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"Subsurface Energy Storage" - Environmental Earth Sciences Topical Collection (with Abstracts)

The Environmantal Earth Sciences (EES) Topical Collection "Subsurface Energy Storage" is a joint initiavtive by ANGUS+ and the EES Editorial Office, guest edited by Sebastian Bauer, Andreas Dahmke, and Olaf Kolditz.


Bauer, S., Dahmke, A., & Kolditz, O. (2017). Subsurface energy storage: geological storage of renewable energy—capacities, induced effects and implications. Environmental Earth Sciences, 76(20), 695. doi:10.1007/s12665-017-7007-9.
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Böttcher, N., Görke, U.-J., Kolditz, O., & Nagel, T. (2017). Thermo-mechanical investigation of salt caverns for short-term hydrogen storage. Environmental Earth Sciences, 76(3), 98. doi:10.1007/s12665-017-6414-2.

To investigate the temperature influence on the cavern capacity, a numerical model was developed in order to simulate the thermo-mechanical behaviour of salt caverns during cyclic hydrogen storage. The model considers the thermodynamic characteristics of the storage medium as well as the heat transport and the temperature-dependent material properties of the host rock. Therefore, a well-known visco-elastic constitutive model was modified to describe temperature effects of rock salt and implemented into the freely available simulator OpenGeoSys. Thermal and mechanical processes are solved using a finite element approach, connected via a staggered coupling scheme. Numerical analyses were performed and evaluated using basic criteria for cavern safety and convergence. The results show that large temperature amplitudes in the working gas may lead to tensile stresses at the cavern boundary. Reducing the frequency of the cyclic loading is a way to reduce temperature variations and to avoid tensile failure. Furthermore, the influence of cavern shape was investigated. Narrow cylindrical caverns converge faster than spherical ones of the same volume and are subjected to a higher risk of structural failure.
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Dethlefsen, F., Nolde, M., Schäfer, D., & Dahmke, A. (2017). Basic parameterization of Schleswig–Holstein’s shallow geological formations for numerical reactive transport simulations: representative groundwater compositions. Environmental Earth Sciences, 76(2), 59. doi:10.1007/s12665-016-6343-5.

Groundwater protection has to remain ensured in spite of the ambition to apply more and various types of subsurface usages in future. In this context, numerical simulations using “Virtual Aquifers” can be suitable for evaluating the general effects of complex induced process interactions, while meaningful simulation results require the appropriate parameterization of scenario analyses, such as regarding representative groundwater compositions. Therefore, this study reviewed the hydrochemical groundwater compositions of the different aquifers in the German state Schleswig–Holstein. To evaluate what aquifers exhibit statistically different compositions, the nonparametric Kruskal–Wallis test, the analysis of variance, the discriminant analysis, and the hierarchical cluster analysis were applied. These showed that between the free aquifers at pH < 6, the free aquifers at pH > 6, and the group of confined aquifers, significant differences in the dissolved constituents exist, but also that among the confined aquifers these differences are not significant, except for saline groundwaters that can be present near underground salt structures and the North Sea. Furthermore, the two methods applied for deducting representative compositions were the nearest neighbor method, where the monitoring wells accessing the groundwater most similar to the median compositions in the respective aquifers were identified, and the cluster center analysis. The calculated representative groundwater compositions for four aquifer groups (“Acidic” shallow aquifers, “Neutral” shallow aquifers, confined freshwater aquifers, saline aquifers) using both methods were very similar. Thus, this study provides a methodology and a basis for parameterizing Virtual Aquifer studies and discusses the limits of representativeness based on the regional data set.


Kabuth, A., Dahmke, A., Beyer, C., Bilke, L., Dethlefsen, F., Dietrich, P., … Bauer, S. (2017). Energy storage in the geological subsurface: dimensioning, risk analysis and spatial planning: the ANGUS+ project. Environmental Earth Sciences, 76(1), 23. doi:10.1007/s12665-016-6319-5.

New techniques and methods for energy storage are required for the transition to a renewable power supply, termed “Energiewende” in Germany. Energy storage in the geological subsurface provides large potential capacities to bridge temporal gaps between periods of production of solar or wind power and consumer demand and may also help to relieve the power grids. Storage options include storage of synthetic methane, hydrogen or compressed air in salt caverns or porous formations as well as heat storage in porous formations. In the ANGUS+ project, heat and gas storage in porous media and salt caverns and aspects of their use on subsurface spatial planning concepts are investigated. The optimal dimensioning of storage sites, the achievable charging and discharging rates and the effective storage capacity as well as the induced thermal, hydraulic, mechanical, geochemical and microbial effects are studied. The geological structures, the surface energy infrastructure and the governing processes are parameterized, using either literature data or own experimental studies. Numerical modeling tools are developed for the simulation of realistically defined synthetic storage scenarios. The feasible dimensioning of storage applications is assessed in site-specific numerical scenario analyses, and the related spatial extents and time scales of induced effects connected with the respective storage application are quantified. Additionally, geophysical monitoring methods, which allow for a better spatial resolution of the storage operation, induced effects or leakages, are evaluated based on these scenario simulations. Methods for the assessment of such subsurface geological storage sites are thus developed, which account for the spatial extension of the subsurface operation itself as well as its induced effects and the spatial requirements of adequate monitoring methods.


