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Intrusion of deep saline waters into freshwater aquifers does not only endanger the regional drinking water supply, but also rivers and stagnant waters and their fauna are threatened by salinisation. The upwelling of highly mineralised saline waters in large parts of the North German Basin is favoured by the presence of Elsterian glacial erosion windows in the Lower Oligocene Rupelian Clay, the most important hydraulic confining unit in this region. Lower precipitation rates and decreasing groundwater levels as a consequence of global climate change, but also anthropogenic interventions, such as increasing extraction rates or the use of the deep geologic subsurface as a reservoir, decrease the pressure potential in the freshwater column and may possibly accelerate this primarily geogenic salinisation process in the coming years. Density-driven flow and transport modelling was performed in the scope of the present study to investigate the upwelling mechanisms of deep saline waters across Quaternary window sediments in the Rupelian. Simulation results show that the interactions between the groundwater recharge rate and anthropogenic interventions such as extraction rates of drinking water wells or the utilisation of the deep subsurface, have a significant influence on the groundwater pressure potential in the freshwater aquifer and associated saltwater upwelling. In all scenarios, salinisation is most severe in the sediments of the erosion windows. Hydraulically conductive faults also intensify salinisation if located nearside erosion windows or induce a more distributed or localised salinisation in aquifers with drinking water relevance in areas that do not intersect with erosion windows. A decline in groundwater recharge thereby significantly favours upward saltwater migration. The simulation scenarios further show that a decrease in groundwater recharge also results in freshwater salinisation occurring up to 10 years earlier, which underlines the need for waterworks to initiate effective countermeasures quickly and in time.

Large-scale energy storage is becoming more important due to the increase in electricity generation from renewable sources and the related grid balancing requirements. In this context, Pumped Hydropower Storage (PHS) in former open-pit lignite mines can substantially contribute to energy supply safety. Assuming an average storage capacity of 150 MW per open-pit mine, PHS could generate a power output of at least 6 GW in European mines which will be abandoned in the next two decades. Experiences from mine-flooding across Europe demonstrate that hydrogeochemical processes can become a critical environmental and economic factor for the realisation of such projects. Depending on sulphide and oxygen availability, buffer capacities and dilution processes, mine waters with increased acidity as well as elevated sulphate and metal concentrations can pose a threat to adjacent ecosystems, groundwater resources and the installed PHS infrastructure.We present a generic parameter study by means of numerical simulations to predict changes in the mine water composition as a result of PHS operation in different hydrogeochemical settings. Published datasets on hydrogeochemical, hydrogeological and technical conditions with a focus on German mines were applied for model parametrisation. A reaction path model was developed that accounts for initial mine flooding, inflows and outflows as well as pumping and release cycles between the two reservoirs. The simulations were run until chemical equilibrium was achieved in the lower reservoir.Simulation results indicate that the long-term availability of buffer capacities in the reservoir water and adjacent sediments determine the development of acidic or neutral mine waters. Sulphate concentrations are mainly influenced by dilution processes, emphasizing the relevance of considering additional in- and outflows. Depending on these fluxes as well as oxygen availability and initial sulphide concentration in the mine sediments, the time to reach chemical equilibrium in the lower reservoir varies significantly from several weeks to months. Furthermore, the dissolution of sulphides and carbonates as well as the precipitation of iron (oxy)hydroxides may affect the properties of the open-pit slope sediments. Their long-term stability may be altered, based on their initial mineral concentration and hydraulic conductivity.In summary, potential impacts on water quality in the PHS reservoirs have been investigated under different hydrogeochemical settings. We conclude that, under specific boundary conditions such as the availability of sufficient buffer capacities and dilution by controlled inflows and outflows, PHS operation in abandoned open-pit coal mines can be realised from an environmental perspective.

Inland salinisation due to the upwelling of highly mineralised deep waters formed by leaching of Upper Permian salt diapirs is a typical phenomenon in the North German Basin. In the German State of Brandenburg, the local absence of the regionally most important aquiclude, the Lower Oligocene Rupelian Clay, separating deep saline waters from the overlying freshwater aquifers, is considered to be the main cause of local salinisation in the freshwater column.The present study uses density-driven 3D flow and transport simulations to assess saltwater upwelling across Quaternary window sediments in the Rupelian for an area in southeastern Brandenburg with detectable salt concentrations in the freshwater column. Previous simulations along a 55 km long transect in Brandenburg using a 2D model have already demonstrated the potential negative impact of groundwater extraction, the use of the deep subsurface as a storage reservoir or lower precipitation rates and decreasing groundwater levels as a consequence of global climate change on the degree of upper aquifer salinisation (Chabab et al., 2022; Tillner et al., 2016; Wetzel et al., 2016).The presented simulation results show that 3D flow strongly affects the temporal and spatial distribution of upper aquifer salinisation due to the varying extent of layers and erosion windows in the Rupelian Clay. The location of groundwater extraction sites, hydraulically conductive faults and spatial variations in groundwater recharge additionally influence the location and degree of shallow aquifer salinisation, and must therefore be carefully considered. Depending on topographic gradients and density variations occurring due to differences in pressure and temperature, convective cells with descending flow and freshwater lenses in the saltwater column also develop locally. We show that 3D flow simulations are essential for site-specific analysis to represent the dynamics of the system with many different hydrogeologic interacting and controlling factors. LiteratureChabab, E., Kühn, M., Kempka, T. (2022): Upwelling mechanisms of deep saline waters via Quaternary erosion windows considering varying hydrogeological boundary conditions. Advances in Geosciences, 58, 47-54.Tillner, E., Wetzel, M., Kempka, T., Kühn, M. (2016): Fault damage zone volume and initial salinity distribution determine intensity of shallow aquifer salinisation in subsurface storage. Hydrology and Earth System Sciences, 20, 1049-1067.Wetzel, M., Kühn, M. (2016): Salinization of Freshwater Aquifers Due to Subsurface Fluid Injection Quantified by Species Transport Simulations. Energy Procedia, 97, 411-418.

Power-to-Methanol is considered as an additional option to Power-to-Gas to convert surplus energy from renewable sources and the electric grid into storable energy carriers. In this context, methanol is an alternative fuel to power combustion engines, and it can be applied to produce chemical feedstock such as formaldehyde required for polymer production, hydrocarbons, gasoline and olefines, as well as gasoline additives and especially as an energy carrier and carbon sink.As long-term storage of energy carriers is required to realise the transition of the energy sector to renewable sources scheduled in the European Union, the fact that storage of methanol requires less operational and safety efforts compared to natural gas or hydrogen is a significant benefit, i.e. methanol does not require any compression prior to its injection into geologic subsurface reservoirs, while being biodegradable and of generally low environmental toxicity. Existing hydrocarbon transport and storage infrastructure can be directly applied to transport and store methanol in the geologic subsurface. In this context, a major concern besides methanol biodegradability is its high miscibility with water, potentially resulting in relevant storage losses that may favour uneconomic storage operations in active groundwater aquifers. Hence, the present study aims at a quantitative assessment of the mixing behaviour of methanol and water based on a reference numerical simulation benchmark previously applied to investigate that of CH4 stored in a CO2 cushion gas within a depleted natural gas reservoir (Oldenburg et al., 2003, Ma et al., 2019, and others). For that purpose, the TRANSPORTSE numerical simulator (Kempka, 2020), applicable to simulate fluid flow as well as transport of heat and reactive transport of chemical species (Kempka et al., 2022) is used in the present study. Mixing ratio-dependent density and viscosity changes as well as different reservoir dipping angles are considered to determine the chemical storage efficiency in view of mixing losses. Simulation results demonstrate that methanol fraction-driven variations in fluid density and viscosity of up to 20 % and 30 %, respectively, as well as the relatively low diffusion coefficients compared to those of gases result in low mixing degrees of both liquid components. Structural geological features need to be considered in the selection of methanol storage sites, since these directly affect the spatial extent of the mixing region, and thus methanol recovery efficiency. Kempka, T., Steding, S., Kühn, M. (2022) Verification of TRANSPORT Simulation Environment coupling with PHREEQC for reactive transport modelling. Advances in Geosciences, 58, 19-29. https://doi.org/10.5194/adgeo-58-19-2022Kempka, T. (2020) Verification of a Python-based TRANsport Simulation Environment for density-driven fluid flow and coupled transport of heat and chemical species. Advances in Geosciences, 54, 67-77. https://doi.org/10.5194/adgeo-54-67-2020Ma, J., Li, Q., Kempka, T., Kühn, M. (2019) Hydromechanical Response and Impact of Gas Mixing Behavior in Subsurface CH4 Storage with CO2-Based Cushion Gas Energy & Fuels 33 (7), 6527-6541. https://doi.org/10.1021/acs.energyfuels.9b00518Oldenburg, C. M. (2003) Carbon Dioxide as Cushion Gas for Natural Gas Storage. Energy Fuels 17(1), 240−246. https://doi.org/10.1021/ef020162b

