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.

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