Kasina, M., Bock, S., Würdemann, H., Pudlo, D., Picard, A., Lichtschlag, A., … Meister, P. (2017). Mineralogical and geochemical analysis of Fe-phases in drill-cores from the Triassic Stuttgart Formation at Ketzin CO2 storage site before CO2 arrival. Environmental Earth Sciences, 76(4), 161. doi:10.1007/s12665-017-6460-9.

Reactive iron (Fe) oxides and sheet silicate-bound Fe in reservoir rocks may affect the subsurface storage of CO2 through several processes by changing the capacity to buffer the acidification by CO2 and the permeability of the reservoir rock: (1) the reduction of three-valent Fe in anoxic environments can lead to an increase in pH, (2) under sulphidic conditions, Fe may drive sulphur cycling and lead to the formation of pyrite, and (3) the leaching of Fe from sheet silicates may affect silicate diagenesis. In order to evaluate the importance of Fe-reduction on the CO2 reservoir, we analysed the Fe geochemistry in drill-cores from the Triassic Stuttgart Formation (Schilfsandstein) recovered from the monitoring well at the CO2 test injection site near Ketzin, Germany. The reservoir rock is a porous, poorly to moderately cohesive fluvial sandstone containing up to 2–4 wt% reactive Fe. Based on a sequential extraction, most Fe falls into the dithionite-extractable Fe-fraction and Fe bound to sheet silicates, whereby some Fe in the dithionite-extractable Fe-fraction may have been leached from illite and smectite. Illite and smectite were detected in core samples by X-ray diffraction and confirmed as the main Fe-containing mineral phases by X-ray absorption spectroscopy. Chlorite is also present, but likely does not contribute much to the high amount of Fe in the silicate-bound fraction. The organic carbon content of the reservoir rock is extremely low (<0.3 wt%), thus likely limiting microbial Fe-reduction or sulphate reduction despite relatively high concentrations of reactive Fe-mineral phases in the reservoir rock and sulphate in the reservoir fluid. Both processes could, however, be fuelled by organic matter that is mobilized by the flow of supercritical CO2 or introduced with the drilling fluid. Over long time periods, a potential way of liberating additional reactive Fe could occur through weathering of silicates due to acidification by CO2.
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Lienen, T., Lüders, K., Halm, H., Westphal, A., Köber, R., & Würdemann, H. (2017). Effects of thermal energy storage on shallow aerobic aquifer systems: temporary increase in abundance and activity of sulfate-reducing and sulfur-oxidizing bacteria. Environmental Earth Sciences, 76(6), 261. doi:10.1007/s12665-017-6575-z.

Aquifer thermal energy storage may result in increases in the groundwater temperature up to 70 °C and more. This may lead to geochemical and microbiological alterations in the aquifer. To study the temperature effects on the indigenous microbial community composition, sediment column experiments at four different temperatures were carried out and the effluents were characterized geochemically and microbiologically. After an equilibrium phase at groundwater temperature of 10 °C for 136 days, one column was kept at 10 °C as a reference and the others were heated to 25, 40 and 70 °C. Genetic fingerprinting and quantitative PCR revealed a change in the bacterial community composition and abundance due to the temperature increase. While at 25 °C only slight changes in geochemical composition and gene copy numbers for bacteria were observed, increasing concentrations of total organic carbon in the 40 °C column were followed by a strong increase in bacterial abundance. Thermophilic bacteria became dominant at 70 °C. Temporary sulfate reduction took place at 40 and 70 °C and this correlated with an increased abundance of sulfate-reducing bacteria (SRB). Furthermore, a coexistence of SRB and sulfur-oxidizing bacteria (SOB) at all temperatures indicated an interaction of these physiological groups in the sediments. The results show that increased temperatures led to significant shifts in the microbial community composition due to the altered availability of electron donors and acceptors. The interplay of SRB and SOB in sedimentary biofilms facilitated closed sulfur cycling and diminished harmful sulfur species.
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Pellizzari, L., Lienen, T., Kasina, M., & Würdemann, H. (2017). Influence of drill mud on the microbial communities of sandstone rocks and well fluids at the Ketzin pilot site for CO2 storage. Environmental Earth Sciences, 76(2), 77. doi:10.1007/s12665-016-6381-z.

At a pilot site for CO2 storage in Ketzin (Germany), a drastic decrease in injectivity occurred in a well intended for injection. This was attributed to an obstruction of the pore throats due to microbial degradation of the organic drill mud and subsequent iron sulfide (FeS) precipitation in the highly saline brine (240 g L−1). To better understand the biogeochemical processes, the response of the autochthonous microbial community to drill mud exposure was investigated. Pristine cores of two aquifers with different salinity were incubated under simulated in situ conditions (50 bar, 40 °C and 45 bar, 25 °C, respectively) and CO2 atmosphere. For the first time, rock cores obtained from the CO2 plume of the storage formation were investigated. The influence of acetate as a biodegradation product of drill mud polymers and the effectiveness of a biocide were additionally tested. Increased microbial diversities were observed in all long-term (8–20 weeks) incubations, even including biocide. Biofilm-like structures and small round-shaped minerals of probable microbiological origin were found. The results indicate that the microbial community remains viable after long-term CO2 exposure. Microorganisms hydrolyzing cellulose polymers (e.g., Burkholderia spp., Variovorax spp.) biodegraded organic components of the drill mud and most likely produced low molecular weight acids. Although the effects of drill mud were less strong as observed in situ, it was demonstrated that acetate supports the growth of sulfate-reducing bacteria (i.e., Desulfotomaculum spp.). The microbial-induced precipitation of amorphous FeS reduced the injectivity in the near-well area. Therefore, when using organic drill mud, the well must be cleaned intensively to minimize the hazards of bacterial stimulation.


