Open Access

ARTICLE

Determining the Energy Potential of Deep Borehole Heat Exchangers in Croatia and Economic Analysis of Oil & Gas Well Revitalization

Marija Macenić, Tomislav Kurevija^*, Tin Herbst
Department of Petroleum and Gas Engineering and Energy, Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb, Zagreb, 10000, Croatia
* Corresponding Author: Tomislav Kurevija. Email:
(This article belongs to the Special Issue: Selected Papers from the SDEWES 2024 Conference on Sustainable Development of Energy, Water and Environment Systems)

Energy Engineering https://doi.org/10.32604/ee.2025.067067

Received 24 April 2025; Accepted 12 November 2025; Published online 03 December 2025

Abstract

The increased interest in geothermal energy is evident, along with the exploitation of traditional hydrothermal systems, in the growing research and projects developing around the reuse of already-drilled oil, gas, and exploration wells. The Republic of Croatia has around 4000 wells, however, due to a long period since most of these wells were drilled and completed, there is uncertainty about how many are available for retrofitting as deep-borehole heat exchangers. Nevertheless, as hydrocarbon production decreases, it is expected that the number of wells available for the revitalization and exploitation of geothermal energy will increase. The revitalization of wells via deep-borehole heat exchangers involves installing a coaxial heat exchanger and circulating the working fluid in a closed system, during which heat is transferred from the surrounding rock medium to the circulating fluid. Since drilled wells are not of uniform depth and are located in areas with different thermal rock properties and geothermal gradients, an analysis was conducted to determine available thermal energy as a function of well depth, geothermal gradient, and circulating fluid flow rate. Additionally, an economic analysis was performed to determine the benefits of retrofitting existing assets, such as drilled wells, compared to drilling new wells to obtain the same amount of thermal energy.

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Keywords

Geothermal energy; deep coaxial borehole heat exchangers; deep BHE; heat extraction; abandoned wells; retrofitted wells

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