Abstract. The importance of research into clean and renewable energy solutions has increased over the last decade. Geothermal energy provision is proven to meet both conditions. Therefore, conceptual models for deep geothermal applications were developed for different field sites regarding different local conditions. In Bavaria, Germany, geothermal applications were successfully carried out in carbonate horizons at depths of 4000 to 6000 m. Matrix permeability and thermal conductivity was mainly studied in karstified carbonates from the Late Jurassic reef facies. Similar to Bavaria, carbonates are located in the east of the Rhenohercynian Massif, in North Rhine-Westphalia (NRW), for which quantification of the geothermal potential is still lacking. Compared to Bavaria, a supraregional carbonate mountain belt is exposed at the Remscheid-Altena anticline (in NRW) from the Upper Devonian and Lower Carboniferous times. The aim of our study was to examine the potential geothermal reservoir by field and laboratory investigations. Therefore, three representative outcrops in Wuppertal, Hagen-Hohenlimburg, and Hönnetal were studied. During field surveys, 1068 discontinuities (139 open fractures without any filling, 213 joints, 413 veins filled with calcite, and 303 fractures filled with debris deposits) at various spatial scales were observed by scanline surveys. These discontinuities were characterized by trace length, true spacing, roughness, aperture, and filling materials. Discontinuity orientation analysis indicated three dominant strike orientations in NNW–SSE, NW–SE, and NE–SW directions within the target horizon of interest. This compacted limestone layer (Massenkalk) is approximately 150 m thick and located at 4000 to 6000 m depth, dipping northwards at a dip angle of about 30 to 40∘. An extrapolation of the measured layer orientation and dip suggests that the carbonate reservoir could hypothetically extend below Essen, Bochum, and Dortmund. Our combined analysis of the field and laboratory results has shown that it could be a naturally fractured carbonate reservoir. We evaluated the potential discontinuity network in the reservoir and its orientation with respect to the prevailing maximum horizontal stress before concluding with implications for fluid flow: we proposed focusing on prominent discontinuities striking NNW–SSE for upcoming geothermal applications, as these (1) are the most common, (2) strike in the direction of the main horizontal stress, (3) have a discontinuity permeability that significantly exceeds that of the reservoir rock matrix, and (4) only about 38 % of these discontinuities were observed with a calcite filling. The remaining discontinuities either showed no filling material or showed debris deposits, which we interpret as open at reservoir depth. Our results indicate that even higher permeability can be expected for karstified formations related to the reef facies and hydrothermal processes. Our compiled data set, consisting of laboratory and field measurements, may provide a good basis for 3D subsurface modelling and numerical prediction of fluid flow in the naturally fractured carbonate reservoir.

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Digital rock physics combines microtomographic imaging with advanced numerical simulations of effective material properties. It is used to complement laboratory investigations with the aim to gain a deeper understanding of relevant physical processes related to transport and effective mechanical properties. We apply digital rock physics to reticulite, a natural mineral with a strong analogy to synthetic open-cell foams. We consider reticulite an end-member for high-porosity materials with a high stiffness and brittleness. For this specific material, hydro-mechanical experiments are very difficult to perform. Reticulite is a pyroclastic rock formed during intense Hawaiian fountaining events. the honeycombed network of bubbles is supported by glassy threads and forms a structure with a porosity of more than 80%. Comparing experimental with numerical results and theoretical estimates, we demonstrate the high potential of in situ characterization with respect to the investigation of effective material properties. We show that a digital rock physics workflow, so far applied to conventional rocks, yields reasonable results for high-porosity rocks and can be adopted for fabricated foam-like materials with similar properties. Numerically determined porosities, effective elastic properties, thermal conductivities and permeabilities of reticulite show a fair agreement to experimental results that required exeptionally high experimental efforts.

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<p>Microtomographic imaging techniques and advanced numerical simulations are combined by digital rock physics (DRP) to obtain effective physical material properties. The numerical results are typically used to complement laboratory investigations with the aim to gain a deeper understanding of physical processes related to transport (e.g. permeability and thermal conductivity) and effective elastic properties (e.g. bulk and shear modulus). The present study focuses on DRP and laboratory techniques applied to a rock called reticulite, which is considered as an end-member material with respect to porosity, stiffness and brittleness of the skeleton. Classical laboratory investigations on effective properties, such as ultrasonic transmission measurements and uniaxial deformation experiments, are very difficult to perform on this class of high-porosity and brittle materials.</p><p>Reticulite is a pyroclastic rock formed during intense Hawaiian fountaining events. The open honeycombed network has a porosity of more than 80 % and consists of bubbles that are supported by glassy threads. The natural mineral has a strong analogy to fabricated open-cell foams. By comparing experimental with numerical results and theoretical estimates we demonstrate the potential of digital material methodology with respect to the investigation of porosity, effective elastic properties, thermal conductivity and permeability</p><p>We show that the digital rock physics workflow, previously applied to conventional rock types, yields reasonable results for a high-porosity rock and can be adopted for fabricated foam-like materials. Numerically determined effective properties of reticulite are in good agreement with the experimentally determined results. Depending on the fields of application, numerical methods as well as theoretical estimates can become reasonable alternatives to laboratory methods for high porous foam-like materials.</p>

