High-temperature aquifer thermal energy storage (HT-ATES) can contribute in balancing the spatiotemporal mismatch that arises between periods of excess energy supply in contrast to phases of high energy demand. Excess energy can be stored under the form of thermal energy in the subsurface by utilizing methods stemming from geothermal engineering applications. In order to increase the efficiency of operating geothermal systems at the German Molasse Basin, such concepts are currently considered for the storage of high-temperature fluids in the Upper Jurassic Reservoir (Malm) of the North Alpine Foreland Basin. The karstified and fractured Malm aquifer comprises a site of extensive and continuously increasing investigation and implementation of geothermal projects. Nevertheless, the suitability of this reservoir for the development of ATES systems has not been yet considerably investigated. In this work we present our initial approach to evaluate the potential for thermal energy storage application in the Upper Jurassic reservoir.Due to the high structural and geological heterogeneity of the Malm aquifer, a subset of this reservoir, with favourable temperatures for heat storage, is investigated here that corresponds to segments governed by karst-dominated fluid flow. The numerical analysis builds upon three currently operating geothermal systems that exhibit such a characteristic karst-controlled fluid flow in depths of ca. 2000–3000 m TVD. In fact, a comprehensive analysis of borehole log data shows that several stratigraphic units contribute as inflow zones in those systems, however the main proportion of inflow results from the karstified zones. The model domain is, therefore, subdivided into three homogeneous units with the shallower layer representing a karstified unit, while the deeper units correspond to the less productive limestone and dolostone sequences of the Malm reservoir. Thermal and hydraulic properties are deciphered by field tests performed in the considered geothermal systems, their respective well logs as well as investigations of rock cores from two wells penetrating into the Malm reservoir (Bohnsack et al., 2020).While those enhanced-permeability reservoirs may represent good candidates for subsurface heat storage due to high injectivity, they simultaneously enable high fluid fluxes that may in turn induce considerable thermal losses. A numerical analysis is performed here to capture and describe the governing physical processes, and to assess the potential of HT-ATES application in such reservoirs. Synthetic numerical models are hence developed that are based on the three considered geothermal systems of the Upper Jurassic reservoir. This approach enables to quantify thermal and hydraulic effects of heat storage, to identify potential hydraulic and thermal interference between injection and production, and to assess developing advective heat fluxes which may trigger heat losses and thus impede long-term sustainable operation of HT-ATES systems. Numerical results contribute into a better understanding of the reservoir behaviour and further into prediction of the system response under different background conditions. ReferencesBohnsack, D., Potten, M., Pfrang, D., Wolpert, P., Zosseder, K. Porosity–permeability relationship derived from Upper Jurassic carbonate rock cores to assess the regional hydraulic matrix properties of the Malm reservoir in the South German Molasse Basin. Geothermal Energy 8, 12 (2020).

The North Alpine Foreland Basin in SE Germany is a post-mature petroleum basin and today Germany's most prolific deep geothermal energy play. Drilling of deep wells is often challenged by the complex pore pressure distribution, which has been studied in the past, but quality and reliability of individual pore pressure measurements and indicators have so far been barely addressed. This is particularly critical, since most datasets originate from old hydrocarbon wells and often display limited availability and poor quality. This paper analyses pore pressure measurements and indicators from 315 deep hydrocarbon and geothermal wells. The dataset covers pressure measurements, drilling mud weights, caliper logs, drilling events and gas readings. A large number of pressure measurements are exposed to uncertainties, resulting predominantly from incomplete pressure build-ups. In addition, investigation of drilling mud weights combined with wellbore instabilities, gas readings and pore pressure-related drilling problems suggest that many wells were subject to underbalanced drilling and mud weight alone is not a reliable pore pressure indicator. The study provides a recommendation for pre-drill pore pressure prediction based on the investigated datasets, which also presents a reference case for other post-mature petroleum basins transitioning to new industries, such as deep geothermal. Thematic collection: This article is part of the Earth as a thermal battery: future directions in subsurface thermal energy storage systems collection available at: https://www.lyellcollection.org/topic/collections/thermal-energy

Summary Due to the constantly increasing menace of global climate change, Stadtwerke München (SWM) has set the ambitious goal of supplying Munich with 100% district heating from CO2 -neutral sources by 2040 at the latest. Hydrothermal geothermal energy, which needs to be extracted for an efficient supply directly in urban areas, accounts for a large part of the projects. A fractured and karstified carbonate reservoir, buried 2–3 km below Munich, offers the best conditions for the extraction of 90–120 °C hot thermal water. However, the injection of chilled thermal water carries the risk of triggering small seismic events caused by changes in the local stress field. Therefore, SWM has implemented a comprehensive life-cycle risk management system to investigate, prevent, and, if necessary, mitigate any seismic events during the operation of hydro-geothermal heating plants. A variety of innovative techniques and installations (e.g., fiber optic cables) will provide information for the development of a safe strategy already in the planning and construction phase. In addition, permanent seismic monitoring mechanisms and a sophisticated risk management system will ensure early detection of potential hazards during the test/operation phase and enable the prevention or mitigation of seismic events through a holistic reservoir management system.

