The Danube basin pilot area
The pilot area of Danube basin is situated in Slovakia, Hungary and partly at Austria. The Danube basin pilot area covers around 12,170 km2 (Figure 1). The Danube Basin is geographically represented by the Danube Lowland in Slovakia and by the Little Hungarian Plain in Hungary. On the west it is bordered by the Eastern Alps, Leitha Mts. and Male Karpaty Mts. On the north the basin has finger like extensions which penetrate among the core mountains of Male Karpaty, Povazsky Inovec and Tribec. On the northeast it is bounded by the Middle Slovakian Neovolcanics and the Burda volcanics. On the southeast, there are emerging units of the Transdanubian Central Range.
Figure 1. Delineation of pilot model area with the production wells.
From the hydrogeological build-up of the Danube Basin it can be assumed that the Carpathian crystalline basement of the Danube basin does not contain relevant geothermal aquifers. However, suitable aquifers are bound to sands or sandstones of Pannonian, Pontian and Dacian age within the Tertiary basin fillings.
The highest heat flow densities have been recorded in the middle of the depression (q > 85-90 mW/m2) and do not correspond to lower temperatures (T < 45°C) nor thermal gradients. Whereas heat flow decreases towards the margins of the Danube basin, temperature increases. This irregularity is caused by a cold water body, which is bound to the uppermost hydro-geological unit showing a maximum thickness of 460 m. The colder zone gradually perishes downward and the temperature field corresponds to the heat flow. Badenian volcanoclastics at depths of 5000 – 6000 m may contain geothermal waters with aquifer temperature exceeding 200°C. They can be utilized by applying reinjection for reasons of sustainability. Because of its post-Sarmatian evolution, the Danube basin has a bowl-like brachy-synclinal shape.
The upper boundary of the geothermal water body is located at depths of 1000 m below the surface and at the bottom it is confined by a fairly impervious substratum - an aquitard (clays) which plunges from the surrounding area towards the middle of the basin to a depth of up to 3400 m below surface.
Figure 2. Groundwater temperature (color) at the base of the main thermal reservoir – upper Pannonian sediments (depth contours, m a.s.l.).
This hydrogeothermal system is likely to have interlayer leakage, inter-granular permeability and confined groundwater levels. It bears geothermal waters with registered temperatures between 42°C and 92°C warm, which are bound to sands and sandstones of Dacian, Pontian and Pannonian age. Figure 2 shows the temperatures and the depth of the upper Pannonian geothermal aquifer base.
The aquifer that is of main interest is late Miocene age (Pannonian, Pontian, Dacian) characterized by clastic rocks and sediments that belong to the Beladice, Volkovce and Ivánka formations on the Slovak side and Újfalu formation, Somló-Tihany formation and Zagyva formation on the Hungarian side. The geothermal aquifer consists of sandstones, sands and clays intercalations of different ratios with intergranular permeability, interlayer leakage and confined ground water level. The geothermal aquifer is explored with few tens of geothermal wells on both sides at different depths. These wells tap geothermal water of different temperature (42°C – 92°C) and different content of dissolved solids (chemical type Na-Cl, Na-HCO3-Cl, Na-HCO3, Na-Ca-HCO3, Ca-Mg-HCO3).
Nowadays there is no existing utilization conflict in Danube basin geothermal area. Though future problems in excessive utilization can happen as shown on model results (Figure 3). Modeled groundwater temperatures and pressure may significantly drop after long-term exploitation.
Figure 3 Map of hypothetic decrease of temperature (A) and pressure (B) due to infinite pumping (color) and piesometric head contours at the base of upper Pannonian at natural state (black, m a.s.l.) overlain with proposed monitoring areas (red).
Main objectives and questions to be answered
The aquifer is recharged based on interlayer leakage from higher horizons. Independent aquifer monitoring and monitoring of utilization is needed for maintaining optimal utilization of geothermal groundwater. Is reinjection into the intergranular aquifers feasible (based on today technology)? If yes, under what conditions?
