The aim of our research is to use abandoned deep-hydrocarbon reservoirs and dry wells in the Croatian part of the Pannonian Basin as a geothermal energy source. Croatia has been exploring and exploiting hydrocarbon reserves in the Pannonian Basin from the mid-20th century. Therefore, many oil and gas wells are reaching the end of their production phase and many are already abandoned. These wells could be considered for geothermal energy production through the coaxial heat exchanger principle, which is usually used in shallow geothermal energy extraction. Using the abandoned deep well Pčelić located in the Drava subbasin as a case study, we have derived the available energy and fluid temperature changes during 20 years of operation for two cases: one with a constant base heat load throughout the year and the second as a variable heat load depending on outside air temperatures. We determined that the maximum potential heat extraction in a variable system is 1750 MWh per year, with 1.5 MW of peak heating power in winter, depending on the sink temperature, climate, and consumer input data. The maximum theoretical constant heat extraction for possible industrial direct heating could be 400 kW during the entire period of 20 years, with fluid temperature reaching steady state at a favorable 50°C. To define steady-state ratio between extracted heat and consumed energy at the circulating pump, we evaluated seasonal performance factor (SPF) analysis similar to heat pump systems. Lower values of SPF linked to a higher flow rate implies higher energy extraction. Our results show that when using a lower flow, steady-state SPF ratio is as high as 280, and for a higher flow, steady-state SPF ratio drops to only 25.

Aquifer Thermal Energy Storage (ATES) systems are a promising solution for sustainable energy storage, leveraging underground aquifers to store and retrieve thermal energy for heating and cooling. As the global energy sector faces rising energy demands, climate change, and the depletion of fossil fuels, transitioning to renewable energy sources is imperative. ATES systems contribute to these efforts by reducing greenhouse gas (GHG) emissions and improving energy efficiency. This review uses the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analysis) methodology as a systematic approach to collect and analyze relevant literature. It highlights trends, gaps, and advancements in ATES systems, focusing on simulation methods, environmental impacts, and economic feasibility. Tools like MODFLOW, FEFLOW, and COMSOL Multiphysics are emphasized for optimizing design and system performance. Europe is identified as a continent with the most favorable predispositions for ATES implementation due to its diverse and abundant aquifer systems, strong policy frameworks supporting renewable energy, and advancements in subsurface energy technologies.

At three locations in Zagreb, classical and extended thermal response test (TRT) was conducted on installed coaxial heat exchangers. With classic TR test, thermogeological properties of the ground and thermal resistance of the borehole...

Utilization of shallow geothermal energy is well known in several European countries. Even though large parts of Republic of Croatia show significant potential for it use, installation of ground source heat pump systems (heat pump and...