Geothermal energy applications are seen as one key element for a successful heat transition in Bavaria. But there are still some barriers for a further development. To minimize these barriers the joint research project Geothermal Alliance Bavaria (GAB) is established. One important issue to foster the implementations of geothermal projects is the assessment of geothermal load prediction in the South German Molasse Basin (SGMB). This includes, aside from a reservoir temperature prognosis, an accurate description of the hydraulic properties of the Upper Jurassic Malm reservoir. Hydraulic test analyses are conducted in the framework of the GAB to obtain specific information about the hydraulic productivity of the reservoir. Results from these analyses show a decrease of rock permeability in southern direction within the reservoir. Because the spatial distribution of hydraulic test data is limited, the porosity of the reservoir is assessed by borehole core tests and logs interpretation. A trend of matrix porosity decrease with depth is recognised and correlates with the hydraulic test results. Based on these findings and combined with further information the Upper Jurassic reservoir could be classified in separated zones of similar production rates, which can now be used for a thermal output prognosis for the Bavarian part of the SGMB. To spatially expand these prognoses more data must be investigated in the next research phase of the GAB.

To increase the use of geothermal energy in Bavaria, the subsurface, which serves as a reservoir, must be explored as precisely as possible. In the projects of the Geothermal‐Alliance Bavaria, the most promising areas for geothermal exploration in Bavaria were characterised based on an extensive geomechanical laboratory testing program which was carried out on both drill cores and analogue samples from quarries. In the North Alpine Foreland Basin (SE Germany), the geomechanical test results on Upper Jurassic carbonate rocks show a high heterogeneity. On the contrary, in the Franconian Basin (NE Bavaria) the geomechanical properties of granite analogues are rather homogenous. For the numerical simulation of the borehole stability, the determined parameters from ultrasonic‐ and compression tests serve as input parameters for different scenarios. For both locations, the determination of the failure depth around the borehole and the stress distribution in the near‐field of the borehole were accomplished. In the North Alpine Foreland Basin, the borehole stability decreases with increasing depth. For all scenarios in NE Bavaria, the borehole stability is very low. In the future, the determined parameter ranges will allow to validate already existing models and to develop new ones. This will enable a better knowledge of the sedimentary and crystalline reservoirs and a more effective use of geothermal energy in Bavaria.

Pore pressure prediction is a well-developed key discipline for well planning in the hydrocarbon industry, suggesting a similar importance for deep geothermal wells, especially, since drilling cost is often the largest investment in deep geothermal energy projects. To address the role of pore pressure prediction in deep geothermal energy, we investigated pore pressure-related drilling problems in the overpressured North Alpine Foreland Basin in SE Germany – one of Europe's most extensively explored deep geothermal energy plays. In the past, pore pressure was mainly predicted via maximum drilling mud weights of offset hydrocarbon wells, but recently more data became available, which led to a re-evaluation of the pore pressure distribution in this area. To compare the impact of pore pressure and its prediction, 70% of all deep geothermal wells drilled have been investigated for pore pressure-related drilling problems and two deep geothermal projects are given as more detailed examples. Thereby, pore pressure-related drilling problems were encountered in one third of all wells drilled, resulting in several side-tracks and an estimated drilling rate decrease of up to 40%, highlighting the importance of accurate pore pressure prediction to significantly reduce the cost of deep geothermal drilling in overpressured environments. Thematic collection: This article is part of the Geopressure collection available at: https://www.lyellcollection.org/cc/geopressure

AbstractTo increase the use of geothermal energy in Bavaria, the subsurface, which serves as a reservoir, must be explored as precisely as possible. In the projects of the Geothermal‐Alliance Bavaria, the most promising areas for geothermal exploration in Bavaria were characterised based on an extensive geomechanical laboratory testing program which was carried out on both drill cores and analogue samples from quarries. In the North Alpine Foreland Basin (SE Germany), the geomechanical test results on Upper Jurassic carbonate rocks show a high heterogeneity. On the contrary, in the Franconian Basin (NE Bavaria) the geomechanical properties of granite analogues are rather homogenous. For the numerical simulation of the borehole stability, the determined parameters from ultrasonic‐ and compression tests serve as input parameters for different scenarios. For both locations, the determination of the failure depth around the borehole and the stress distribution in the near‐field of the borehole were accomplished. In the North Alpine Foreland Basin, the borehole stability decreases with increasing depth. For all scenarios in NE Bavaria, the borehole stability is very low. In the future, the determined parameter ranges will allow to validate already existing models and to develop new ones. This will enable a better knowledge of the sedimentary and crystalline reservoirs and a more effective use of geothermal energy in Bavaria.