Conversion of static geologic models into numerical simulation grids is a pre-requisite to undertake site-specific assessments of geologic subsurface utilisation in terms of risk assessments, design and operational optimisations as well as long-term predictions.GEOMODELATOR is a Python-based Open Source software package which enables modellers to translate static geologic models into regular structured simulation grids with element partitions following a complex model geometry.For that purpose, geologic models generated by means of Geographic Information Systems (GIS), Computer-Aided Design (CAD) or other specific geologic modelling software packages are integrated in form of point cloud data together with the desired structured simulation grid geometry.GEOMODELATOR maps geometric features such as lithologic horizons, faults and any kind of other geometric data by 3D Delaunay triangulation onto the pre-defined grid element centres, and provides the modeller with Visualization Toolkit (VTK) data and Python numpy arrays for visual model inspection and their direct application in numerical simulations, respectively.The present contribution shows the application of GEOMODELATOR to different numerical simulation studies addressing fluid flow as well as transport of heat and chemical species in geological subsurface utilisation.

This paper presents a series of surface experimental simulations of methane-oriented underground coal gasification using hydrogen as gasification medium. The main aim of the experiments conducted was to evaluate the feasibility of methane-rich gas production through the in situ coal hydrogasification process. Two multi-day trials were carried out using large scale gasification facilities designed for ex situ experimental simulations of the underground coal gasification (UCG) process. Two different coals were investigated: the “Six Feet” semi-anthracite (Wales) and the “Wesoła" hard coal (Poland). The coal samples were extracted directly from the respective coal seams in the form of large blocks. The gasification tests were conducted in the artificial coal seams (0.41 × 0.41 × 3.05 m) under two distinct pressure regimes - 20 and 40 bar. The series of experiments conducted demonstrated that the physicochemical properties of coal (coal rank) considerably affect the hydrogasification process. For both gasification pressures applied, gas from “Six Feet” semi-anthracite was characterized by a higher content of methane. The average CH4 concentration for “Six Feet” experiment during the H2 stage was 24.12% at 20 bar and 27.03% at 40 bar. During the hydrogasification of “Wesoła" coal, CH4 concentration was 19.28% and 21.71% at 20 and 40 bar, respectively. The process was characterized by high stability and reproducibility of conditions favorable for methane formation in the whole sequence of gasification cycles. Although the feasibility of methane-rich gas production by underground hydrogasification was initially demonstrated, further techno-economic studies are necessary to assess the economic feasibility of methane production using this process.

In the context of a potential utilisation of coal resources located in the Mecsek mountain area in Southern Hungary, an assessment of groundwater pollution resulting from a potential water-borne contaminant pool remaining in in situ coal conversion reactors after site abandonment has been undertaken in the scope of the present study. The respective contaminants may be of organic and inorganic nature. A sensitivity analysis was carried out by means of numerical simulations of fluid flow as well as contaminant and heat transport including retardation to assess spatial contaminant migration. Hereby, the main uncertainties, e.g., changes in hydraulic gradient and hydraulic contributions of the complex regional and local fault systems in the study area, were assessed in a deterministic way to identify the relevant parameters. Overall 512 simulations of potential groundwater contamination scenarios within a time horizon exceeding the local post-operational monitoring period were performed, based on maximum contaminant concentrations, cumulative mass balances as well as migration distances of the contaminant plume. The simulation results show that regional faults represent the main contaminant migration pathway, and that contamination is unlikely assuming the given reference model parametrisation. However, contamination within a simulation time of 50 years is possible for specific geological conditions, e.g., if the hydraulic conductivity of the regional faults exceeds a maximum value of 1 × 10−5 m s−1. Further, the parameter data analysis shows that freshwater aquifer contamination is highly non-linear and has a bimodal distribution. The bivariate correlation coefficient heatmap shows slightly positive correlations for the pressure difference, the fault permeability and the simulation time, as well as a negative correlation for the retardation coefficient. The results of this sensitivity analysis have been integrated into a specific toolkit for risk assessment for that purpose.

The geothermal hot water reservoir below the small town of Waiwera in New Zealand has been known to the indigenous Māori for many centuries. Overproduction by European immigrants led to a water level decrease and consequently artesian flow from the wells and the seeps on the beach ceased. The Te Kaunihera o Tāmaki Makaurau Auckland Council established the Waiwera Thermal Groundwater Allocation and Management Plan to allow the geothermal system to recover. For a sustainable operation, the management regime can be informed by hydrogeological models based on monitoring data. The underlying geological model has been revised according to field observations and an existing numerical model transferred to the newly developed software package TRANSPORTSE. Monitoring and digitally derived data have been integrated in a geographic information system (GIS).

Many types of geological subsurface utilisation are associated with fluid and heat flow as well as simultaneously occurring chemical reactions. For that reason, reactive transport models are required to understand and reproduce the governing processes. In this regard, reactive transport codes must be highly flexible to cover a wide range of applications, while being applicable by users without extensive programming skills at the same time. In this context, the TRANsport Simulation Environment (Kempka, 2020) was coupled with the geochemical reaction module PHREEQC (Parkhurst & Appelo, 2013), providing multiple features that make it applicable to complex reactive transport problems in various fields. Code readability is ensured by the applied high-level programming language Python which is relatively easy to learn compared to low-level programming languages. In the present study, common geochemical benchmarks are used to verify the numerical code implementation.Currently, the coupled simulator can be used to investigate 3D single-phase fluid and heat flow as well as multicomponent solute transport in porous media. In addition to that, a wide range of equilibrium and nonequilibrium reactions can be considered. Chemical feedback on fluid flow is provided by adapting porosity and permeability of the porous media as well as fluid properties. Thereby, users are in full control of the underlying functions and equations of state. Both, the solution of the system of the partial differential equations and PHREEQC module, can be easily parallelised to increase computational efficiency.The benchmarks used in the present study include density-driven flow as well as advective and diffusive reactive transport of solutes. Furthermore, porosity, permeability and diffusivity changes caused by kinetically controlled dissolution-precipitation reactions are considered to verify the main features of our reactive transport code. In future, the code implementation may be used to quantify processes encountered in different types of subsurface utilisation, such as geothermal energy production, geological energy, CO2 and nuclear waste storage.References:Kempka, T. (2020). Verification of a Python-based TRANsport Simulation Environment for density-driven fluid flow and coupled transport of heat and chemical species. Adv. Geosci. 54, 67–77. (https://doi.org/10.5194/adgeo-54-67-2020)Parkhurst, D.L.; Appelo, C.A.J. (2013). Description of Input and Examples for PHREEQC Version 3 - a Computer Program for Speciation, Batch-reaction, One-dimensional Transport, and Inverse Geochemical Calculations. In Techniques and Methods; Publisher: U.S. Geological Survey; Book 6, 497 pp. (https://pubs.usgs.gov/tm/06/a43/)

