How an unsuccessful exploration well became a geothermal oasis – the case of the Podhájske baths in Slovakia

Today Podhájske is one of the most known and best developed thermal baths of Slovakia, but its story begun with an unsuccessful exploration project for hydrocarbon deposits in the first half of the ’60-ties. The exploration wells were drilled as P-1 (1335 m), P-2  (1601 m), P-3 (1602 m) and P-4 (1317 m) and expected Miocene bituminose shale deposits. In these times there were no seismic profiles available to prepare the projectants for the surprise, thus, instead of bituminose shales they discovered a relic of the Hronic nappe built by Triassic dolomites and Permian clastics – and geothermal water of (lower-middle Miocene) seawater origin.

Chemical composition of the geothermal water from the Po-1 well of Na-Cl type showing its seawater origin.

Since the original wells were not suitable for geothermal purposes, a new well was projected and drilled in 1973, the Podhájska-1 (or simply Po-1). The well reached 1900 m depth with yield between 45-53 l/s and 81°C temperature. Soon the building of the thermal baths and greenhouses begun. However, the used and cooled water was released to the stream at the time.

The original Po-1 well during drilling (1973).
3D scheme of the geological setting of the Po-1 well. The lower layer shows the geology of the basin basement (light blue are the mentioned Triassic dolomites), upper layer shows the heat flow density on the surface.

The collector of the thermal water is in the Triassic dolomites of about 190 km2 area and 360 m average thickness, which is a closed and hydrogeological structure with limited volume. After a few years a decrease of pressure and also temperature was detected as signs of overexploitation of the sources. For this reason, in 1986 a new well, the GRP-1, was drilled which serves for reinjection of the used water and thus our story has a happy ending: since 1996 the whole amount of thermal water is pumped back into the structure giving life for the spa, heating the neighbouring hotels and greenhouses for a long time to come.

Well log and correlation of the Po-1 and GRP-1 wells with the direction of the geothermal water cycle. Numbers in the map: 1) the Po-1 well, 2) thermal spa, 3) hotel heating 4) greenhouses 5) the GRP-1 well – reinjection.

Balazs Kronome
SGIDS, State Geological Institute of Dionýz Štúr

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