Deep geothermal energy exploits reservoirs that are accessed at depths greater than 400 meters below ground level. The types of reservoirs are basically the same as those used in shallow geothermal energy. In deep geothermal energy, a distinction is also made between high-enthalpy and low-enthalpy reservoirs. This classification is based on temperature, with 200 °C generally considered the boundary between low-enthalpy (lower temperatures) and high-enthalpy (higher temperatures) reservoirs.
Hydrothermal reservoirs are areas at depths of over 400 meters in which thermal water circulates. This water can occur in karst cavities, fractures, fault zones, or porous aquifers. Hydrothermal reservoirs are widely developed in Germany at considerable depths. The reservoirs are used for balneological purposes in spa towns such as Bad Staffelstein. District heating supply is based on hydrothermal geothermal energy in cities like Munich-Riem, Neustadt-Glewe, Erding, and many others. Munich aims to convert its district heating supply to 100 % renewable energy by 2040, largely relying on geothermal energy. Electricity and heat are also generated, for example, in Grünwald near Munich. For electricity generation, temperatures above 120 °C are required.
A prerequisite for a hydrothermal system is the presence of a productive water-bearing rock layer (productive horizon) with extensive vertical and lateral distribution to ensure long-term utilization. The thermal water circulating in this natural reservoir can be used for both electricity and heat production or for heat alone, depending on the extraction rate and temperature.
Thermal water is usually exploited with two or more wells. A so-called doublet consists of a production and an injection well. When complemented by an additional well for extraction or reinjection, it is called a triplet. Deviated well paths allow multiple wells to be installed within a small power plant site while avoiding a geothermal short circuit. Since only one drilling site is needed and relocating the drilling rig is less complex, this approach saves costs.
Natural reservoirs with sufficient water availability are widespread in Germany. In the geothermal provinces of the Molasse Basin in the Alpine foreland, the Upper Rhine Graben, and the North German Basin, hydrothermal reservoirs are also present at sufficient depths to economically generate electricity and provide heat.
In northern Germany, sedimentary pore reservoirs from the Jurassic, Triassic, and Permian periods are primarily developed for heat supply. In southern Germany, secondary-fractured and/or cavernous rocks are mainly used. These are referred to as fracture or karst aquifers. To achieve the highest possible extraction rates, these rocks are developed in the areas of fault zones. An example of this is the Malm carbonates of the Bavarian Molasse Basin.
In the Upper Rhine Graben, large quantities of water are primarily associated with young, deep-reaching fault zones. These must be accessed precisely via deviated wells. To connect a well to a fault zone or improve the connection, stimulation measures can be employed, including hydraulic and chemical stimulation. In particular, the carbonate rocks of the Muschelkalk in the Upper Rhine Graben and the Malm limestone in the Molasse respond successfully to acid stimulation. If inflow or injection is best achieved via an extension or a branch of an existing well, this is referred to as drilling-based stimulation.
The basis for precise drilling is prior seismic exploration. This method allows the identification of individual layers as reflectors as well as fault zones.
The disadvantage of hydrothermal geothermal energy lies in the spatially limited distribution of reservoir rocks. As a result, heat utilization is restricted to specific regions. Therefore, hydrothermal geothermal energy accounts for less than 10 % of the technical potential.
The term petrothermal reservoir generally refers to hot deep rock that is free of circulating thermal waters. Essentially, all low-porosity and poorly fractured deep rocks and sedimentary rocks are suitable. In terms of utilization, petrothermal systems account for about 90 % of Germany’s potential for geothermal electricity generation. Current petrothermal projects include the EU research project Soultz-sous-Forêts, which is now in economic use, as well as the project in Bad Urach.
Crystalline and densely layered sedimentary rocks at sufficient depth and with appropriately high temperatures (over 150 °C) can serve as reservoir formations. Development is carried out via two or more wells drilled into the deep, dense rock. Stimulation measures are then used to create artificial pathways for water flow.
