A number of wells exist that produce thermal water for thermal spas (e.g. Bad Füssing, Bad Wörishofen). To be accredited as spa water, certain criteria concerning chemical composition and/or water temperature have to be fulfilled. In geothermal projects for energy use, the thermal water looses some heat, as heat exchangers inside the heating plant transfer it to water that is running into the district heating system. Due to this process, direct use of the thermal water in the form of a spa is not possible.
In the Bavarian Alpine foothill several hundred wells for the oil, gas and geothermal industry have already been drilled. Vorderriß 1, with a true vertical depth of 6,468 m, is the deepest well. With depths > 5,500 m the wells in Sauerlach and Holzkirchen are the deepest in the geothermal sector in Bavaria. The procedures that ensure safety and environmental protection and avoid emissions have to be followed by the drilling companies. This is monitored by the mining authority of South Bavaria.
The well diameters gradually become smaller with increasing depths. Typical end diameters of a well are 8.1/2” (21.6 cm) and 6.1/8” (15.5 cm). The diameter at the beginning of the drilling is for example 23” (58.4 cm). Geothermal wells have significantly bigger end diameters than gas or oil wells do.
Modern drilling rigs work very quietly. In a distance of several hundred meters the drilling noise usually cannot be heard anymore. Loud noises can be produced e.g. by handling the drill pipes. Depending on distance to residential development the installation of a noise preventer might be required. The boundary values of the “TA-Lärm” (technical protections against noise) have to be kept.
The Bavarian mining authorities require and control the compliance of very strict safety and environmental standards for any kind of deep well, no matter what it will be used for. All safety, environment and water rights issues are examined through an operating plan. At a well site construction provisions must guarantee that neither water, oil or other substances contaminate the groundwater or the environment. Therefore, the inner area of the well site has a complete drainage system including a catch basin and oil/water separator. Wastewater and cuttings are regularly tested for their composition and disposed of by waste disposal companies. Special locking safety systems, so called Blowout-Preventers, protect the wells. Before drilling starts, a conductor casing is installed tens of meters into the ground, this protects the shallow groundwater from the well. After the drilling, the well site has to be deconstructed and recultivated. If a heating or power plant is planned to be constructed, compensation areas have to be provided.
The occurrence of low magnitude seismic events is in principle also possible with hydrothermal use in Bavaria. The vibrations of the subsurface, known as induced seismicity, can be caused by a wide variety of human interventions in the subsurface, e.g. mining, oil and gas production, tunneling and spring construction or geothermal energy. Whether and if so to what extent induced seismicity can be observed depends to a large extent on the boundary conditions (local stress field, disturbance geometry, thermal conditions, etc.). Basically, the Alpine foothills are hardly seismically active. Only with very fine measuring instruments can the smallest earth movements be detected in the subsurface. At a few Bavarian geothermal sites, weak seismic signals (occasionally perceptible, but mostly below the tactility threshold) could be registered. In the case of hydrothermal energy generation in the Bavarian Alpine foothills, however, the occurrence of damaging earthquakes caused by hydrothermal use is excluded by the experts. More than 20 years of operating experience with hydrothermal geothermal energy in Bavaria and Upper Austria confirm this assessment.
The cooled thermal water flows back into the Malm-Aquifer through a reinjection well. The wells are lined by a cemented steel tube that is perforated only in the target horizon. Where required a pump can be used to increase the injection pressure, should the water not flow easily back into the aquifer.
All groundwater and thermal water contains dissolved minerals (especially salt and chalk) and dissolved gases (mainly CO2, sometimes slight proportions of CH4 (Methane) and H2S (hydrogen sulfide)). The thermal water is flowing in a closed cycle during operation, therefore should the thermal water contain traces of environmentally significant components, these substances cannot escape. The reinjection well returns these pollutants into the target horizon. Only during the pumping tests (testing of the well immediately after drilling), steam can escape for a short period into the atmosphere. It's composition is monitored the whole time during testing, but in general it is harmless.