Schelenz, S., Vienken, T., Shao, H., Firmbach, L., & Dietrich, P. (2017). On the importance of a coordinated site characterization for the sustainable intensive thermal use of the shallow subsurface in urban areas: a case study. Environmental Earth Sciences, 76(2), 73. doi:10.1007/s12665-016-6331-9.

Shallow geothermal applications have become standard solutions for heating and cooling in many newly built or redeveloped residential neighborhoods, but current urban development practices do not yet consider the new demands that result from the intensive thermal use of the shallow subsurface. A coordinated site characterization is of great importance as a sound basis for an optimized planning of geothermal systems that brings together user requirements (heating, cooling, and/or seasonal energy storage) and (hydro)geological subsurface conditions. The aim of this study is to raise awareness and to demonstrate the relevance of a coordinated site characterization. Therefore, this study quantifies the advantages of a site-specific over a desktop-based site characterization in reducing uncertainty for calculation of borehole heat exchanger length and predicted induced temperature changes in the subsurface for a newly developed residential neighborhood in the city of Taucha, Germany. Results show that savings of over EUR 1850 per house (EUR 98,050 for the entire neighborhood) can be achieved by a coordinated exploration and prediction accuracy of temperature plume development was substantially improved. Although being more cost intensive, exploration costs for this case study are <3% of the assumed individual geothermal system costs of EUR 16,000 if divided equally among geothermal users. Three different options are presented to implement coordinated exploration concepts into site development practice.


Westphal, A., Kleyböcker, A., Jesußek, A., Lienen, T., Köber, R., & Würdemann, H. (2017). Aquifer heat storage: abundance and diversity of the microbial community with acetate at increased temperatures. Environmental Earth Sciences, 76(2), 66. doi:10.1007/s12665-016-6356-0.
The temperature affects the availability of organic carbon and terminal electron acceptors (TEA) as well as the microbial community composition of the subsurface. To investigate the impact of thermal energy storage on the indigenous microbial communities and the fluid geochemistry, lignite aquifer sediments were flowed through with acetate-enriched water at temperatures of 10, 25, 40, and 70 °C in sediment column experiments. Genetic fingerprinting revealed significant differences in the microbial community compositions with respect to the different temperatures. The highest bacterial diversity was found at 70 °C. Carbon and TEA mass balances showed that the aerobic degradation of organic matter and sulfate reduction were the primary processes that occurred in all the columns, whereas methanogenesis only played a major role at 25 °C. The methanogenic activity corresponded to the highest abundance of an acetoclastic Methanosaeta concilii-like archaeon and the most efficient degradation of acetate. This study suggests a significant impact of geothermal energy storage on the natural microbial community and various metabolic activities because of increased temperatures in sediments with a temperature-related sediment organic matter release.


al Hagrey, S. A., Schäfer, D., Köhn, D., Wiegers, C. E., Chung, D., Dahmke, A., & Rabbel, W. (2016). Monitoring gas leakages simulated in a near surface aquifer of the Ellerbek paleo-channel. Environmental Earth Sciences, 75(14), 1083. doi:10.1007/s12665-016-5784-1.

Renewable energy resources are intermittent and need buffer storage to bridge the time-gap between production and demand peaks. The North German Basin has a very large capacity for compressed air/gas energy storage (CAES) in porous saltwater reservoirs and salt cavities. Even though these geological storage systems are constructed with high caution, accidental gas leakages occurred in the past. Stored gases migrated from deep reservoirs along permeable zones upwards into shallow potable aquifers. These CAES leakages cause changes in the electro-elastic properties, and density of the aquifers, and therefore justify investigations with the application of different geophysical techniques. A multiphase flow simulation has been performed to create a realistic virtual CAES leakage scenario into a shallow aquifer in Northern Germany. This scenario is used to demonstrate the detecting resolution capability of a combined geophysical monitoring approach, consisting of acoustic joint waveform inversion (FWI) of surface and borehole data, electrical resistivity tomography (ERT) and gravity. This combined approach of geophysical multi-techniques was able to successfully map the shape and determine the physical properties of the simulated gas phase body at a very early stage after leakage began. Techniques of FWI and ERT start to resolve CAES leakage anomalies only a few years and gravity even a few months after leakage began. Geophysical monitoring of vast areas may start by conducting time-effective aero-surveys (e.g. electromagnetic induction or gravity gradient methods) to isolate anomalous subareas of potential leakage risks. These subareas are then studied in detail using our combined high-resolution approach. In conclusion, our approach is sensitive to CAES leakages and can be used for monitoring.
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Berta, M., Dethlefsen, F., Ebert, M., Gundske, K., & Dahmke, A. (2016). Surface passivation model explains pyrite oxidation kinetics in column experiments with up to 11 bars p(O2). Environmental Earth Sciences, 75(16), 1175. doi:10.1007/s12665-016-5985-7.

Despite decades of research in numerous experimental and field studies, the reaction kinetics of pyrite oxidation is still not characterized for high partial pressures of oxygen and near-neutral pH-levels. These conditions potentially exist in aquifers where oxidative site remediation, temporary water storage, or a leakage from a compressed air energy storage facility is present. For planning and monitoring of such field operations, their potential side effects on protected natural resources like groundwater have to be characterized. Thereby, site-scale assessments of such side effects of subsurface use by numerically modeling geochemical changes caused by the presence of oxygen need parametrization. Also, a function transferring results from simple, low pressure experiments to high pressure environments requires experimental bases. Pyrite oxidation can be the main consequence of oxygen intruding reduced aquifers. In this study, pyrite oxidation kinetics was examined at oxygen partial pressures from 0 to 11 bars, corresponding to an air intrusion in up to 500 m depth, at neutral pH-levels in high and low pressure flow-through column experiments representing aquifer conditions. A reaction rate equation was developed and evaluated with 1D PHREEQC numerical reactive transport models using experimental data as transfer function between high pressure and low pressure experiments. This model development included an improvement of established rate laws with a passivation term, which is, in contrast to previously published functions, dependent on the partial pressure of oxygen. The resulting model on passivated oxidation kinetics of pyrite at high oxygen partial pressures was able to reproduce independent experimental results acquired using different experimental set-ups. This assessment found the passivation to overcome the theoretical increase in pyrite oxidation kinetics caused by elevating oxygen partial pressure. These findings contribute to future experimental and modeling efforts for risk assessment and monitoring of oxygen-rich plumes in the subsurface.
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Beyer, C., Popp, S., & Bauer, S. (2016). Simulation of temperature effects on groundwater flow, contaminant dissolution, transport and biodegradation due to shallow geothermal use. Environmental Earth Sciences, 75(18), 1244. doi:10.1007/s12665-016-5976-8.

The quantitative prognosis and assessment of possible impacts of temperature changes in groundwater due to the geothermal use of the shallow subsurface in urban regions requires process-based numerical models of coupled non-isothermal groundwater flow, heat and mass transport processes and biogeochemical reactive processes. This work therefore aims at developing and implementing numerical methods as well as the required parameterizations to simulate the effects of temperature increases due to heat injection in a groundwater aquifer. Parameter and process models for fluid flow, solute transport, mass transfer processes between aqueous and non-aqueous phases, and microbial growth coupled to contaminant biodegradation are expressed as functions of temperature for this purpose. The developed model is implemented in the OpenGeoSys code and applied in a set of benchmark simulations, where thermal impacts of borehole heat exchangers (BHE) are simulated in an aquifer with a TCE contamination in a residual NAPL source zone. The thermal plumes emitted by the BHEs result in a focusing of groundwater flow due to a viscosity reduction of the heated water. The local increase in groundwater flow as well as an increase in TCE solubility with temperature leads to increased TCE emissions from the source zone. At the same time, increases in microbial growth rates allow for higher TCE degradation rates by reductive dechlorination. Results of the benchmark simulations allow insights into the interactions of the individual processes and potential benefits or conflicts of geothermal use of the subsurface and natural attenuation processes at contaminated sites. Also, the benchmark simulations can be used as test cases for intercomparison and validation of reactive transport codes.
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Boockmeyer, A., & Bauer, S. (2016). Efficient simulation of multiple borehole heat exchanger storage sites. Environmental Earth Sciences, 75(12), 1–13. doi:10.1007/s12665-016-5773-4.

In this paper, an adapted model is developed for borehole heat exchangers (BHEs) to simulate geothermal applications such as heat storage on a large scale efficiently and with high accuracy. The adapted numerical model represents all BHE components, allowing for a detailed representation of the governing processes. The approach is calibrated and validated for a single U-tube BHE using a high-resolution experimental data set from a laboratory thermal response test. It is found that the computational effort can be reduced by factors of ~50, ~50 and ~25 for single U-tube, double U-tube and coaxial BHEs, respectively, if an absolute deviation of less than 1 % compared to a conventional fully discretised model is allowed. Computation times can be reduced further by accepting higher deviations. The adapted modelling approach allows for a detailed and correct representation of the temporal and spatial temperature distribution under highly transient conditions by applying it to a high-temperature heat storage scenario using multiple BHEs. The model is especially suited to represent coupled flow and heat transport processes, to account for groundwater flow in the BHE region as well as geological heterogeneities and especially interaction between a large number of BHEs.
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Delfs, J.-O., Nordbeck, J., & Bauer, S. (2016). Upward brine migration resulting from pressure increases in a layered subsurface system. Environmental Earth Sciences, 75(22), 1441. doi:10.1007/s12665-016-6245-6.

