Feasibility study of repurposing oil and gas wells in Hungary for geothermal production

During the 20th century, Hungary developed a significant oil and gas industry due to the discovery of large hydrocarbon reserves, first in the southwestern and then in the southeastern regions. The legacy of this history, marked by numerous drillings, is a substantial amount of geological information about the country's geology, as well as many currently unused wells.
Combining the abandoned or partially abandoned wells and the favorable geological characterization of the area, the concept of my thesis was born. The Net-Zero European Union strategy, the striving for sustainability, and the ongoing energy crisis are driving the adoption of green energy solutions, and the time has come to act and capitalize on this opportunity. Repurposing oil and gas wells provides an opportunity to produce clean energy. The well owner saves the cost of the abandonment operation, and the investor seeking to produce green energy avoids the drilling costs of a new well.
One problem with the abandoned hydrocarbon wells is that they were designed for oil and gas exploration and production purposes, which differs from the basis of design for geothermal wells. The other technical limitation is that most of the unused wells are either partially or entirely abandoned. Hence, several criteria had to be established to evaluate the status of the wells and their relevance to my thesis.
In this study, an old exploration well was selected based on geological and technical evaluation. Using the well’s end-of-well report, a theoretical 3D geological model was built in FEFLOW. In the software production simulations were held in four different scenarios, starting with the initial conditions and conducting three theoretical acid stimulations on the well. The changes in the production parameters were compared and evaluated. The idea behind the stimulations is that the well could overcome the production limits due to its unfavorable structure.
The selection of the well, based on predetermined criteria, was successful, and a geological model was built using the well’s data. However, due to the lack of data from the well, which was drilled in the 70s, the missing information had to be filled with average values from literature research.
The simulations confirmed the concept, since the selected Nab-1 well’s production parameters were increased due to the acid treatment. However, the geothermal energy output is not increased as much as expected, with increases of only 6%, 16%, and 20% in the different scenarios.
Sensitivity analyses were conducted to determine the increased yield and gross thermal energy output of the well after each treatment. The unit price of energy remains significantly lower after the treatment procedures than in the case of drilling a new well, but the technical risks are high. The feasibility of repurposing oil and gas wells depends on the desired amount of energy and the conditions of the well. To determine whether a well is a good candidate for acid treatment in practice and can produce enough geothermal energy, a deep technical analysis shall be conducted regarding the well’s structural conditions.