will be useful both for improving underground storage integrity control strategies and for the search for natural H2-producing zones.