Upward displacement of brine from deep geological formations poses a potential threat to near-surface drinking water resources. In this work, the impact of a layered sequence of hydraulically permeable and impermeable layers connected by a vertical fluid pathway like, e.g., a fault is investigated using an idealized scenario and numerical process simulation. Long-term upward brine migration is induced by overpressure in the lowest permeable formation, and the upward migration through the vertical pathway and the interaction with the intermediary permeable layers is investigated. The simulations show that brine displaced upwards through the vertical fluid pathway moves into the intermediary permeable formations, settling into the lower parts of the permeable layers and displacing the resident less salty formation brine from this layer further upwards through the vertical pathway. Thus, formation brine from different depths displaces each other rather than mixing along the pathway or rising along the full length of the vertical pathway. An effect of the gradual upward displacement is a decrease in the salt concentrations along the pathway such that brine intrusion into the groundwater aquifer is reduced. However, if the hydraulic connection between the vertical pathway and the intermediary layers is low, higher-density brine accumulates in the vertical pathway and upward movement of the brine is impeded due to its own weight.
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Dethlefsen, F., Beyer, C., Feeser, V., & Köber, R. (2016). Parameterizability of processes in subsurface energy and mass storage. Environmental Earth Sciences, 75(10), 1–25. doi:10.1007/s12665-016-5626-1.

The numerical simulation of scenarios is a promising approach when quantifying the potential hydraulic, thermal, geomechanical, and chemical effects of subsurface energy and mass storage. Particularly, the coupling of processes is a strong point in numerical simulations. This study defines the geoscientific parameter demand as well as the demand for process understanding for simulating subsurface energy and mass storage, describes the existing numerical codes to conduct the simulations, provides generally valid parameter values, and emphasizes on discussing parameters where only few values exist. In this context, it is exemplified that the parameterizability of the regarded processes is determined by an uncertainty in parameter values (variability or aleatory uncertainty) as well as in the understanding of processes (epistemic uncertainty) as it was suggested by Walker et al. (Integr Assess 4:5–17, 2003). The study categorizes the knowledge about parameter values and processes into these uncertainty groups and exemplarily evaluates the impacts of the uncertainties. Using this approach illustrates the concepts needed for calculating prediction errors of numerical scenario simulations, such as sensitivity analyses in the case of statistical data uncertainty and laboratory or field studies in the case of scenario uncertainties.
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Feldmann, F., Hagemann, B., Ganzer, L., & Panfilov, M. (2016). Numerical simulation of hydrodynamic and gas mixing processes in underground hydrogen storages. Environmental Earth Sciences, 75(16), 1165. doi:10.1007/s12665-016-5948-z.

The storage of hydrogen in underground reservoirs comprises a potential solution for balancing the fluctuating energy production from wind and solar power plants. In this concept, electrolysers are used to transform excessively produced electrical energy into chemical energy in the form of hydrogen. The resulting large volumes of hydrogen are temporarily stored in subsurface formations purely or in mixture with other gases. In times of high energy demand, the chemical energy is transformed back into electricity by fuel cells or engine generators. Key aspects in the development period and the subsequent cyclic operations of such a storage are the hydrodynamic behavior of hydrogen and its interaction with residual fluids in the reservoir. Mathematically, the behavior can be described by a compositional two-phase flow model with water and gas as phases and all relevant chemical species as components (H2, H2O, CH4, CO2, N2, H2S, etc.). The spatial variation of the gas phase composition between injected and initial gas leads to density and viscosity contrasts which influence the displacement process. The mixing of gases with different compositions is governed by molecular diffusion or mechanical dispersion dependent on the flow velocity. In the present paper, a numerical case study in a depleted gas reservoir was performed. The storage was charged with hydrogen for 5 years. Subsequently, 5 years of seasonal cyclic operation were simulated to predict injection and production rates, pressure response and composition of the produced gas stream .
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Khaledi, K., Mahmoudi, E., Datcheva, M., & Schanz, T. (2016). Analysis of compressed air storage caverns in rock salt considering thermo-mechanical cyclic loading. Environmental Earth Sciences, 75(15), 1149. doi:10.1007/s12665-016-5970-1.

Exploring the material response of rock salt subjected to the variable thermo-mechanical loading is essential for engineering design of compressed air energy storage (CAES) caverns. Accurate design of salt caverns requires adequate numerical simulations which take into account the most important processes affecting the development of stresses and strains. To fulfill this objective, this paper presents a two-step simulation to analyze the thermo-mechanical behavior of rock salt in the vicinity of CAES caverns. In the first step, the changes in air temperature and pressure resulted from injection and withdrawal processes are estimated using an analytical thermodynamic model. Then, in the second step, the temperature and pressure variations obtained from the analytical model are utilized as the boundary condition for a finite element model of CAES cavern. An elasto-viscoplastic creep model is employed to describe the material behavior of rock salt. In the numerical section, a computational model to simulate the thermo-mechanical behavior of rock salt around the cavern is presented. Finally, the stability and long-term serviceability of the simulated cavern are evaluated considering two extreme loading scenarios: (1) low-pressure working condition and (2) high-temperature operation. Obtained results show that both stability and serviceability of the cavern are highly affected by the internal operating pressure. Dilatancy, damage propagation, tensile failure and increasing the rate of cavern closure are the unfavorable consequences of low-pressure working condition. Similarly, the increased creep rate due to the elevated temperature accelerates the volume convergence and subsequently endangers the serviceability of the system.
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Köhn, D., Nil, D. D., al Hagrey, S. A., & Rabbel, W. (2016). A combination of waveform inversion and reverse-time modelling for microseismic event characterization in complex salt structures. Environmental Earth Sciences, 75(18), 1235. doi:10.1007/s12665-016-6032-4.