Various terms are used for the utilization of petrothermal geothermal energy. In Europe, the term “Enhanced Geothermal System” (EGS) has become established, which can be translated as “improved geothermal systems.” In the United States, the term “Engineered Geothermal Systems” (also EGS) is commonly used. These terms also cover projects in which low natural permeability is increased through stimulation. Outdated and technically obsolete terms include Hot-Dry-Rock (HDR), Hot-Wet-Rock (HWR), and Deep Heat Mining (DHM). The latter term gives the impression that, similar to a mineral deposit, the resource is “exploited” and depleted. This is incorrect for geothermal energy, as the resource is effectively inexhaustible from a human perspective.
Hydraulic stimulation (hydraulic fracturing) is the main technique for developing petrothermal reservoirs, involving the widening of existing fractures and the artificial fracturing of compact rock. This is achieved by injecting large volumes of water into the rock under high pressure. The injection of cold water cools the rock, and the resulting contraction helps to open fractures. Hydraulic stimulation is the most important stimulation method. Chemical stimulation, using substances such as acids, can enlarge fractures in soluble rocks like limestone and remove drill cuttings and deposits.
Heat extraction is carried out using circulating water, which is fed into the artificially enlarged or newly created fracture system through the injection well. The water absorbs heat from the rock and is then brought to the surface via the production well, where it is used for energy generation before being returned to the system through the injection well.
The advantage of using petrothermal reservoirs lies in their vast potential, as they are not dependent on locally confined thermal water reservoirs. This makes development location-independent.
Deep geothermal probes are a closed system for geothermal energy extraction that reach greater depths than the probes used in shallow geothermal energy. Both systems operate in a similar way, but deep probes achieve higher temperatures, allowing the heat to be used directly for heating without the need for a heat pump. All potential heat utilization options are available, ranging from high-temperature process heat for industry and commerce to low-temperature agricultural applications.
The heat transfer area with the surrounding rock corresponds to the borehole’s surface area. The possible extraction capacity ranges from 150 to 250 watts per meter of borehole depth, which for a 2,000 to 3,000-meter borehole corresponds to several hundred kilowatts.
The deep geothermal probe system consists of a single borehole at depths of over 400 meters up to several thousand meters. In its simplest form, a coaxial pipe is installed. A heat transfer fluid circulates through this pipe: it is pumped down through the outer annulus of the borehole, absorbs heat at depth, and then rises back to the surface via a thinner, insulated inner pipe. The surrounding rock serves as the heat exchanger. Water (possibly with additives) is primarily used as the heat transfer medium in the probe.
The thermal performance of a deep geothermal probe primarily depends on geological conditions: the temperature, described by the local geothermal gradient, and the heat transport, which occurs either slowly via conduction through the rock or relatively quickly via convection through groundwater.
Technical measures within the probe can influence the maximum heat output. Large heat pumps can further increase energy yield at lower input temperatures.
As an alternative to circulating water, probes with direct evaporators (heat pipes) can be used. These employ a working fluid with a low boiling point, or a mixture such as ammonia and water. Such probes can also operate under pressure, for example with carbon dioxide. Heat pipes can achieve significantly higher extraction rates than conventional probes. This technology is already used in geothermal applications but is still in the early stages of development.
Deep geothermal probes also offer the opportunity to repurpose existing, no-longer-used oil and gas wells, reducing construction costs. For example, in 1988 in Prenzlau, an existing borehole was deepened to around 2,800 meters and converted into a deep geothermal probe. Since 1994, around 300 kW of thermal energy has been supplied to the municipal district heating network.
The advantage of deep geothermal probes compared to open systems is that, due to the closed loop, there is no contact with groundwater, preventing any material exchange with the surrounding rock. Geochemical processes such as dissolution and mineralization are entirely avoided. Furthermore, deep probes can be installed at any location, as they do not rely on natural thermal water reservoirs and are not bound to specific geological structures. This eliminates the exploration risk associated with other deep geothermal systems.