Mineral dissolution and precipitation can substantially affect rock permeability, which is a critical parameter for a broad range of geological subsurface applications. Virtual experiments on digital pore-scale samples represent a powerful and flexible approach to understand the impact of microstructural alterations on evolving hydraulic rock behaviour and quantify trends in permeability. In the present study, porosity-permeability relations are simulated for a precipitation-dissolution cycle within a typical reservoir sandstone. A hysteresis in permeability is observed depending on the geochemical process and dominating reaction regime, whereby permeability of the six investigated reaction paths varies by more than two orders of magnitude at a porosity of 17 %. Controlling parameters for this hysteresis phenomenon are the closure and re-opening of micro-scale flow channels, derived from changes in pore throat diameter and connectivity of the pore network. In general, a transport-limited regime exhibits a stronger impact on permeability than a reaction-limited regime, which uniformly alters the pore space. In case of mineral precipitation, higher permeability reduction results from successive clogging of pore throats, whereas in case of dissolution, permeability significantly increases due to a widening of existing flow paths. Both, the geochemical process and dominating reaction regime govern characteristic microstructural alterations, which cannot be simply reversed by the inversion of the geochemical processes itself. Hence, permeability evolution clearly depends on the hydrogeochemical history of the sample.

Understanding the mechanical behaviors of granite after thermal treatment under loading and unloading conditions is of utmost relevance to deep geothermal energy recovery. In the present study, a series of loading and unloading triaxial compression tests (20, 40 and 60 MPa) on granite specimens after exposure to different temperatures (20, 200, 300, 400, 500 and 600 °C) was carried out to quantify the combined effects of thermal treatment and loading/unloading stress conditions on granite strength and deformation. Changes in the microstructure of granite exposed to high temperatures were revealed by optical microscopy. The experimental results indicate that both, thermal treatment and loading/unloading stress conditions, degrade the mechanical behaviors and further decrease the carrying capacity of granite. The gradual degradation of the mechanical characteristics of granite after thermal treatment is mainly associated with the evolution of thermal micro-cracks based on optical microscopy observations. The unloading stress state induces the extension of tension cracks parallel to the axial direction, and thus, the mechanical properties are degraded. Temperatures above 400 °C have a more significant influence on the mechanical characteristics of granite than the unloading treatment, whereby 400 °C can be treated as a threshold temperature for the delineation of significant deterioration. This study is expected to support feasibility and risk assessments by means of providing data for analytical calculations and numerical simulations on granite exposed to high temperatures during geothermal energy extraction.

In the context of a potential utilisation of coal resources located in the Mecsek mountain area in Southern Hungary (Püspöki et al.. 2012), an assessment of groundwater pollution resulting from a potential water-borne contaminant pool remaining in in-situ coal conversion reactors after site abandonment has been undertaken. The respective contaminants may be of organic (i.e., phenols, benzene, polycyclic aromatic hydrocarbons, etc.) and inorganic nature (i.e., ammonia, mercury, zinc, cyanide, heavy metals, etc.), whereby data for the Mecsek coal has been derived from extensive laboratory testing.The probability assessment was carried out by means of numerical simulations of fluid flow as well as contaminant and heat transport including retardation using the TRANsport Simulation Environment (Kempka, 2020). Hereby, the main uncertainties, e.g., changes in hydraulic gradient and hydraulic contributions of the complex regional and local fault systems in the study area, were assessed in a deterministic way to identify the parameters relevant for the overall sensitivity study. Using Monte-Carlo analyses and Latin hypercube sampling, the uncertainty bandwidths of water table, retardation factors, dispersion coefficients, hydraulic conductivities of aquitards, faults and aquifers as well as groundwater recharge were considered.The simulation results demonstrate that fluid flow via the regional faults is the main driver for a potential contamination of the shallow groundwater aquifers. Consequently, the numerical simulation results on potential fault reactivation due to coal extraction (Hedayatzadeh et al., 2022) were taken into account in view of probable hydraulic conductivity changes in the regional fault systems and the rock matrix surrounding the abandoned reactors. The probabilities of groundwater aquifer contamination within a time horizon of 50 years are presented based on maximum contaminant concentrations, cumulative mass balances as well as migration distances of the contaminant plume. The results of this analysis are essential for mining authorities as well as potential stakeholders to improve the understanding on potential environmental impacts, and have been integrated into a specific toolkit for risk assessment (Tranter et al., 2022) for that purpose.ReferencesHedayatzadeh, M. et al. (2022) Ground subsidence and fault reactivation during in-situ coal conversion assessed by numerical simulations, https://meetingorganizer.copernicus.org/EGU22/EGU22-11736.htmlKempka, T. (2020) Verification of a Python-based TRANsport Simulation Environment for density-driven fluid flow and coupled transport of heat and chemical species. Adv. Geosci. 54, 67–77. https://doi.org/10.5194/adgeo-54-67-2020 Püspöki, Z. et al. (2012) Stratigraphy and deformation history of the Jurassic coal bearing series in the Eastern Mecsek (Hungary). International Journal of Coal Geology 102, 35–51. https://doi.org/10.1016/j.coal.2012.07.009 Tranter, M. et al. (2022) Environmental hazard quantification toolkit based on modular numerical simulations, https://meetingorganizer.copernicus.org/EGU22/EGU22-10115.html

Comprehensive risk assessments for subsurface utilisation projects such as in-situ coal conversion, deep geothermal energy, geological storage, and waste disposal are implemented to a limited extent in common practice. The impacts of subsurface processes on environmental hazards (e.g., migration of groundwater-borne contaminants, induced seismicity, and subsidence) are often convoluted and therefore not trivially to predict. Furthermore, decisions on project feasibilities are commonly based on expert knowledge subject to non-standardised approaches. However, an objectively and transparently developed risk assessment is imperative for a publicly accepted, long-term economic and environmentally friendly design of future subsurface utilisation.We propose a new environmental hazard quantification framework based on modular simulations. The aim is to create a uniform basis for both project developers and authorities to carry out risk analyses. The approach streamlines state-of-the-art numerical models [1,2], accounting for multiphase flow, geomechanics, geochemistry, and heat transport, to determine the likelihood and severity of hazards. The method uses the results of the computational expensive Monte Carlo simulations of each module to train gradient boosting machine learning algorithms. These surrogate models facilitate loose coupling within the framework and a seamless integration into a graphical user interface for demonstrating hazard probability distributions.The approach was applied to two example study areas with complex geological settings as part of a risk assessment for in-situ coal conversion. A substantial rock volume is extracted during this operation, and a contaminant pool is potentially left behind, which may put the environment at risk. With our presented approach, the shortcoming of using conceptually simplified models are substantially reduced, since subsurface complexities are accounted for. The transparency of the assessment basis should generally increase the acceptance of geoengineering projects, which is considered one of the crucial aspects for the further development and dissemination of geological subsurface utilisation.[1] Hedayatzadeh et al.: Ground subsidence and fault reactivation during in-situ coal conversion assessed by numerical simulations, EGU22, https://meetingorganizer.copernicus.org/EGU22/EGU22-11736.html, 2022.[2] Kempka et al.: Probability of contaminant migration from abandoned in-situ coal conversion reactors, EGU22, https://meetingorganizer.copernicus.org/EGU22/EGU22-11204.html, 2022.