How can the elaborated steady state models help to manage future utilization?
The elaborated steady state and transient geothermal models can serve as a tool in future joint management of transboundary thermal reservoirs. Possible future influence was predicted based on the current utilization magnitude (Figure 4). The steady state models allowed to identify sensitive areas were future monitoring of geothermal aquifer should be established. Scenarios of additional direct use of thermal ground water proved the necessity of geothermal water reinjection.
Future joint resource assessment, monitoring and reporting strategies
The monitoring status of the geothermal water in the active wells is not sufficient. Monitoring of geothermal aquifers is based on reported abstraction (yield and temperature) on an annual basis with monthly reported values in both countries. Independent monitoring of the geothermal aquifer (through monitoring wells constructed exclusively for this purpose, monitored by independent authority) of the geothermal aquifer is not established in any of the countries. Detailed cross-border monitoring and reporting should be a part of an effective, reliable transboundary monitoring system equipped with pressure/head transducers, temperature and electrical conductivity probes. Ideally monitoring should be performed on daily basis. Active monitoring is expected in the operating thermal wells with remote control and web-based data presentation.
Mitigation of environmental impact
Geothermal water with high amount of TDS is not utilized. In case of well Po-1 in Podhájska, geothermal water is partly reinjected back to the aquifer and partly disposed to the surface water. Most of the utilized geothermal water after their thermal use is discharged to surface streams and has acceptable values (temperature and TDS limits) for environment. Some installations include heat pumps for better thermal efficiency or use methane (if present in water after the gas separation, site Zlatná na Ostrove, well VZO-13) and thus mitigating thermal pollution of the streams or production of green house gases.
Towards a future joint thermal water management
Recharge of geothermal water has been evaluated in a number of studies in Slovakia, evaluating the regional conditions for geothermal water circulation and water regime along with calculations of water sources and reserves. They lack periodic updates based on monitored data in the geothermal aquifer. The water balance calculations are not representative on the Hungarian side of the pilot area. Although the amount of abstraction is defined for most of the wells and is stated in permission for abstraction, there is lack of data verification and independent monitoring of the aquifer to prevent depletion of geothermal resources. The delineation of geothermal water body that is used for reporting on national levels is not compatible across the national border of both countries and different attitudes of geothermal water body delineation are used.
Future management strategies.
To forecast the future effects of thermal water production and utilization in Slovakia and Hungary, common regulations are needed to control water production in both countries. For the better and most efficient management, the delineation of a transboundary geothermal groundwater body is recommended based on same rules for delineation. The better understanding of the thermal aquifer requires some additional research.
The Web based Information Tool on hydrogeothermal resources in the Danube Basin Pilot Area aims to give an overview about identified relevant hydrogeothermal reservoirs, summarized at so called “Hydrogeothermal Plays” in order to support decisions towards a possible future transnational management of natural thermal water resources. It addresses governmental authorities, decision makers, possible investors as well as experts and the interested public. However, it does not aim to replace detailed surveys or feasibility studies, as most of the published results are presented at a regional geographical level.
The Web Information Tool consists of the following products:
The Web Based Information Tool summarizes the main results of Transenergy and intends to inform, raise awareness and provide sound base data and models.
The individual Web Map Service for the Danube Basin Pilot Area shows relevant reservoir data for the Hydrogeothermal Play.
For all pilot areas the following uniform list of contents concerning the Web-Map-Service has been chosen:
All presented layers are derived from regional geological and numerical models achieved during Transenergy. They intend to give an overview about the regional scale geothermal conditions at the identified Hydrogeothermal Plays.
The pilot area Web-Map-Service is very similar to the Transenergy Web-Map-Service for the supra-regional project area. In order to view the manual click here.
Data available within the web map application is provided only for a regional overview of geological and geothermal conditions of the TRANSENERGY pilot area Danube Basin and should not be used for more local potential/reservoir assessment. No permission is granted for use of the data outside this application without written permission of the owner of the data. For further information please contact our project partners.
Videos of scenario modelling: