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Die vorliegende Erfindung betrifft ein System zur Gewinnung geothermischer Energie, umfassend a.) ein druckdichtes Kühlungsreservoir und einen tiefer gelegenen druckdichten Gesteinshohlraum als Wärmereservoir, wobei im Wärmereservoir gegenüber dem Kühlungsreservoir eine erhöhte Temperatur und optional ein erhöhter Druck vorliegt, b.) eine Injektionsleitung, über die ein im Kühlungsreservoir vorliegendes flüssiges Energietauschermedium dem Wärmereservoir zugeführt werden kann, c.) eine Steigleitung, über die das in seinen flüssigen oder überkritischen Aggregatzustand übergegangene Energietauschermedium aus dem Wärmereservoir in das Kühlungsreservoir zurückgeführt werden kann, wobei die Steigleitung eine Vorrichtung zur Energieumwandlung umfasst. Zudem wird ein Verfahren zur Gewinnung geothermischer Energie beansprucht.

This very young thesis is so simple that it is hard to imagine that it is new and that no one had this idea before. The thesis applies to the Green Karst only. It promises to be an essential addition to the understanding of karstification. NERO assumes that CO2 not only participates passively in the water cycle, but also has an independent, in‐depth mobility (Greek: nerochytis = sink). NERO could be highly relevant to karstification. CO2 has a higher specific density than air. Carbonated water has a higher specific density than non‐carbonated water. Overall, CO2 has an in‐depth mobility vector. The NERO thesis means turning the focus away from only the water cycle and towards both, water cycle and cave‐air CO2 mobility. There are stations and transitions: the soil, the gaseous transition from the soil to the vadose zone, the vadose zone with cave‐air ventilation, density‐driven dissolution at the karst water level, the phreatic zone and the karst spring. The NERO thesis offers a new approach to understanding cave formation in the Green Karst: At the karst water level, water is enriched with additional CO2 from the cave air. In the following new solubility arises. Subterranean waterways with their typically round, phreatic wall and ceiling shapes can be understood as a result of that process. Perhaps even giant halls can be explained by this process (SCHERZER et al. 2017, 2020).

We present a dataset of in-situ measurements in the marginal area of a CO2- and brine-rich cavernous structure in an underground salt mine. The data were collected within the framework of the BMBF-project ProSalz. One aim was to reveal the sources and dynamics of fluid movement as well as temporal and spatial distribution of fluids in a potentially weakened cavern rim. Over a period of three years pressure and gas monitoring was carried out along a transect from a cavernous structure to undisturbed rock salt. In addition, temperature and relative humidity data from the underground gallery were recorded. The gas inflow into isolated borehole sections provided an insight into short- and long-term changes of gas migration patterns in rock salt. Pressure increases of up to 4kPa/day and CO2 concentrations of up to 1.2%, especially at the start of the campaign were measured. The gas migration is coupled to discrete fractures and was limited spatially and temporary. Overall, gas occurrences were not correlated to their distance to the cavern, suggesting no wide-ranging fluid-rock interaction within the rim of the investigated natural cavernous structure in rock salt.

Data of the "density-driven dissolution of CO2 in karst water" column experiment. A laboratory column was filled with tapwater (water level 5,55 m) and exposed it to an elevated gaseous CO2-concentration, roughly 50 times the current atmospheric concentration (20000 +/- 5000 ppm). After stripping the water with ambient air, it was initially in equilibrium with atmospheric conditions. Then, the concentration of dissolved CO2 was measured over a time period of 60 days in two different depths in the column. Measuring points are in the column air, 1 m below water level and 0,15 m above ground. CO2-air concentration was provided using an air-membrane pump (KNF N86 KTE) and a 100 l TEDLAR bag (for more detailed information look into the related publication Class 2021 et al.)

The relevance of CO2 emissions from geological sources to the atmospheric carbon budget is becoming increasingly recognized. Although geogenic gas migration along faults and in volcanic zones is generally well studied, short-term dynamics of diffusive geogenic CO2 emissions are mostly unknown. While geogenic CO2 is considered a challenging threat for underground mining operations, mines provide an extraordinary opportunity to observe geogenic degassing and dynamics close to its source. Stable carbon isotope monitoring of CO2 allows partitioning geogenic from anthropogenic contributions. High temporal-resolution enables the recognition of temporal and interdependent dynamics, easily missed by discrete sampling. Here, data is presented from an active underground salt mine in central Germany, collected on-site utilizing a field-deployed laser isotope spectrometer. Throughout the 34-day measurement period, total CO2 concentrations varied between 805 ppmV (5th percentile) and 1370 ppmV (95th percentile). With a 400-ppm atmospheric background concentration, an isotope mixing model allows the separation of geogenic (16–27%) from highly dynamic anthropogenic combustion-related contributions (21–54%). The geogenic fraction is inversely correlated to established CO2 concentrations that were driven by anthropogenic CO2 emissions within the mine. The described approach is applicable to other environments, including different types of underground mines, natural caves, and soils.