Intrusion of deep saline waters into freshwater aquifers does not only endanger the regional drinking water supply, but also rivers and stagnant waters as well as their fauna and flora are threatened by salinisation. The upconing of highly mineralised saline waters in large parts of the North German Basin is favoured by the presence of Elsterian glacial erosion windows in the Lower Oligocene Rupelian Clay, the most important confining unit in this region. Lower precipitation rates and decreasing groundwater levels as a consequence of global climate change, but also anthropogenic interventions, such as increasing extraction rates or the utilisation of the geological subsurface, decrease the pressure potential in the freshwater column and may possibly accelerate primarily geogenic salinisation processes in the coming years [1, 2].In this study, density-driven flow and transport modelling [3] was performed to investigate the upconing mechanisms of deep saline waters across Quaternary window sediments in the Rupelian. First, the main variables influencing the dynamics of the freshwater/saltwater boundary were determined using generic 2D models. For a site-specific analysis along a 20 km long transect in the Federal State of Brandenburg, Germany, the geological/hydrogeological conditions were then integrated into the 2D models, starting from the Mesozoic strata in the bedrock of the Rupelian sequence as the model basis, up to the Quaternary unconsolidated rock series at the ground surface. At site, the Rupelian Clay has been partially eroded and salinisation in the hanging freshwater column is already detectable.Simulation results show that the interactions between influencing variables, e.g., the regional groundwater flow and seasonal dynamics of the groundwater recharge rate, as well as anthropogenic interventions such as extraction rates of drinking water wells, have a significant influence on the groundwater pressure potential in the freshwater aquifer and associated saltwater upconing. The temporal development of saltwater intrusion shows up quite differently, depending on boundary conditions and also strongly depends on flow rates and cross-section of the Rupelian windows. Depending on the topography, the fluid density gradient and its effect on flow conditions and pressure potential, creates a dynamic between deep saline and shallow freshwater aquifers, with ascending flow occurring locally in larger discharge areas. The next steps will comprise a 3D extension of the model as well as consideration of chemical rock-water interactions.Literature[1] Tillner, E., Wetzel, M., Kempka, T., Kühn, M. (2016): Fault damage zone volume and initial salinity distribution determine intensity of shallow aquifer salinisation in subsurface storage. Hydrology and Earth System Sciences, 20, 1049-1067.[2] Wetzel, M., Kühn, M. (2016): Salinization of Freshwater Aquifers Due to Subsurface Fluid Injection Quantified by Species Transport Simulations. Energy Procedia, 97, 411-418.[3] Kempka, T. (2020): Verification of a Python-based TRANsport Simulation Environment for density-driven fluid flow and coupled transport of heat and chemical species. Advances in Geosciences, 54, 67-77.

The geothermal hot water reservoir below the small town of Waiwera in New Zealand has been known to the indigenous people, the Maori, for many centuries. Its use by European immigrants began in the 19th century. Until the end of the 1960s, all production wells drilled for the warm water were artesian. But, triggered by overproduction, the water which has a temperature of up to 50 °C, has to be pumped up since then. In the 1970s, the warm water springs on Waiwera beach also dried up. Therefore, the Auckland Council implemented a water management plan for a future sustainable use of the area in the 1980s (Kühn and Altmannsberger 2016). Just recently, there have been reports about recovered, temporary artesian flow from several wells. Further, there is indication for a renewed activity of the hot springs at the beach (Präg et al. 2020). For a comprehensive understanding and an environmentally friendly and balanced long-term usage of the aquifer, a fairly complex hydrogeological model is required.Various approaches for a quantified hydrogeological description of the Waiwera reservoir have been implemented since the 1980s. Some are data driven (Kühn and Schöne 2017, Kühn and Grabow 2021) and others process based (Kühn and Altmannsberger 2016, Somogyvári et al. 2019) to finally understand and assess the constraints and impacts on the system (Kühn and Schöne 2018). However, none of the models directly delivers all the results needed for an all-encompassing water management. Disadvantage of all previous work is the independent model set-up and usage of only some of the acquired monitoring and simulation results. We present now a Geographic Information System (GIS) as a data base which integrates all geoscientific information known about the geothermal area of Waiwera combined with software tools for management. This will be the basis of the next generation of hydrogeological models for the geothermal area.LiteratureKühn, M., Altmannsberger, C. (2016): Assessment of data driven and process based water manage-ment tools for the geothermal reservoir Waiwera (New Zealand). - Energy Procedia, 97, 403-410. https://doi.org/10.1016/j.egypro.2016.10.034Kühn, M., Grabow, L. (2021): Deconvolution well test analysis applied to a long-term data set of the Waiwera geothermal reservoir (New Zealand). - Advances in Geosciences, 56, 107-116. https://doi.org/10.5194/adgeo-56-107-2021Kühn, M., Schöne, T. (2018): Investigation of the influence of earthquakes on the water level in the geothermal reservoir of Waiwera (New Zealand). - Advances in Geosciences, 45, 235-241. https://doi.org/10.5194/adgeo-45-235-2018Kühn, M., Schöne, T. (2017): Multivariate regression model from water level and production rate time series for the geothermal reservoir Waiwera (New Zealand). - Energy Procedia, 125, 571-579. https://doi.org/10.1016/j.egypro.2017.08.196Präg, M., Becker, I., Hilgers, C., Walter, T. R., Kühn, M. (2020): Thermal UAS survey of reactivated hot spring activity in Waiwera, New Zealand. - Advances in Geosciences, 54, 165-171. https://doi.org/10.5194/adgeo-54-165-2020Somogyvári, M., Kühn, M., Reich, S. (2019): Reservoir-scale transdimensional fracture network inversion. - Advances in Geosciences, 49, 207-214. https://doi.org/10.5194/adgeo-49-207-2019

