Geological risk analysis of geothermal developments in the Buda thermal karst area

The Buda Thermal Karst system is one of the best-known hypogenic karst areas. In addition, it is located below the city of Budapest. The groundwater of the confined and unconfined carbonate system is characterised by complex gravity and buoyancy-driven flow. It is associated with a complex geological setup characterised by significant heterogeneity in permeability distribution, resulting in exploration uncertainty. The utilisation of thermal water in the area goes back to the Roman Empire through springs, but from the 19th century, wells have been deepened. In Budapest, outstanding thermal wells and projects have been developed. However, several unsuccessful attempts also occurred in SE Budapest due to geological uncertainty, lack of knowledge and proper pre-analysis.

Motivated by the green transition and the energy crisis, the geothermal utilisation of the thermal water in the area has become the focus of interest nowadays. The geothermal risk assessment can be a helpful tool to quantify the uncertainties of the exploration of such a complex system. In this research, we develop a novel methodology to analyse and quantify the geological risk of geothermal developments, focusing on the deep and thick carbonate reservoir. The new approach is adapted from the petroleum industry and follows the risk assessment scheme of petroleum play analysis.

The methodology was tested in a local study area in northeast Budapest, where the required geothermal capacity for heating was previously determined. Here, the Neogene siliciclastic sediments cover the Triassic carbonates, which serve as thermal water reservoirs. To estimate whether a future geothermal system can provide sufficient capacity, in this study, the probability of success is assessed by evaluating potential geological risk factors which can hinder geothermal production and reinjection. This is done by combining and evaluating datasets on aquifer and water quality, temperature and flow conditions using lithological data, geophysical logs (cavities), temperature-elevation, and pressure-elevation profiles. The outcomes of the research help in decision-making while preparing new geothermal developments.

This research was supported and financed through the KDP-2021 Cooperative Doctoral Programme of the Ministry of Culture and Innovation of Hungary from the source of the National Research, Development and Innovation Fund, grant number: KDP 2021 _ELTE_C 1789026. The research is funded by the National Multidisciplinary Laboratory for Climate Change, RRF-2.3.1-21-2022-00014 project.