In eastern Hesse and western Thuringia, Germany hosts significant potassium-bearing salt deposits industrially excavated in the Werra-Fulda mining district. The salt belongs to the upper Permian (Zechstein) and was deposited around 258 to 252.5 Ma ago. In the Werra-Fulda mining district, the halite rocks (Werra-Rocksalt, z1NA) contain two minable potash seams, potash Seam Thüringen (z1KTh) and potash Seam Hessen (z1KHe), with an average thickness between 2 m and 3 m (Figure 1). To investigate the chemical and isotopic composition of the gas phase of Seam Hessen, gaseous samples were collected from five, 2-meter deep, horizontal boreholes drilled in the potash horizon which is mined at a depth of 540 m. About 4 weeks in advance of the gas sampling, the packer-closed boreholes were evacuated to about 3kPa and the pressure gradient inside the holes was continuously monitored in the boreholes D1 to D5. Selected gas samples were analyzed for their noble gas isotopic composition and the δ13C values of CO2 and CH4. The noble gas isotopic compositions were determined using a VG 5400 noble gas mass spectrometer after purification in a preparation line. The carbon isotope compositions were analysed with a GC-IRMS, comprising a GC 6890N connected to a GC-C/TC III combustion device and coupled to a MAT 253 mass spectrometer. The standard deviation of the δ13C values (in ‰ vs. VPDB) is ± 0.5 ‰.

Fluid inclusions are voids enclosed in the rock matrix and contain, depending on their origin and development, various amounts of gaseous, liquid or solid phases. Depending on their occurrence within the crystalline structure or in healed micro-fractures, primary and secondary inclusions can be distinguished. Their characteristics are utilized in various geological settings to reconstruct rock history and fluid involvement. Fluid inclusions could also be considered to be small equivalents to large cavities. As salt is regarded a favorable host rock for the storage of natural gas and other materials in artificial caverns, knowledge on gas migration and retention is crucial. Here, we present results of a fluid inclusion study in various salt rocks using Raman spectroscopy in addition to conventional microscopic characterization and gas analysis on whole rock samples. This approach allows for a better understanding of fluid generation and migration in different salt lithologies over geological times. Various salt minerals (halite, sylvite, kieserite and carnallite) from an area of potential overprint of CO2-dominated gas migration were investigated. Numerous fluid inclusions exhibit chevron structure and are small sized. Large single- or two-phased inclusions are observed with irregular shapes, often indicative for leakage or necking down. Interestingly, although the CO2 concentrations in whole rock samples were high, fluid inclusions were dominated by an aqueous phase and often contain numerous daughter minerals. This suggests that CO2-rich gas is stored along distinct fractures or grain boundaries within an otherwise intact rock.

Natural gas hydrates occur at all active and passive continental margins and are supposed to contain tremendous amounts of methane, which makes them attractive as a potential source of energy. During the last two decades several methods have been developed and tested to produce methane from gas hydrates in lab experiments and field trials. In principle, three different approaches, namely depressurization, thermal stimulation and chemical stimulation have been tested. The injection of CO2 into hydrate-bearing sediments as a variant of a chemical stimulation appears to be particularly favorable because it combines the production of methane from the hydrate phase with the storage of CO2 as a solid clathrate. Despite the fact that numerous experimental and modelling studies have been conducted worldwide in order to understand the process on different scales and to evaluate its efficiency, some issues remain pending. We obtained laboratory data from micro-scale experiments using analytical methods such as Raman spectroscopy and X-ray diffraction providing information about the structural changes of the hydrate phase and the gas exchange processes on a molecular scale. The results indicate that the processes related to the exchange of the guest molecules are quite complex. It is generally accepted that the driving force for the release of methane from the hydrate lattice and the incorporation of CO2 into the hydrate structure is the chemical potential gradient between the hydrocarbon hydrate phase and the injected CO2 phase. Raman spectroscopic and X-ray diffraction measurements indicate that this process correlates with a (partial) decomposition or opening of the hydrate cavities. In case of mixed hydrocarbon hydrates with structure II the exchange of the guest molecules with CO2 also induces a change of the hydrate structure (sII to sI). In general, the exchange of the guest molecules results in the formation of a secondary mixed hydrate phase containing CO2 besides methane and other hydrocarbons depending on the composition of the surrounding gas phase. We also obtained laboratory data from large-scale experiments providing information about the methane recovery and fluid migration, hydrate dissociation rates and the formation of a secondary hydrate phase, heat and mass transfer. The experimental results show a wide variation indicating that the recovery rate of methane strongly depends on the experimental conditions such as the volume of the sample, hydrate saturation and thus permeability of the hydrate bearing sediment, hydrate morphology as well as pressure and temperature of the injected CO2 phase. To avoid clogging due to the formation of a CO2 hydrate in the vicinity of the injection area we used gas mixtures containing N2 besides CO2 which also has an impact on the recovery rate of methane and the potential formation of a secondary hydrate phase. In any case, the injection of CO2 results in the production of a gas mixture rather than the extraction of pure methane because the injected CO2 forms a mixed hydrate with a certain amount of methane remaining in the hydrate phase. In addition, the long-term stability of the secondary formed CO2-rich hydrate phase also depends on the potential changes of the chemical environment which may result in the release of CO2 due to a re-exchange of the hydrate bonded gas molecules or dissociation of the hydrate phase. Maybe these results make the approach of methane production from natural gas hydrate deposits via CO2 injection less attractive with regard to efficiency and safe CO2 sequestration than originally expected. This contribution will present the experimental results of our investigations on various scales using different analytical methods and the overarching interpretation and conclusions based on these data.

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