Mineral dissolution and precipitation can substantially affect rock permeability, which is a critical parameter for a broad range of subsurface applications, including geothermal energy production, geological storage, petroleum engineering and subsurface contaminant transport. In order to quantify trends in rock properties, virtual experiments on digital pore-scale samples represent a powerful and flexible approach to fundamentally understand the impact of microstructural alterations on evolving permeability.In the present study, cycles of secondary mineral precipitation and subsequent dissolution are simulated on a synthetic sandstone sample [1]. For that purpose, the flow velocity magnitude is used as a proxy for solvent flux to depict characteristic transport-limited alteration patterns, whereas the inner surface area is used to constrain reaction-limited processes [2,3]. The corresponding hydraulic property evolutions are computed for combinations of reaction- and transport-limited precipitation and subsequent dissolution. Hysteresis can be observed for most of the geochemical reaction pathways, where the permeability trend for the dissolution differs significantly from that of precipitation. Transport-limited mineral dissolution initially shows a considerably higher permeability increase due to the widening of existing main flow paths, whereas the subsequent dissolution of new flow paths leads to a comparably lower permeability increase. The determined discontinuity in permeability evolution clearly demonstrates that microstructural changes as the opening or closure of flow paths might not be simply an inversion of the geochemical processes on an identical reaction pathway. The simulated porosity-permeability relationships are further discussed in the context of property trends observed in nature. Current analytical approaches are not able to reflect the evolution for these dynamic processes, since they describe permeability as a simple function of porosity. Hence, pore-scale modelling approaches are required to describe permeability trends and further develop understanding of reservoir behaviour, since hydraulic property changes resulting from mineral precipitation and dissolution clearly depend on geochemical processes and their history.[1] Wetzel M., Kempka T., Kühn M. (2021): Diagenetic trends of synthetic reservoir sandstone properties assessed by digital rock physics. Minerals, 11, 2, 151. DOI: 10.3390/min11020151[2] Wetzel M., Kempka T., Kühn M. (2020): Digital rock physics approach to simulate hydraulic effects of anhydrite cement in Bentheim sandstone. Advances in Geosciences, 54, 33-39. DOI: 10.5194/adgeo-54-33-2020[3] Wetzel M., Kempka T., Kühn M. (2020): Hydraulic and mechanical impacts of pore space alterations within a sandstone quantified by a flow velocity-dependent precipitation approach. Materials, 13, 4, 3100. DOI: 10.3390/ma13143100

As the demand for water supply increases with population growth, the quality of ground and surface water resources is deteriorating rapidly in many regions worldwide, particularly in Ghana. This situation has put supply systems under severe pressure as many of the available water resources are polluted by anthropogenic activities such as mining, agriculture, domestic and industrial sewage. Ghana's water quality problems are not different from current global challenges, as many surface waters and some aquifers have been polluted by mining activities and to some extent also by agriculture and industrial seepage. The Pra Basin is one of the most affected basins in Ghana with a total area of around 2,300 km2 and a population of over five million people. The economic history of the basin is unparalleled as it is home to the country's major mineral deposits, including gold, bauxite, manganese, and diamonds. Recent studies have shown significant amounts of water pollutants including mercury (Hg), arsenic (As), lead (Pb), iron (Fe), manganese (Mn), cadmium (Cd), selenium (Se) and nitrate (NO3). The underlying geology of the Pra Basin consists mainly of metasediments and granitoids. The occurrence of groundwater is controlled by the development of secondary porosities through fractures, joints, and faults. This study provides insights into the evolution and hydrogeochemical processes that control the groundwater quality in the Pra Basin. The methodology applied here includes field sample collection, statistical analysis of hydrochemical data, petrographic and mineralogical analysis of rock outcrops and geochemical modelling. Groundwater samples were taken from shallow (mainly hand-dug wells with depths 30 m) throughout the basin. Samples were analysed for major ions, and trace metals using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), Ion Chromatography (IC), and a Picarro L-2140i Ringdown Spectrometer. Multivariate statistical analyses, inverse and forward geochemical modelling were applied to the hydrochemical data of around 100 water samples. The mineral phases used as model input were obtained from X-ray Diffraction (XRD) measurements of rock outcrops from the study area and mainly include chlorite, albite, muscovite, biotite, and calcite. The analysis of the results shows that the geochemistry of the groundwater resources in the Pra Basin is mainly controlled by water-rock-interaction. Within the given uncertainty limits, the dissolution of carbonates and weathering of silicates are the drivers for the chemical development of the groundwater in the basin. The presented findings will support the development of sustainable water resources management strategies and contribute to mitigating future contamination.

The Mackenzie Delta (MD) is a permafrost-bearing region along the coasts of the Canadian Arctic which exhibits high sub-permafrost gas hydrate (GH) reserves. The GH occurring at the Mallik site in the MD is dominated by thermogenic methane (CH4), which migrated from deep conventional hydrocarbon reservoirs, very likely through the present fault systems. Therefore, it is assumed that fluid flow transports dissolved CH4 upward and out of the deeper overpressurized reservoirs via the existing polygonal fault system and then forms the GH accumulations in the Kugmallit–Mackenzie Bay Sequences. We investigate the feasibility of this mechanism with a thermo–hydraulic–chemical numerical model, representing a cross section of the Mallik site. We present the first simulations that consider permafrost formation and thawing, as well as the formation of GH accumulations sourced from the upward migrating CH4-rich formation fluid. The simulation results show that temperature distribution, as well as the thickness and base of the ice-bearing permafrost are consistent with corresponding field observations. The primary driver for the spatial GH distribution is the permeability of the host sediments. Thus, the hypothesis on GH formation by dissolved CH4 originating from deeper geological reservoirs is successfully validated. Furthermore, our results demonstrate that the permafrost has been substantially heated to 0.8–1.3 °C, triggered by the global temperature increase of about 0.44 °C and further enhanced by the Arctic Amplification effect at the Mallik site from the early 1970s to the mid-2000s.

As recent developments regarding the increasing demand of renewable energy sources in the European energy sector demonstrate, the need for large-scale energy storage technologies intensifies. Since the availability of wind and photovoltaic energy are undergoing high fluctuations, excess energy has to be stored to be available at times of high energy demand. Implementation of pumped hydro power storage (PHS) plants in abandoned underground reservoirs are intensively studied as potential storage solution (e.g. Pickard, 2012), whereby open-pit lignite mines are also expected to contribute to this issue (Thema and Thema, 2019), but are hardly investigated, yet. PHS follows the concept of pumping and releasing water between two reservoirs located at different elevations.The success of energy storage by PHS in abandoned mines highly depends on the geo- and hydrochemical processes in the reservoirs and the surrounding porous media (Pujades et al., 2018). Oxidation of sulphur bearing minerals, especially of pyrite, might trigger the generation of Acid Mine Drainage (AMD; Akcil and Koldas, 2006), which can impact groundwater chemistry as well as slope stability, and further induce corrosion at critical technical infrastructure (Pujades et al., 2018).In the scope of the present study, we have investigated the major chemical reaction paths by numerical modelling to conceptualise comprehensive reactive transport simulations for environmental risk assessments. For that purpose, we considered available research findings from studies on the Lusatian and Rhenish lignite mining areas, and applied these to other European mining sites. Calcite buffering, mineral dissolution-precipitation balances, heavy metal contamination as well as mixing processes between the potential reservoirs and groundwater have been taken into account. In summary, geochemical impacts potentially occurring with PHS operation under hydrochemical boundary conditions representative for European open-pit lignite mines were investigated and quantified. ReferencesAkcil, A., and Koldas, S: Acid Mine Drainage (AMD): causes, treatment and case studies, Improving Environmental, Economic and Ethical Performance in the Mining Industry - Part 2, Life cycle and process analysis and technical issues, J. Clean. Prod., 14, 1139-1145, 2006.Pickard, W. F.: The History, Present State, and Future Prospects of Underground Pumped Hydro for Massive Energy Storage, Proc. IEEE, 100, 473–483, 2012.Pujades, E., Jurado, A., Orban, P., and Dassargues, A.: Hydrochemical changes induced by underground pumped storage hydropower: Influence of aquifer parameters in coal mine environments, Advances in Geosciences, 45, 45-49, 2018.Thema, J., and Thema, M.: Pumpspeicherkraftwerke in stillgelegten Tagebauen am Beispiel Hambach-Garzweiler-Inden, Wuppertal Paper, 2nd ed., 194, 2019.

