There is a huge variation in the conditions that exist for the development of deep geothermal energy in Austria’s geological units (surface area 83,871 km2, 8.9 M inhabitants as at 2019). Because of the low permeability of the rock and the generally low heat flux density, apart from a few local anomalies, the units of the Bohemian Massif and large parts of the Central Alps are of limited use for deep geothermal development. In some parts of the Northern Limestone Alps the geothermal gradient is significantly reduced by descending cold meteoric water resulting from the convective imprinting of the temperature field. The effects generated by this convection were discernible to depths of 6,000 m in a hydrocarbon wellbore drilled on the edge of the Vienna Basin in the Northern Limestone Alps.
The basins bordering the Alps, and especially the Upper Austrian Molasse Basin and the Styrian Basin, exhibit the best geological and thermal conditions for exploiting deep geothermal energy and also boast the highest utilisation rates. The geothermal resources of the Vienna Basin are generally estimated to be fairly high and this will have important economic implications, especially in connection with the supply of geothermal district heating to the metropolitan area of Vienna.
By late 2020 Austria had recorded the drilling of 77 deep geothermal wellbores, representing a cumulative length of some 135 km (Figure 1).
About two thirds of this figure, both in terms of number and cumulative length, comprised boreholes sunk in sedimentary basins, while the remainder were drilled in the Eastern Alpine units and consisted exclusively of wells intended for balneology and spa tourism. Because of the complex depositional features involved, combined with the low permeability and small size of the aquifers, a considerable proportion of these drilling projects proved unsuccessful.
At the end of 2019 the installed output of deep geothermal installations totalled 95 MW, to which can be added an estimated 10 MW from balneological systems. This combined output of 105 MW represents about one tenth of that of near-surface installations that operate using heat pumps.
Upper Austrian Molasse Basin
The Upper Austrian Molasse Basin is part of the northern Alpine foredeeps and constitutes the eastern extension of the South German Molasse Basin, which is the most important area for geothermal exploration in Germany. In both these basins the most significant deep aquifer is contained in the basement carbonate rocks of the Late Jurassic (Malm, also locally Berriasian) period. The platform sediments were originally more than 1,000 m thick but were subsequently worn away by erosion phases during the Lower Cretaceous period, when they constituted the land surface, to levels of thickness of less than 100 m in some places. These erosion processes were also associated with karstification phases that contributed significantly to local increases in permeability that reached maximum values in the region of 10 Darcy. The impact of tectonics is discernible in many of the successful geothermal projects and fault zones are of significance here because of their increased transmissivity, which is frequently connected with the dolomitisation of the limestones.
The water circulating in the Late Jurassic aquifer has a total solution content of 1 to 1.2 g/l and is consistently of the sodium hydrogen carbonate type. According to isotopic labelling this water exhibits a very high percentage of atmospheric precipitation, which suggests a connection to surface catchment areas.
The carbonates of the Late Jurassic lie partly below thick Upper Cretaceous deposits that are mostly of clay marl and calcareous marl in origin. These beds act like a hydraulic and thermal cap rock and are overlain by Palaeogene and Neogene sediments that can reach thicknesses of over 3,000 m in the southern part of the Basin. They contain important oil and gas bearing beds, though apart from local balneological applications the strata have no real significance for the development of deep geothermal energy.
The maximum values of the terrestrial heat flow of 95 mW/m2 produce the highest thermal gradients of around 4 K/100 m, local convection systems excluded, with the highest pumping temperatures that can be expected being in the region of 120 °C.
The commercially successful geothermal projects that have already been set up in the Upper Austrian Molasse Basin demonstrate the significance of deep geothermal energy for the decarbonisation of heat supplies. By the end of 2020 seven geothermal district heating networks were in operation with a cumulative installed output of 86 MW and a total volume flow of 380 l/s. These systems are currently concentrated on the northern Innviertel where there is a high density of geothermal installations clustered around the thermal springs of Lower Bavaria, north of the Inn river (Bad Füssing, Bad Birnbach and Bad Griesbach) (Figure 2).
The maximum working temperatures of the Upper Austrian installations are around 107 °C, while the highest installed output (18 MW) is currently that of the Ried-Mehrnbach district heating system. Along with the Simbach-Braunau operation, which is a joint German-Austrian project, Ried-Mehrnbach offers the greatest potential for further district heat connections. At the Geinberg thermal project, which provides district heating and supplies a spa resort and hotel complex, plans are now being made to extend and diversify the consumer base by supplying heat energy to a major horticultural centre.