Both for the pure use of geothermal heat for power production and for direct heating the thermal water, which cools down during usage, is reinjected into the ground. To ensure the groundwater remains balanced, it flows back into the same water bearing rock formation where it has been taken from.
This is dependent on geological conditions, in some regions soils and rocks exist which have a higher content of naturally occurring radioactive minerals. Under certain conditions they can dissolve into the ground - or thermal water. If the minerals precipitate on pipelines, radioactive pollutants can concentrate. Increased geogenic concentrations of radiogenic pollutants occur for example in the Keuper-Sandstone in Franconia or in the granite of the Bavarian Forest. The Malm-Carbonates in the Bavarian Molasse Basin do not show any signs of increased concentrations of radiogenic substances.
As well as maps for the earth´s surface, on which mountains, rivers and forests are noted, there exist maps and three-dimensional models for the earth´s interior. For these models information from different sources is needed: Firstly provided from the composition and spatial distribution of the rocks on earth´s surface in combination with knowledge about the regional geological history of the earth. Secondly there are thousands of deep wells worldwide (exploration wells, wells for water, oil or gas, …) and there are already several hundreds in South Bavaria that provide information on the geological structure in detail. Thirdly there are different methods of surveying that give a good view from earth´s surface into its interior. In the Molasse Basin seismic campaigns surveys are commonly executed. Through these sources the structure of the sub-surface, the temperature and the location of the reservoir are known quite well before drilling starts.
Firstly it is important to understand that the earth´s interior heat comes from natural radioactive decompositions of elements from the earth´s crust, mantle (Uranium, Thorium, 40Kalium) and also from residual heat from the creation of the earth (accretionary heat). Furthermore, changes in temperature due to rain water play an important role for near-surface geothermal energy. In general, a steady heat flow from the earth´s hot interior to the cold(er) atmosphere takes place. This happens all over the world (in South Bavaria the heat flow is about 60-90 mW/m2). A geothermal well in combination with a geothermal plant channels and uses this heat flow, which sooner or later gets radiated into space anyway. This is the main difference between geothermal energy and fossil energy sources, which means that geothermal energy is definitely regenerative. 99 % of earth´s mass has a temperature far above 1000 °C and still 90 % of the remaining 1 % is more than 100 °C warm. The heat is not only stored in the thermal water, but mainly in the rocks the thermal water is flowing through. Regarding the use of geothermal energy, water is just the medium of transportation that brings the high temperatures from the sub-surface to the geothermal plant. After using it for district heating or power generation, the water is being reinjected into the ground with lower temperature, where it heats up slowly to its initial temperature. The area surrounding the reinjection well is cooled. It's extend can be predicted by computer simulations. The temperature of the reinjected water depends on the way it is used on the surface. There are high energy fields, where the water is reinjected with temperatures about 170 °C, but there also exist direct usages (greenhouses, fish-farms, de-icing of motorways, etc.) with reinjection temperatures of 10 °C or less. The thermally influenced area inside the reservoir close to the reinjection well can be described as a more or less spherically or cylindrically shaped body around the reinjection well. The coldest temperatures come about in the central area of the thermally influenced area. Outwardly temperatures rise. With the extension of the cold body the surface at which the water can heat up enlarges by the increase of pores and cracks in the soil matrix.
