Hydro-thermal modeling of geothermal energy extraction from Soultz-sous-Forêts, France using supercritical CO2

Climate change requires immediate action, and for sustainable development, and uninterrupted energy supply is necessary. Since anthropogenic emission of CO2 in the atmosphere has a major role in climate change, carbon negative energy solutions are the necessity of the time. Geothermal energy is one such renewable source that can assist in achieving an economic solution to low carbon energy. Engineered geothermal systems or enhanced geothermal systems (EGS) are more suitable from an industrial perspective and can supply uninterrupted energy supply for a long duration. In conventional EGS systems, water is the heat transfer fluid. However, the use of supercritical CO2 as the heat-carrying fluid has significant advantages over water including less chemical reactivity, low fluid viscosity, and comparatively higher thermal conductivity for shallow systems. Fluid loss is the major issue in any EGS operation. However, CO2 loss during the EGS operation could lead to carbon geosequestration, and therefore a carbon-negative energy solution is possible when using CO2 in EGS operations. A case study of Soultz-sous-Forêts geothermal site is considered in this work to investigate the feasibility of CO2 usage as the heat-carrying medium. Soultz-sous-Forêts is present in the Upper Rhine Graben, France. Geologically Soultz-sous-Forêts geothermal site comprises three layers: 1.5 km of thick quarternary and tertiary sediments, 350 m thick Buntsandstein and the basement is granite. Presently, three wells (GPK-3, GPK-4: injection wells, and GPK-2: production well) are operating at this site up to a depth of approximately 5 km. In this work, a three-dimensional Soultz-sous-Forêts site is considered with five major faults. In the present model, supercritical CO2 is injected through GPK3 and GPK4 and produced using GPK-2. This work investigates the coupled hydro-thermal processes occurring in the fractures and the rock matrix. The local thermal non-equilibrium (LTNE) approach is considered to account for the heat exchange between the rock matrix and supercritical CO2 flowing through the faults. Recent studies have reported fluid loss along the wellbore casing in all three wells. Therefore, a wellbore leakage model is also coupled along these well trajectories and its impact on final production temperature is assessed. Results obtained from different injection rate strategy at different injection temperature indicates that even 100 years of geothermal energy extraction operation will not have much impact on the production well temperature and therefore, a sustainable energy supply is feasible at the Soultz-sous-Forêts site.