In geothermal reservoir systems, changes in pore pressure due to production (depletion), injection or temperature changes result in a displacement of the effective stresses acting on the rock matrix of the aquifer. To compensate for these intrinsic stress changes, the rock matrix is subjected to poroelastic deformation through changes in rock and pore volume. This in turn may induce changes in the effective pore network and thus in the hydraulic properties of the aquifer. Therefore, for the conception of precise reservoir models and for long-term simulations, stress sensitivity of porosity and permeability is required for parametrization. Stress sensitivity was measured in hydrostatic compression tests on 14 samples of rock cores stemming from two boreholes of the Upper Jurassic Malm aquifer of the Bavarian Molasse Basin. To account for the heterogeneity of this carbonate sequence, typical rock and facies types representing the productive zones within the thermal reservoir were used. Prior to hydrostatic investigations, the hydraulic (effective porosity, permeability) and geomechanical (rock strength, dynamic, and static moduli) parameters as well as the microstructure (pore and pore throat size) of each rock sample were studied for thorough sample characterization. Subsequently, the samples were tested in a triaxial test setup with effective stresses of up to 28 MPa (hydrostatic) to simulate in-situ stress conditions for depths up to 2000 m. It was shown that stress sensitivity of the porosity was comparably low, resulting in a relative reduction of 0.7–2.1% at maximum effective stress. In contrast, relative permeability losses were observed in the range of 17.3–56.7% compared to the initial permeability at low effective stresses. Stress sensitivity coefficients for porosity and permeability were derived for characterization of each sample and the different rock types. For the stress sensitivity of porosity, a negative correlation with rock strength and a positive correlation with initial porosity was observed. The stress sensitivity of permeability is probably controlled by more complex processes than that of porosity, where the latter is mainly controlled by the compressibility of the pore space. It may depend more on the compaction of precedented flow paths and the geometry of pores and pore throats controlling the connectivity within the rock matrix. In general, limestone samples showed a higher stress sensitivity than dolomitic limestone or dolostones, because dolomitization of the rock matrix may lead to an increasing stiffness of the rock. Furthermore, the stress sensitivity is related to the history of burial diagenesis, during which changes in the pore network (dissolution, precipitation, and replacement of minerals and cements) as well as compaction and microcrack formation may occur. This study, in addition to improving the quality of input parameters for hydraulic–mechanical modeling, shows that hydraulic properties in flow zones largely characterized by less stiff, porous limestones can deteriorate significantly with increasing effective stress.

For the successful realization and productivity prediction of new hydrothermal projects in the South German Molasse Basin, the hydraulic matrix properties of the Upper Jurassic Malm reservoir have to be determined as accurately as possible. To obtain specific information on the distribution of the petrophysical parameters (e.g., rock density, porosity, and permeability) 363 samples of rare drilling cores from the reservoir northeast of Munich (wells Moosburg SC4 and Dingolfing FB) were investigated using different experimental methods. Additionally, porosity was calculated by a downhole resistivity log of a nearby borehole close to Munich for comparison and the attempt of transferability of the data set to other locations within the Central Molasse Basin. Core data were divided into groups of different stratigraphic and petrographic units to cover the heterogeneity of the carbonate aquifer and provide data ranges to improve reservoir and prediction models. Data for effective porosity show a high variance from 0.3 to 19.2% throughout this heterogeneous aquifer. Permeability measured on core samples is scattered over several orders of magnitude (10 −4 –10 2 mD). Permeability models based on the porosity–permeability relationship were used to estimate permeability for the whole aquifer section and identify possible flow zones. A newly developed empirical model based on distinct lithofacies types allows a permeability estimation with a deviation < 10 mD. However, fractured, karstified, and vuggy zones occurring in this typically karstified, fractured, and porous reservoir cannot yet be taken into account by the model and result in an underestimation of permeability on reservoir scale. Overall, the dominant permeability trends can be mapped well using this model. For the regional transfer and the correlation of the results, a core-related porosity/permeability log for the reservoir was compiled for a well close to Munich showing similarities to the core investigations. The validation of the regional transferability of the parameter set to other locations in the Molasse Basin was carried out by correlation with the interpreted log data of a well near Munich.

Pregrouting in TBM tunnelling / Vorauseilende Injektion bei TBM‐Vortrieben