For economical reasons the distance between production and injection well should not be too long (distance of the thermal pipeline on the surface should be as short as possible). After some years of operation the front of the cooled thermal water could reach the production well. This is called a “thermal breakdown”. In addition to the distance between production and injection well, the production rate and the injection temperature, also the preferred flow direction is of great importance (implying the local geology, e.g. direction of faults and karst zones) for the spread of the cooling front. By means of numerical modelling and simulating it is possible to predict the thermal and hydraulic conditions of operation. These conditions can be calibrated over and over again by analyzing pump tests and operational data. The predictions that were calculated based on simulations confirm the existence of the heat source over a long period of time. Solely the energy stored in the 500-600 m thick Malm is of gigantic extend. In case of a thermal breakdown temperature does not drop down suddenly to the reinjection temperature. In fact, it is a very slowly ongoing process (tenths of a degree). In Paris some of the geothermal doublets have been in use since the 1970s. A thermal breakthrough has not been observed yet, though the distance between production and injection well is often less than 1 km. At present, many new geothermal plants are being planned or constructed in Paris (e.g. airport Paris-Orly). The use of geothermal energy can be compared to the use of biomass, where there is also a certain period necessary for the regeneration of the resource depending on the chosen scale of the project. The same applies to the warmth in the area around the reinjection well of a geothermal plant. On the one hand though, the “resource heat” is used and on a local scale sometimes faster “mined” than it can reheat around the reinjection well, but on the other hand it regenerates - contrary to other fossil energy sources – in periods of time that are manageable in human time scales. After finishing geothermal use, the area around the injection well regenerates thermally. This proceeds quickly at first, because the heat flow aligns itself in direction of the cold pole of the reinjection well (not only from below but also from all sides). In the Malm layer to a certain extend convective heat flow is also possible, because it is an aquifer. This accelerates the regeneration process. If heat flow is only conductive, the regeneration process is asymptotic, which means that at the beginning the temperature increases sharply because of a high thermal difference between the two areas. When the thermal difference diminishes, the process slows down, too. According to that, the operating life of a geothermal plant in the Molasse basin is rather determined by the lifespan of the operating equipment (plant, tubes, district heating network).
Especially on the edges of huge tectonic plates, geothermal heat concentrates due to thinning in the earth´s crust and magmatic processes. The heat flow in these areas is significantly above the worldwide average. In these locations geothermal heat has been used on a large scale and with relatively small outlay for many years (e. g. Indonesia, Philippines, Iceland, Italy, USA, New Zealand, Central America, China). In Germany (also in South Bavaria) the heat conditions are of modest values, apart from the Upper Rhine Graben and the formerly active magmatic region of Urach. Using geothermal energy makes sense in Germany not only because of the high population density and the resulting high energy requirements, but also because energy generators and consumers can be brought close to each other. There are two ways of using geothermal energy in Germany: 1. The low temperature level is lifted by a heat pump to a usable level (additional energy for the drive of the compressor is required) --> near-surface geothermal energy. 2. Deep drilling ensures the required temperature level --> deep geothermal energy. The use of geothermal energy requires a “medium of transportation”, which transports heat to the surface. Ideally, there are thermal water-bearing geological layers in the deeper sub-surface, in which heat is stored on the one hand in the water itself and on the other hand in the surrounding rocks. This situation exists in the southern part of the Alpine foothills. The thermal water horizon in this area consists of limestones from the Upper Jurassic (Malm). Inside its pores and joints the thermal water can circulate. The energy already stored in this rock layers is significant and is continuously supplied by regenerative heat flow from the earth´s interior.
Geothermal energy is independent of weather conditions, season and time of day. This means that geothermal energy is able to provide the base load of an energy system. Wind turbines need enough wind to work, hydroelectric power stations only work with sufficient supplies of water. Although photovoltaic conversion plants work the whole year, they produce less energy in the winter season. Bioenergy like wood, vegetable oil and biogas are available in every season, but they need a lot of space. Photovoltaic conversion, wind and hydraulic electric power plants are solely able to produce electricity, solar collectors and near-surface geothermal energy can only supply heat. Deep geothermal energy and bioenergy can do both. The area that is needed for a geothermal plant is very small compared to areas of plants of other regenerative energy sources. On the one hand the investment costs are very high, on the other hand the costs for operating are low and a free resource in form of thermal water is always available. Furthermore, no additional cost for storing capacity arise, contrary to solar or wind power. Hence, geothermal energy can also take part in the mixture of regenerative energy sources for the supply of heat and electricity in Germany.