Hydrogen storage might be key to the success of the hydrogen economy, and hence the energy transition in Germany. One option for cost-effective storage of large quantities of hydrogen is the geological subsurface. However, previous experience with underground hydrogen storage is restricted to salt caverns, which are limited in size and space. In contrast, pore storage facilities in aquifers -and/or depleted hydrocarbon reservoirs- could play a vital role in meeting base load needs due to their wide availability and large storage capacity, but experiences are limited to past operations with hydrogen-bearing town gas. To overcome this barrier, here we investigate hydrogen storage in porous storage systems in a two-step process: 1) First, we investigate positive and cautionary indicators for safe operations of hydrogen storage in pore storage systems. 2) Second, we estimate hydrogen storage capacities of pore storage systems in (current and decommissioned) underground natural gas storage systems and saline aquifers. Our systematic review highlights that optimal storage conditions in terms of energy content and hydrogen quality are found in sandstone reservoirs in absence of carbonate and iron bearing accessory minerals at a depth of approx. 1,100 m and a temperature of at least 40°C. Porosity and permeability of the reservoir formation should be at least 20% and 5 × 10−13 m2 (∼500 mD), respectively. In addition, the pH of the brine should fall below 6 and the salinity should exceed 100 mg/L. Based on these estimates, the total hydrogen storage capacity in underground natural gas storages is estimated to be up to 8 billion cubic meters or (0.72 Mt at STP) corresponding to 29 TWh of energy equivalent of hydrogen. Saline aquifers may offer additional storage capacities of 81.6–691.8 Mt of hydrogen, which amounts to 3.2 to 27.3 PWh of energy equivalent of hydrogen, the majority of which is located in the North German basin. Pore storage systems could therefore become a crucial element of the future German hydrogen infrastructure, especially in regions with large industrial hydrogen (storage) demand and likely hydrogen imports via pipelines and ships.

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<p>Medium-depth and deep geothermal systems hosted in carbonate rocks are amongst the most promising geothermal resources in the world due to their favorable geological and stress- and temperature-sensitive petrophysical heterogeneity. In general, structural heterogeneities such as natural fractures, karstifications, cavities or entire fracture networks mainly dominate geothermal fluid flows and storages and, thus, dictate the reservoir quality. However, especially for carbonate reservoirs, it is even more complex, as a profound understanding of the links between diagenetic processes, facies, deformations, porosities, and fluid flow properties is essential to estimate the distribution of reservoir quality. This assessment is further complicated by the spatially and temporally varying pressure and temperature conditions (e.g., recharge and discharge of geothermal systems).</p><p>In the wake of Bavaria’s (southern Germany) success story in exploiting the geothermal systems hosted in deep carbonates, there are extensive investigations to determine the geothermal potential of Devonian carbonates in North Rhine-Westphalia (western Germany). The geothermal potential of these Devonian carbonates strongly depends upon how and to what extent the tectonically influenced burial history and diagenetic processes have modified the pore network and promoted heterogeneities such as fractures and karstifications. In our triaxial experiments, we examined the influence of in-situ stress and temperature and their histories as well as the influence of brittle faulting on porosities and hydraulic properties of different Devonian carbonate rocks. From analogue outcrops limestone, dolomitic limestone, dolostone, and fractured carbonates were sampled and petrophysically investigated. The stresses simulated covered both hydrostatic and triaxial states. Furthermore, the influence of elevated temperature and stress on the hydraulic properties of samples triaxially compressed at effective confining pressures was also studied systematically. Our results show that the interplay of temperature and stress state, and their histories, are fundamental for the evolution of hydraulic properties in the reservoir. Depending on the rock’s mineralogy, the mineral expansion caused by the increased temperature can surpass the effect of microcracking due to heating, resulting in a significant decrease in hydraulic properties. Our results were supplemented by micro-CT images of the microstructure of the samples before and after triaxial testing. It is shown that the interaction of temperature and stress is fundamental for the assessment of the geothermal potential of both intact and fractured carbonate reservoirs.</p>