The geothermal hot water reservoir underlying the coastal township of Waiwera, northern Auckland Region, New Zealand, has been commercially utilized since 1863. The reservoir is complex in nature, as it is controlled by several coupled processes, namely flow, heat transfer and species transport. At the base of the aquifer, geothermal water of around 50°C enters. Meanwhile, freshwater percolates from the west and saltwater penetrates from the sea in the east. Understanding of the system’s dynamics is vital, as decades of unregulated, excessive abstraction resulted in the loss of previously artesian conditions. To protect the reservoir and secure the livelihoods of businesses, a Water Management Plan by The Auckland Regional Council was declared in the 1980s [1]. In attempts to describe the complex dynamics of the reservoir system with the goal of supplementing sustainable decision-making, studies in the past decades have brought forth several predictive models [2]. These models ranged from being purely data driven statistical [3] to fully coupled process simulations [1].Our objective was to improve upon previous numerical models by introducing an updated geological model, in which the findings of a recently undertaken field campaign were integrated [4]. A static 2D Model was firstly reconstructed and verified to earlier multivariate regression model results. Furthermore, the model was expanded spatially into the third dimension. In difference to previous models, the influence of basic geologic structures and the sea water level onto the geothermal system are accounted for. Notably, the orientation of dipped horizontal layers as well as major regional faults are implemented from updated field data [4]. Additionally, the model now includes the regional topography extracted from a digital elevation model and further combined with the coastal bathymetry. Parameters relating to the hydrogeological properties of the strata along with the thermophysical properties of water with respect to depth were applied. Lastly, the catchment area and water balance of the study region are considered.The simulation results provide new insights on the geothermal reservoir’s natural state. Numerical simulations considering coupled fluid flow as well as heat and species transport have been carried out using the in-house TRANSport Simulation Environment [5], which has been previously verified against different density-driven flow benchmarks [1]. The revised geological model improves the agreement between observations and simulations in view of the timely and spatial development of water level, temperature and species concentrations, and thus enables more reliable predictions required for water management planning.[1] Kühn M., Stöfen H. (2005): Hydrogeology Journal, 13, 606–626, https://doi.org/10.1007/s10040-004-0377-6[2] Kühn M., Altmannsberger C. (2016): Energy Procedia, 97, 403-410, https://doi.org/10.1016/j.egypro.2016.10.034[3] Kühn M., Schöne T. (2017): Energy Procedia, 125, 571-579, https://doi.org/10.1016/j.egypro.2017.08.196[4] Präg M., Becker I., Hilgers C., Walter T.R., Kühn M. (2020): Advances in Geosciences, 54, 165-171, https://doi.org/10.5194/adgeo-54-165-2020[5] Kempka T. (2020): Adv. Geosci., 54, 67–77, https://doi.org/10.5194/adgeo-54-67-2020

Understanding the geochemistry of water resources is a prerequisite in the development of sustainable water resource management strategies. The Pra Basin is one of the few basins in Ghana with economic importance. The Basin is constituted by three river systems (Birim, Offin and Pra) and covers a total land size of approximately 2,300 km2. It traverses several towns and serves as the main water supply for communities and industry. Currently, the quality of water resources in the Pra Basin especially surfacewaters have been affected negatively as a result of activities such as illegal mining (e.g., the use of mercury for the extraction of gold), indiscriminate waste disposal, and poor farm management practices (e.g., inappropriate application of fertilizers and pesticides). Specific contaminants include mercury (Hg), arsenic (As), lead (Pb), iron (Fe), manganese (Mn), cadmium (Cd), selenium (Se), and nitrate (NO3). The Pra Basin is underlain by three rock formations, the Birimian Supergroup, the Tarkwain Formation and the granitoids. The mineral composition of the Birimian Supergroup comprises argillitic/pellitic sediment (plus or minus kerogen), sericite schist, and quartz-sericite schist. The granitoids comprise biotite (hornblende, muscovite), biotite gneiss, biotite schist, amphibolite partly of contact metamorphism, K-feldspar rich granitoid, two-mica or muscovite granite and monzonite, serecite schist, quartz-serecite, and garnet. The Tarkwaian rocks mineralogy also includes basaltic flow/subvolcanic rock and minor interbedded volcaniclastics, detrital sediment mainly sandstone and conglomerate ultramafic and minor mafic igneous rock. Samples of groundwater were collected from shallow (mainly hand-dug wells of depths < 10 m) and deep (mainly boreholes of depths >30 m) aquifers across the Pra Basin. Surfacewaters were collected from rivers and stream networks. The samples were analysed for major ions, trace metals and stable isotopes (oxygen-18 and deuterium) using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), Ion Chromatography (IC), and Picarro L-2140i Ringdown Spectrometer at the GFZ laboratories. Multivariate statistical analysis and inverse geochemical modelling have been applied to around 100 water samples sourced from boreholes, hand-dug wells, and rivers of the Pra Basin to determine the chemical state of the waters. Specifically, the study seeks to (1) determine the origin and evolution of the geochemistry of both surfacewater and groundwater, (2) identify recharge and discharge areas, and (3) study sources and sinks of minerals including sulphates, carbonates, and silicates. The abundance of cations and anions are in the order of Na>Ca>K>Mg and HCO3>Cl>SO4>NO3 (surfacewater), Na>Ca>Mg>K and HCO3>Cl>NO3>SO4 (hand-dug well), and Na>Ca>Mg>K, and HCO3>Cl>NO3>SO4 (boreholes). Our research findings demonstrate that geochemistry of water resources in the Pra Basin is mainly controlled by rock-water interaction. With the application of hydrogeochemical modelling, including silicate mineral weathering and ion exchange, significant processes controlling the basin’s hydrochemistry variations are quantified. The presented results will support the development of sustainable water resources management strategies and contribute to mitigating future contamination.

Salt deposits host an important industrial raw material and provide storage capacities for energy and nuclear waste. However, leaching zones can seriously endanger the development and utilisation of salt deposits for these purposes, especially if these occur in potash seams. Their increased solubility enables even NaCl-saturated solutions, if present, to deeply penetrate these seams. The resulting salt dissolution processes generate fluid flow paths and affect the mechanical rock integrity. To model the timely evolution of leaching zones and to assess their hazard potential, a reactive transport model has been developed, taking into account not only the complex dissolution and precipitation behaviour of potash salts, but also the resulting porosity and permeability changes as well as density-driven chemical species transport. Additionally, the model makes use of an approach to describe transport and chemical reactions at the interface between impermeable (dry) salt rocks and permeated leaching zones (Steding et al., 2021). In the present study, we focus on the effect of heterogeneity of the mineral distribution within potash seams and on the influence of mineral- and saturation-dependent dissolution rates.The applied reactive transport model is based on a coupling of the geochemical module PHREEQC (Parkhurst & Appelo, 2013) with the TRANSport Simulation Environment (Kempka, 2020) as well as the newly developed extension of an interchange approach (Steding et al., 2021). A numerical model has been developed and applied to simulate the leaching process of a carnallite-bearing potash seam due to natural density-driven convection. The results show that both, the mineral composition and dissolution rate of the original salt rock, strongly influence the shape and evolution of the leaching zone (Steding et al., 2021).In nature, strong variations of the mineralogy occur within potash seams with random or stratified distributions. Furthermore, dissolution rates depend on the mineral itself as well as on its saturation state. Both may considerably influence the growth rate of a leaching zone. Therefore, the reactive transport model has been extended by mineral- and saturation-dependent dissolution rates. A scenario analysis has been undertaken to compare the impact of homogeneous and heterogeneous rock compositions. For that purpose, the carnallite content in the potash seam was varied from 5 to 25 wt. % including different stratifications and random distributions. The simulations were classified by means of the Péclet and Damköhler numbers, and the long-term behaviour as well as hazard potential are discussed. References:Parkhurst, D.L.; Appelo, C.A.J. (2013). Description of Input and Examples for PHREEQC Version 3 - a Computer Program for Speciation, Batch-reaction, One-dimensional Transport, and Inverse Geochemical Calculations. In Techniques and Methods; Publisher: U.S. Geological Survey; Book 6, 497 ppKempka, T. (2020). Verification of a Python-based TRANsport Simulation Environment for density-driven fluid flow and coupled transport of heat and chemical species. Adv. Geosci. 54, 67–77. Steding, S.; Kempka, T.; Zirkler, A.; Kühn, M. (2021). Spatial and temporal evolution of leaching zones within potash seams reproduced by reactive transport simulations. Water 13, 168.

