At the frontiers of deep geothermal energy: the emergence of the 'subsurface' environment in a context of energy transition

As a natural reservoir of fossil fuels extracted to underpin the two industrial revolutions, is the subsurface in the process of changing its status - on the pretext of the energy transition - to become one of the environments in which this transition is possible? This is the bet that is being made with deep geothermal energy. This technology consists in producing heat and electricity from a modification of the so-called enhanced medium-depth geothermal energy that developed in France, in the Paris Basin, after the oil crises of the 1970's. While this form of geothermal energy sought heat in geological layers located at a depth of about 2000 metres, from which fluids were extracted to bring the heat to the surface, deep geothermal energy involves seeking heat much deeper (about 5000 metres) by creating underground networks to circulate fluids. This technique is much more expensive and technically more difficult. However, the 2010's have shown an interest in this technique, with a view to producing low- carbon electricity, as part of the energy transition policies. Nevertheless, the development of this geothermal energy has been complex, generating more industrial failures than successes to date. If the 'lack of social acceptability' is often proposed to justify the failure of these projects, it seemed necessary to us, in the course of this research, to go beyond this explanation, by looking not only at the populations, but also at the socio-technical support and the territorial context in which the technology was deployed. Indeed, social acceptability is often reduced to a procedural and 'social risk' management approach, obscuring the fact that the controversy is a competition between different narratives, and revealing shifting power relations within the territories. The deployment of these technologies is constrained by other limits: the techno-economic model requires a maturation phase, through the mobilization of the techno-scientific promise (notably in the case of the EGS) and the search for a viable economic model (heat, electricity, lithium). 6 / 462 Abstract The most salient result of our work is undoubtedly that these projects also reveal mechanisms of politicization of the subsurface, by questioning the relationship between surface and underground environments and by constructing vertical territories. The subsurface is thus gradually changing its status, thanks to the energy transition, becoming an environment where this transition is manifested. These changes are timid, limited to the spaces where underground projects are deployed and, above all, they are not institutionalized. This is illustrated by the analysis of the governance of the subsurface, through the setting up of hybrid forums and the difficulties of managing this environment by the state authorities, which means that, in terms of public policy at least, we are still far from vertical territories. (author)