The increased emission of greenhouse gases into the atmosphere, causing climate changes, leads to a strong requirement of renewable energy resources. However, they are intermittent and need buffer storage to bridge the time gap between production and public demands. The injection of gas (e.g. compressed air or hydrogen) in sealed underground structures like salt caverns is one approach to solve this problem. Possible risks related to cavern storage are gas leakages from the injection tube into the surrounding sediments, material failure in salt rock surrounding the cavern during irregular operation and in the most extreme case a partial collapse of the cavern. For the early detection of these problems, a geophysical monitoring strategy is required. The objective of this paper was to map possible leakage paths outside of the salt structures and local failures within the cavern walls by the localization of crack-induced microseismic events. Classical methods require arrival time picking and phase identification. An alternative approach is elastic reverse-time modelling (RTMOD), where the recorded microseismic events are numerically backpropagated from the receiver positions into the elastic underground model. The resulting seismic wavefield focuses at the location of the event, which can be subsequently imaged by estimating the maximum of the seismic energy at each underground point. However, the success of this approach highly depends on the used elastic background model. In case of complex salt bodies, the strong velocity contrast between the salt and the surrounding sediments is a major problem. Therefore, we propose a combined monitoring approach, consisting of a seismic full waveform inversion of active source reflection seismic data to accurately image the background velocity model and subsequent RTMOD for the microseismic event localization. Accuracy and sensitivity with respect to the acquisition geometry and random noise will be demonstrated using a complex benchmark model. Furthermore, the localization accuracy is discussed for three different scenarios covering the detection of a partial cavern collapse, a gas leakage and the occurrence of cracks within the cavern wall due to extreme loading conditions during irregular operation.
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Lüders, K., Firmbach, L., Ebert, M., Dahmke, A., Dietrich, P., & Köber, R. (2016). Gas-phase formation during thermal energy storage in near-surface aquifers: experimental and modelling results. Environmental Earth Sciences, 75(21), 1404. doi:10.1007/s12665-016-6181-5.

Heating of groundwater by thermal energy storage (TES) poses a potential for the formation of a separate gas phase. Necessary boundary conditions, potential effects and monitoring feasibility of this process were not focused within previous studies. Since the formation of a gas phase could change groundwater flow conditions, hydrochemistry, porous media properties and thus efficiency of TES applications, improved understanding of the process is needed. The temperature of percolated sediment column tests was adjusted to 10, 25, 40 and 70 °C to quantify temperature-induced physical gas-phase formation and its effect on electrical resistance. Gas-phase formation, its accumulation and effects on hydraulic conductivity, heat conductivity and heat capacity were investigated using scenario calculations based on a closed-loop borehole TES system at 60 °C for different geochemical conditions. Experimentally quantified degassing ratios were within the expected range of thermodynamic calculations. The laboratory time-lapse electrical resistivity measurements proofed as a suitable tool to identify the onset and location of the gas-phase formation. Depending on the geochemical conditions, hydraulic conductivity in the area of the simulated heat storage site decreased between 60% and up to one order of magnitude in consequence of degassing within the scenario calculations. Heat conductivity and heat capacity decreased by maximally 3 and 16%, respectively. The results indicate that gas-phase formation as a result of aquifer heating can have pronounced effects especially on groundwater flow conditions and therefore should be considered particularly for nearly or fully gas-saturated groundwater and aquifers containing gas sources.
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Mahmoudi, E., Khaledi, K., Blumenthal, A. von, König, D., & Schanz, T. (2016). Concept for an integral approach to explore the behavior of rock salt caverns under thermo-mechanical cyclic loading in energy storage systems. Environmental Earth Sciences, 75(14), 1–19. doi:10.1007/s12665-016-5850-8.

The fluctuating nature of renewable energy sources can be managed by storing the surplus of electrical energy in an appropriate reservoir. The excess electricity available during off-peak periods of consumption may be used to compress air or electrolyze hydrogen. Afterward, the pressurized gas is stored in the rock salt cavities and discharged to compensate the shortage of energy when required. During this process, the rock salt surrounding the cavern undergoes thermo-mechanical cyclic loading. In order to achieve a reliable geotechnical design, the stress–strain response of rock salt under such loading condition has to be identified and predicted. In order to investigate the rock salt behavior under such loading, a comprehensive study using three concepts of geotechnical engineering, i.e., experimental investigation, constitutive modeling and numerical analysis, is conducted. A triaxial experimental setup is developed to supplement the knowledge of the cyclic thermo-mechanical behavior of rock salt. The imposed boundary conditions in the experimental setup are assumed to be similar to the stress state obtained from a full-scale numerical simulation. The computational model relies primarily on the governing constitutive model for predicting the behavior of rock salt cavity. Hence, a sophisticated elasto-viscoplastic creep constitutive model is developed to take into account the dilatancy and damage progress, as well as the temperature effects. The contributed input parameters in the constitutive model can be calibrated using the experimental measurements. In the following, the initial numerical simulation is modified based on the calibrated constitutive model. However, because of the significant levels of uncertainties involved in the design procedure of such structures, a reliable design can be achieved by employing probabilistic approaches. Therefore, the numerical calculation is extended by statistical tools such as sensitivity analysis, optimum experimental design, back analysis, probabilistic analysis and robust reliability-based design to get final design parameters of paramount need for practice.
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Miao, X.-Y., Beyer, C., Görke, U.-J., Kolditz, O., Hailemariam, H., & Nagel, T. (2016). Thermo-hydro-mechanical analysis of cement-based sensible heat stores for domestic applications. Environmental Earth Sciences, 75(18), 1293. doi:10.1007/s12665-016-6094-3.