The underlying study addresses the ambitious German Federal Government’s objectives for the transition to a new energy era by proposing the implementation of a low-carbon energy system, improving the electricity grid as well as solar and wind power initiatives. Hereby, the “Power-to-Gas-to-Power” (PGP) approach combines the storage of excess energy from renewable power sources in the form of synthetic hydrocarbons, and their subsequent utilisation in a closed cycle to produce low-carbon electricity [1]. Based on the availability of two adjacent subsurface storage formations for CO2 and CH4 [2], hydrogen gained from excess solar and/or wind power is transformed into methane by means of CO2 captured on-site. When required, electricity is regained in a combined cycle plant by burning the CH4, with CO2 cycled in a closed loop.In a show case study for the two German cities of Potsdam and Brandenburg, the PGP process chain was quantified to a total process efficiency of about 26%, exhibiting costs of 20 eurocent/kWh [2]. These previous assessments referring to energy production and storage technologies economics of the year 2012, have shown that PGP is generally economically competitive compared to conventional storage technologies [2]. Further results show that PGP can compete with global cost bandwidths of hydropower and compressed air storage as well as with upper limit COEs for solar thermal power and photovoltaic. However, PGP is not competitive compared to fossil fuel-based as well as onshore/offshore wind-based energy production [3]. However, cost trends related to energy production and storage technologies significantly correlate with fuel and commodity prices, CO2 emission charges as well as technology improvements that have been rapidly changing in the past few years. Thus, the purpose of the present study is to update the previously published PGP costs and elaborate a general overview on the current status of PGP on the global energy market.[1] Kühn, M. (2013): System and method for ecologically generating and storing electricity. - Patent WO 2013156611 A1: [2] Streibel, M., Nakaten, N. C., Kempka, T., Kühn, M. (2013): Analysis of an Integrated Carbon Cycle for Storage of renewables. - Energy Procedia, 40, pp. 202-211. DOI: [3] Kühn, M., Nakaten, N., Kempka, T. (2020): Geological storage capacity for green excess energy readily available in Germany. - Advances in Geosciences, 54, 173-178. DOI

Natural gas hydrates are considered as one of the most promising alternatives to conventional fossil energy sources, and are thus subject to world-wide research activities for decades. Hydrate formation from methane dissolved in brine is a geogenic process, resulting in the accumulation of gas hydrates in sedimentary formations below the seabed or overlain by permafrost. The LArge scale Reservoir Simulator (LARS) has been developed (Schicks et al., 2011, 2013; Spangenberg et al., 2015) to investigate the formation and dissociation of gas hydrates under simulated in-situ conditions of hydrate deposits. Experimental measurements of the temperatures and bulk saturation of methane hydrates by electrical resistivity tomography have been used to determine the key parameters, describing and characterising methane hydrate formation dynamics in LARS. In the present study, a framework of equations of state to simulate equilibrium methane hydrate formation in LARS has been developed and coupled with the TRANsport Simulation Environment (Kempka, 2020) to study the dynamics of methane hydrate formation and quantify changes in the porous medium properties in LARS. We present our model implementation, its validation against TOUGH-HYDRATE (Gamwo & Liu, 2010) and the findings of the model comparison against the hydrate formation experiments undertaken by Priegnitz et al. (2015). The latter demonstrates that our numerical model implementation is capable of reproducing the main processes of hydrate formation in LARS, and thus may be applied for experiment design as well as to investigate the process of hydrate formation at specific geological settings.Key words: dissolved methane; hydrate formation; hydration; python; permeability.ReferencesSchicks, J. M., Spangenberg, E., Giese, R., Steinhauer, B., Klump, J., & Luzi, M. (2011). New approaches for the production of hydrocarbons from hydrate bearing sediments. Energies, 4(1), 151-172, https://doi.org/10.3390/en4010151Schicks, J. M., Spangenberg, E., Giese, R., Luzi-Helbing, M., Priegnitz, M., & Beeskow-Strauch, B. (2013). A counter-current heat-exchange reactor for the thermal stimulation of hydrate-bearing sediments. Energies, 6(6), 3002-3016, https://doi.org/10.3390/en6063002Spangenberg, E., Priegnitz, M., Heeschen, K., & Schicks, J. M. (2015). Are laboratory-formed hydrate-bearing systems analogous to those in nature?. Journal of Chemical & Engineering Data, 60(2), 258-268, https://doi.org/10.1021/je5005609Kempka, T. (2020) Verification of a Python-based TRANsport Simulation Environment for density-driven fluid flow and coupled transport of heat and chemical species. Adv. Geosci., 54, 67–77, https://doi.org/10.5194/adgeo-54-67-2020Gamwo, I. K., & Liu, Y. (2010). Mathematical modeling and numerical simulation of methane production in a hydrate reservoir. Industrial & Engineering Chemistry Research, 49(11), 5231-5245, https://doi.org/10.1021/ie901452vPriegnitz, M., Thaler, J., Spangenberg, E., Schicks, J. M., Schrötter, J., & Abendroth, S. (2015). Characterizing electrical properties and permeability changes of hydrate bearing sediments using ERT data. Geophysical Journal International, 202(3), 1599-1612, https://doi.org/10.1093/gji/ggv245

In the present study, we apply our validated stoichiometric equilibrium model [1], based on direct minimisation of Gibbs free energy, to predict the synthesis gas compositions produced by in-situ coal conversion at three European coal deposits. The applied modelling approach is computationally efficient and allows to predict synthesis gas compositions and calorific values under various operating and geological boundary conditions, including varying oxidant and coal compositions. Three European coal deposits are assessed, comprising the South Wales Coalfield (United Kingdom), the Upper Silesian Coal Basin (Poland) and the Ruhr District (Germany). The stoichiometric equilibrium models were first validated on the basis of laboratory experiments undertaken at two different operating pressures by [2] and available literature data [3]. Then, the models were adapted to site-specific hydrostatic pressure conditions to enable an extrapolation of the synthesis gas composition to in-situ pressure conditions. Our simulation results demonstrate that changes in the synthesis gas composition follow the expected trends for preferential production of specific gas components at increased pressures, known from the literature, emphasising that a reliable methodology for estimations of synthesis gas compositions for different in-situ conditions has been established. The presented predictive approach can be integrated with techno-economic models [4] to assess the technical and economic feasibility of in-situ coal conversion at selected study areas as well as of biomass and waste to synthesis gas conversion projects.[1] Otto, C.; Kempka, T. Synthesis Gas Composition Prediction for Underground Coal Gasification Using a Thermochemical Equilibrium Modeling Approach. Energies 2020, 13, 1171.[2] Kapusta et al., 2020[3] Kempka et al., 2011[4] Nakaten and Kempka, 2019