The Styrian Basin (Figure 3), which is a marginal branch of the Pannonian Basin, is the second most prospective geothermal province in Austria. Thermal operations commenced in the 1970s and initially focused on balneological exploitation, with three sites (Loipersdorf, Bad Waltersdorf and Bad Blumau) being established at a former failed hydrocarbon exploration well. Extension wells were subsequently drilled at all three locations. These successful thermal projects have provided a much-needed economic boost and have helped bring structural change to these primarily agricultural regions.
As a marginal branch of the Pannonian Basin the Styrian Basin exhibits terrestrial heat flow densities of up to 105 mW/m2, this producing geothermal gradients of between 4 and 5 K/100 m. The most important aquifers for deep geothermal exploitation are primarily situated in the basement carbonate rocks of the Graz Palaeozoic, which generally date from the Devonian Period. This geological unit presents a complex internal nappe structure that has yet to be explored in its entirety owing to the relatively low exposure density to date, a situation that restricts geological predictive reliability. Another important element is the occurrence of post-volcanic CO2 in the Palaeozoic aquifer, this leading to heavy carbonate precipitation (scaling) due to degassing as a result of the release of pressure during the production stage. Gas-water ratios of more than 10:1 were observed at some individual wells.
The basement of the Basin is overlain with Neogene sediments whose stratigraphic reach extends from Carpathian to Pliocene. The mainly clastic sediments of the Lower Badenian and Pliocene contain volcanic rocks, which in the latter case take the form of basalts, while the Badenian volcanites are formed as latite and andesite, these creating extensive buried lava domes that present significant problems for seismic-reflection survey work.
Because of their reduced volumetric flow demand balneological projects can also be set up in the Neogene strata series, with the sandstones of the Sarmatium Stage mainly serving as aquifers. Here the usable temperatures are at a maximum of 60 °C.
Of all the geothermal energy schemes currently running Bad Blumau and Frutura (vegetable production) are especially worthy of mention. The Blumau project is based on an exploratory well that was drilled by an oil prospecting company back in 1979. This hydrocarbon well reached the fractured Graz Palaeozoic at a depth of some 2,600 m (the Blumau 1/1a well). In 1996, during the development of the Bad Blumau geothermal project, a 2,800 m deep geothermal well (Blumau 2) was drilled along with another 1,200 m-deep well that was intended for balneological use. This geothermal doublet consists of production well Blumau 2 and reinjection well Blumau 1/1a. With a free overflow of 30 l/s produced by the gas-lift effect of the degassing CO2 the result is a cascade-type process that uses ORC technology for electrical power generation (250 kWel) and also for heating a hotel complex and thermal spa centre, as well as supplying an outdoor swimming pool. The installed thermal output is around 7.5 MW.
The two deep wells for the Frutura greenhouse project north of Fürstenfeld were drilled in 2014. As at Blumau these wellbores were also targeted at the Palaeozoic limestones and dolomites of the Basin basement. The first well, Frutura GT 1, reached the Palaeozoic sequence at a depth of 2,800 m and drilled through 460 m of limestone and dolomite rock. As the results of the first pumping tests were unsatisfactory a sidetrack (Fru GT 1a) was then drilled on the basis of data obtained from well-seismic surveys. Frutura GT 2 only reached the top of the Palaeozoic strata series marginally higher than GT 1. However, GT 2 had a greater deviation than GT 1/1a and was therefore able to penetrate the completely dolomite-based Palaeozoic sequence over a length of some 560 m. After carrying out tests it was decided to use GT 2 as the production well and GT 1/1a as the reinjection well. The installation of a centrifugal immersion pump at a depth of 900 m, which was well below the bubble point, produced a production rate of 60 l/s without the need for a separator. Frutura GT 2, which has a probe head temperature of 124.5 °C, is now Austria’s hottest geothermal well. The CO2 content of the gas can be as much as 90 % at a gas-water ratio of up to 12:1. Frutura GT 2 has achieved a water mineralisation rate of 70 g/l, the highest yet recorded in the Styrian Basin. Isotope labelling has qualified the sodium-chloride water as connate water without meteoric components.
Geothermal energy was first used in the greenhouse centre in 2016. The installed thermal output currently stands at 16 MW for a greenhouse area of 17.5 ha and an annual energy output of 63 GWh. When the complex is finally completed it will have a greenhouse area of about 24 ha. There are also plans to use the carbon dioxide gas for CO2 supplementation in the greenhouses.