The thermo-hydro-mechanical behaviour of a water-saturated cement-based heat store for domestic applications has been investigated. Numerical simulations have been employed to locate the critical regions during thermal loading, for which analytical solutions have been derived and validated by numerical simulations. The analytical solutions allow a fast screening of materials and design parameters in relation to the stresses induced by thermomechanical loading. Maximum stresses in the system have been quantified based on the thermomechanical properties of three heat exchanger materials selected by design engineers and of the filling material. Sensitivity analyses indicate that the stress distribution is very sensitive to the thermal expansion coefficients of the involved materials. The results of this study can serve as a guide line for the design of the present and similar heat storage systems. The analytical solution developed is a fast and robust method for the evaluation of stresses around heat exchangers embedded in a solid material and can serve as a tool for design optimisation.
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Nagel, T., Görke, U.-J., Moerman, K. M., & Kolditz, O. (2016). On advantages of the Kelvin mapping in finite element implementations of deformation processes. Environmental Earth Sciences, 75(11), 1–11. doi:10.1007/s12665-016-5429-4.

Classical continuum mechanical theories operate on three-dimensional Euclidian space using scalar, vector, and tensor-valued quantities usually up to the order of four. For their numerical treatment, it is common practice to transform the relations into a matrix–vector format. This transformation is usually performed using the so-called Voigt mapping. This mapping does not preserve tensor character leaving significant room for error as stress and strain quantities follow from different mappings and thus have to be treated differently in certain mathematical operations. Despite its conceptual and notational difficulties having been pointed out, the Voigt mapping remains the foundation of most current finite element programmes. An alternative is the so-called Kelvin mapping which has recently gained recognition in studies of theoretical mechanics. This article is concerned with benefits of the Kelvin mapping in numerical modelling tools such as finite element software. The decisive difference to the Voigt mapping is that Kelvin’s method preserves tensor character, and thus the numerical matrix notation directly corresponds to the original tensor notation. Further benefits in numerical implementations are that tensor norms are calculated identically without distinguishing stress- or strain-type quantities, and tensor equations can be directly transformed into matrix equations without additional considerations. The only implementational changes are related to a scalar factor in certain finite element matrices, and hence, harvesting the mentioned benefits comes at very little cost.
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Nolde, M., Schwanebeck, M., Dethlefsen, F., Duttmann, R., & Dahmke, A. (2016). Utilization of a 3D webGIS to support spatial planning regarding underground energy storage in the context of the German energy system transition at the example of the federal state of Schleswig–Holstein. Environmental Earth Sciences, 75(18), 1284. doi:10.1007/s12665-016-6089-0.

When decarbonizing a state-wide energy system by introducing a growing share of renewable energies, underground energy storage can help to deal with fluctuating electric grid feed-in from renewables like wind power. Since besides energy storage other subsurface usages can claim or effect possible scarce suited underground spaces and interact with other usages at the surface, subsurface spatial planning is a growing field of interest for state authorities and in science now. Combining two-dimensional surface geodata on concerned fields like regional planning and energy infrastructure with three-dimensional geological data into one coherent data model could therefore support spatial planners in identifying and locating underground entities suited for energy storage. In this paper, a volumetric grid-based concept to integrate two- and three-dimensional geodata into one coherent data framework is implemented, including available data sets on geology, energy infrastructure and existing spatial plans. Missing spatial data on regional electric energy production and heat energy demand are derived from available primary data. Upon this data basis, a self-developed open source-based 3D webGIS prototype is utilized to identify and visualize potential underground spaces for a compressed air energy storage use case scenario at the example of the federal state of Schleswig–Holstein in North Germany. A first basic and a subsequently extended query via the 3D webGIS on the developed data model provide spatial information on search domains for potential energy storage sites in salt rock structures that could be integrated into emerging subsurface spatial planning.
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Pfeiffer, W. T., al Hagrey, S. A., Köhn, D., Rabbel, W., & Bauer, S. (2016). Porous media hydrogen storage at a synthetic, heterogeneous field site: numerical simulation of storage operation and geophysical monitoring. Environmental Earth Sciences, 75(16), 1177. doi:10.1007/s12665-016-5958-x.

