Enhanced Geothermal Systems (EGS) involve artificially creating permeability in geothermal systems through engineering and stimulation methods. Despite over 30 EGS projects initiated since the pioneering experiment at Los Alamos National Lab in 1974, technical complexities, financial constraints, and seismicity challenges have hindered their success. This work examines all EGS projects established from 1974 to 2024, focusing on developments over the last decade. Presently, Europe leads with nine commercial and research projects, followed by North America with seven EGS endeavors. Most projects have been conducted in igneous formations. While some projects, such as Desert Peak and Fervo Energy in the USA or Landau and Rittershofen in Europe, achieved high flow rates after stimulation, many projects failed to meet commercial viability thresholds, resulting in project abandonment. Challenges also persist due to induced seismicity, especially in naturally fractured reservoirs near large fault zones. Recent advancements, like Fervo Energy's multistage fracking in non-fractured rock, offer promise in mitigating induced seismicity. A deep understanding of thermo-hydro-mechanical behaviors, careful control over injection rates, and comprehensive risk assessments remain crucial for operational safety and the further development of EGS.

As the hydrocarbon industry declines in central Europe, countless wells are left behind, providing an opportunity to use existing infrastructure and expertise in accelerating the green energy transition. TRANSGEO is a regional development project exploring the potential for producing geothermal energy from these abandoned oil and gas wells. TRANSGEO members have produced socio-economic and policy analyses of well reuse in 5 central European countries: Austria, Croatia, Germany, Hungary, and Slovenia. The socio-economic analyses focus on reusing active and abandoned boreholes in municipalities (for district heating systems and thermal baths/spas), agriculture (for greenhouses, drying, and aquaculture), and industry. Heat demand of these specific applications has been matched with the energy production potential of 5 geothermal reuse technologies (Aquifer and Borehole Thermal Energy Storage, Deep Borehole Heat Exchangers, Hydrothermal Energy, and Enhanced Geothermal Systems) to guide future geothermal development projects in choosing the most suitable options for well reuse. The economic analysis provides cost estimates for a variety of reuse situations and compares the cost of well reuse with the cost of drilling new wells, which is often much higher. The policy analysis provides information on the laws related to well ownership and reuse in the 5 countries, guidance on steps required to undertake a reuse project, and national and EU financial support and incentives. TRANSGEO is co-funded by the European Regional Development Fund through Interreg Central Europe.

As more deep hydrocarbon wells are coming to the end of production, interest in opportunities to reuse this valuable infrastructure for geothermal development is increasing. To facilitate repurposing of existing wells, the regional development project TRANSGEO is creating a variety of tools and guidance documents to inform new geothermal redevelopment projects and decrease their technical and financial risk. We have compiled a database of wells in regional sedimentary basins in five central European countries (Austria, Croatia, Germany, Hungary, and Slovenia) as the basis for an online application for selecting candidate wells for geothermal redevelopment. Additionally, engineering workflows for applying 5 geothermal reuse technologies (Aquifer and Borehole Thermal Energy Storage, Deep Borehole Heat Exchangers, Hydrothermal Energy, and Enhanced Geothermal Systems) were created based on literature and numerical modelling studies. The workflows provide information on the steps involved in evaluating and adapting a well for a new purpose. TRANSGEO is co-funded by the European Regional Development Fund through Interreg Central Europe.

The use of carbon dioxide (CO2) in geothermal power plants has gained significant attention in recent years as a means of preventing calcium carbonate scaling in the wells and submersible pumps. Calcium carbonate scaling can cause a number of problems in geothermal power plants, including reduced efficiency, increased energy consumption, and decreased power output.

The PRESUS C technology from Linde is a highly effective and reliable method of injecting CO2 below the submersible pump in geothermal power plants. This technology is capable of reducing the concentration of dissolved calcium in the geothermal fluids, thereby preventing the formation of calcium carbonate scaling.

By injecting CO2 into the geothermal fluid, the pH level of the fluid is lowered, making it less conducive to the formation of calcium deposits. This method has been proven to be highly effective, with some geothermal power plants.

Moreover, the use of CO2 injection in geothermal power plants is a relatively low-cost and environmentally friendly solution to calcium carbonate scaling. Unlike some other methods, such as acidification or the use of chemical or biological inhibitors, CO2 injection does not produce any harmful byproducts or waste.

In conclusion, the use of CO2 injection technology such as PRESUS C from Linde in geothermal power plants is an effective and sustainable solution to calcium carbonate scaling. By preventing the formation of calcium deposits, this technology can help geothermal power plants operate more efficiently and reliably, ultimately contributing to a more sustainable and low-carbon energy future.

The research project Eva-M 2.0 adopts a holistic approach to investigate and compare two methods for mitigating calcium carbonate scaling in geothermal plants located in the Bavarian Molasse basin. The first method involves the injection of an environmentally friendly liquid polymer inhibitor, while the second employs CO₂ injection. The study incorporates comprehensive hydrochemical and microbiological monitoring, alongside assessments of biological and thermal degradability, corrosion, and fluid-rock interaction with the reservoir. This paper presents the results related to the economic efficiency of both scaling mitigation methods, focusing on key performance indicators. The results demonstrate that both methods effectively prevent scaling, thereby enhancing the economic efficiency of medium-enthalpy hydrogeothermal projects.