Large-scale energy storage such as porous media hydrogen storage will be required to mitigate shortages originating from fluctuating power production if renewables dominate the total supply. In order to assess the applicability of this storage option, a possible usage scenario is defined for an existing anticlinal structure in the North German Basin and the storage operation is numerically simulated. A heterogeneous and realistic parameter distribution is generated by a facies modelling approach. The storage operation, which is performed using five wells, consists of an initial filling of the storage with nitrogen used as cushion gas and hydrogen as well as several week-long withdrawal periods each followed by a refill and a shut-in period. Storage performance increases with the number of storage cycles and a total of 29 million m3 of hydrogen gas at surface conditions can be produced in the long term, equating to 186,000 GJ of energy when assuming a re-electrification efficiency of 60 %. In addition to downhole pressure monitoring geophysical techniques such as seismic full waveform inversion (FWI), electrical resistivity tomography (ERT) and gravity methods can be used for site monitoring, if their individual detecting capabilities are sufficient. Investigation of the storage scenario by virtual application of these methods shows that FWI and ERT can be used to map the thin gas phase distribution in this heterogeneous formation with the individual methods conforming each other. However, a high spatial density of receivers in a crosswell geometry with less than 500 m distance between the observation wells is required for this. Gravity mapping also shows anomalies indicating mass changes caused by the storage operation. However, monitoring the filling state of this hydrogen storage site is not possible.
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Popp, S., Beyer, C., Dahmke, A., Koproch, N., Köber, R., & Bauer, S. (2016). Temperature-dependent dissolution of residual non-aqueous phase liquids: model development and verification. Environmental Earth Sciences, 75(11), 1–13. doi:10.1007/s12665-016-5743-x.

The use of heat storages in the subsurface, especially in urbanized areas, may conflict with existing subsurface contaminations of non-aqueous phase liquids (NAPL). In this work, available data and models regarding temperature influences on parameters for kinetic NAPL dissolution of trichloroethene (TCE) are summarized, discussed and implemented into a numerical simulator. As systematic data on temperature-dependent TCE solubility, diffusion coefficients and dissolution rates are sparse, a set of high-resolution quasi-2D laboratory NAPL dissolution experiments using TCE was conducted at 10, 20, 40 and 70 °C. Because the experimental data show incomplete dissolution of the residual TCE–NAPL, two different classes of TCE–NAPL blobs representing fast and slow dissolution kinetics were introduced in the model. A good agreement of model simulations and experimental measurements of TCE mass flow rates could thus be obtained for each temperature investigated. The numerical model thus can be applied to simulate kinetic dissolution of residual NAPL source zones in groundwater under variable temperature conditions.
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Schulte, D. O., Welsch, B., Boockmeyer, A., Rühaak, W., Bär, K., Bauer, S., & Sass, I. (2016). Modeling insulated borehole heat exchangers. Environmental Earth Sciences, 75(10), 1–12. doi:10.1007/s12665-016-5638-x.

In the heating sector, borehole heat exchangers have become popular for supplying renewable energy. They tap into the subsurface to extract geothermal energy for heating purposes. For advanced applications, borehole heat exchangers require insulation in the upper part of the borehole either to meet legal requirements or to improve their performance. A priori numerical heat transport models of the subsurface are imperative for the systems’ planning and design. Only fully discretized models can account for depth-dependent borehole properties like insulated sections, but the model setup is cumbersome and the simulations come at high computational cost. Hence, these models are often not suitable for the simulation of larger installations. This study presents an analytical solution for the simulation of the thermal interactions of partly insulated borehole heat exchangers. A benchmark with a fully discretized OpenGeoSys model confirms sufficient accuracy of the analytical solution. In an application example, the functionality of the tool is demonstrated by finding the ideal length of a borehole insulation using mathematical optimization and by quantifying the effect of the insulation on the borehole heat exchanger performance. The presented method allows for accommodation of future advancements in borehole heat exchangers in numerical simulations at comparatively low computational cost.
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Wang, B., & Bauer, S. (2016). Converting heterogeneous complex geological models to consistent finite element models: methods, development, and application to deep geothermal reservoir operation. Environmental Earth Sciences, 75(20), 1349. doi:10.1007/s12665-016-6138-8.

Static geological models representing complex geological systems are the prerequisite of dynamic model simulations applied for assessing subsurface processes. The corner point grid approach has been applied to represent the complexity in geometry, hydraulic connectivity, and heterogeneity found in these static geological models. Due to the occurrence of faults, pinch-outs, and eroded geological layers, corner point grids easily degenerate, which leads to model inconsistencies. This study introduces a workflow for converting heterogeneous geological models to consistent finite element models, accounting for regular and irregular hexahedral blocks of the corner point grid by converting to a set of hexahedra, prism, pyramid, and tetrahedral elements, based on the individual degeneration situation. Heterogeneous geological data such as permeability or porosity can be transferred layer-wise or on a block-wise basis. Additionally, well trajectories can be accurately mapped to the converted finite element mesh, to place the corresponding source terms. The developed workflow is tested on dedicated test cases and applied to convert a real complex field site from the North German Basin for use in a deep geothermal reservoir operation. The field application demonstrates the robustness and applicability of the newly developed conversion workflow and the suitability of the converted mesh for dynamic finite element reservoir model simulations.
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