Quantifying interactions and dependencies among geometric, hydraulic and mechanical properties of reservoir sandstones is of particular importance for the exploration and utilisation of the geological subsurface and can be assessed by synthetic sandstones comprising the microstructural complexity of natural rocks. In the present study, three highly resolved samples of the Fontainebleau, Berea and Bentheim sandstones are generated by means of a process-based approach, which combines the gravity-driven deposition of irregularly shaped grains and their diagenetic cementation by three different schemes. The resulting evolution in porosity, permeability and rock stiffness is examined and compared to the respective micro-computer tomographic (micro-CT) scans. The grain contact-preferential scheme implies a progressive clogging of small throats and consequently produces considerably less connected and stiffer samples than the two other schemes. By contrast, uniform quartz overgrowth continuously alters the pore space and leads to the lowest elastic properties. The proposed stress-dependent cementation scheme combines both approaches of contact-cement and quartz overgrowth, resulting in granulometric, hydraulic and elastic properties equivalent to those of the respective micro-CT scans, where bulk moduli slightly deviate by 0.8%, 4.9% and 2.5% for the Fontainebleau, Berea and Bentheim sandstone, respectively. The synthetic samples can be further altered to examine the impact of mineral dissolution or precipitation as well as fracturing on various petrophysical correlations, which is of particular relevance for numerous aspects of a sustainable subsurface utilisation.

Quantifying trends in hydraulic and mechanical properties of reservoir sandstones has a wide practical importance for many applications related to geological subsurface utilization. In that regard, predicting macroscopic rock properties requires detailed information on their microstructure [1]. In order to fundamentally understand the pore-scale processes governing the rock behaviour, digital rock physics represents a powerful and flexible approach to investigate essential rock property relations [2]. This was shown, e.g., for hydraulic effects of anhydrite cement in the Bentheim sandstone in relation to an unsuccessful drilling campaign at the geothermal well Allermöhe, Germany [3]. Rock weakening due to decreasing calcite mineral content was also demonstrated by application of numerical simulations [4]. In the present study, a process-based method is used for reconstructing the full 3D microstructure of three typical reservoir reference rocks: the Fontainebleau, Berea and Bentheim sandstones. For that purpose, grains are initially deposited under the influence of gravity and afterwards diagenetically consolidated. The resulting evolution in porosity, permeability and rock stiffness is examined and compared to the respective micro-CT scans of the sandstones. The presented approach enables to efficiently generate synthetic sandstone samples over a broad range of porosities, comprising the microstructural complexity of natural rocks and considering any desired size, sorting and shape of grains. In view of a virtual laboratory, these synthetic samples can be further altered to examine the impact of mineral dissolution and/or precipitation as well as fracturing on various petrophysical correlations, what is of particular relevance for a sustainable exploration and utilisation of the geological subsurface.[1] Wetzel M., Kempka T., Kühn M. (2017): Predicting macroscopic elastic rock properties requires detailed information on microstructure. Energy Procedia, 125, 561-570. DOI: 10.1016/j.egypro.2017.08.195 [2] Wetzel M., Kempka T., Kühn M. (2020): Hydraulic and mechanical impacts of pore space alterations within a sandstone quantified by a flow velocity-dependent precipitation approach. Materials, 13, 4, 3100. DOI: 10.3390/ma13143100[3] Wetzel M., Kempka T., Kühn M. (2020): Digital rock physics approach to simulate hydraulic effects of anhydrite cement in Bentheim sandstone. Advances in Geosciences, 54, 33-39. DOI: 10.5194/adgeo-54-33-2020 [4] Wetzel M., Kempka T., Kühn M. (2018): Quantifying rock weakening due to decreasing calcite mineral content by numerical simulations. Materials, 11, 542. DOI: 10.3390/ma11040542

Water has been used as a working fluid injected into the hot reservoirs during the exploitation of deep geothermal energy, therefore, understanding the thermomechanical properties of reservoir rocks after water cooling is essential. For that reason, we have conducted a series of laboratory tests on air and water cooled granites from normal temperature to 600 °C, to reveal the changes in their thermomechanical properties. At 600 °C, the average values of uniaxial compressive strength, elastic modulus and P-wave velocity of water cooled granite decrease by 84.9%, 73.1% and 66.2%, which are 11.0%, 17.0% and 17.7% larger than those of air cooled granite. Through optical microscopic analysis, the microcrack density and average width of water cooled granite increase with thermal temperature and are 4.18 mm/mm2 and 54.62 μm at 600 °C, while the values of air cooled granite are only 1.97 mm/mm2 and 25.16 μm. We thus combined the deterioration of the macroscopic mechanical characteristics of air and water cooled granites with the propagation and development of microcracks. Supported by data from international literature, the changes in the thermomechanical characteristics of granite has been systematically compared to international literature, which is hoped to provide technical support for the geothermal energy exploitation.

Leaching zones within potash seams generally represent a significant risk to subsurface mining operations and the construction of technical caverns in salt rocks, but their temporal and spatial formation has been investigated only rudimentarily to date. To the knowledge of the authors, current reactive transport simulation implementations are not capable to address hydraulic-chemical interactions within potash salt. For this reason, a reactive transport model has been developed and complemented by an innovative approach to calculate the interchange of minerals and solution at the water-rock interface. Using this model, a scenario analysis was carried out based on a carnallite-bearing potash seam. The results show that the evolution of leaching zones depends on the mineral composition and dissolution rate of the original salt rock, and that the formation can be classified by the dimensionless parameters of P&eacute, clet (Pe) and Damk&ouml, hler (Da). For Pe &gt, 2 and Da &gt, 1, a funnel-shaped leaching zone is formed, otherwise the dissolution front is planar. Additionally, Da &gt, 1 results in the formation of a sylvinitic zone and a flow barrier. Most scenarios represent hybrid forms of these cases. The simulated shapes and mineralogies are confirmed by literature data and can be used to assess the hazard potential.

Potash salts belong to the most soluble minerals and their unintended dissolution can result in a safety risk for the construction and utilisation of salt caverns and mines [1]. One main challenge in modelling the formation of leaching zones within potash seams is the representation of fluid-rock interactions within regions exhibiting highly varying porosities. Chemical reactions cannot take place if small porosities inhibit the inflow of solution, although present solutions may be undersaturated with respect to certain minerals. These porosity variations only occur at the dissolution front in binary systems, such as NaCl solution and solid halite. Its progress can be described by a mass transfer rate, depending on the concentration of present solutions or by assuming a saturated interface between dry rock and solution, subtracting out the diffusive mass transport. In contrast, the dissolution of potash salt results in the formation of a porous rock matrix, consisting of undissolved and precipitated minerals that can further react with the surrounding solution. Accordingly, fluid-rock interactions and largely varying porosities also occur remote from the dissolution front. The interchange approach [2] was developed to describe these interactions. Coupled with a reactive transport model including PHREEQC [3] and TRANSE [4] this approach is capable to quantify, e.g., the leaching process of carnallite-bearing potash seams due to natural density-driven convection. The dissolution rate is essential for both, the timely progress and geometric shape of evolving leaching zones in the potash seam. Therefore, the interchange approach has been adapted in the scope of the present study to consider saturation-dependent dissolution rates for each mineral. In this contribution, we discuss the feasibility and limitations of our approach to represent fluid-rock interactions between brine and different types of potash salts at the metre scale.

Many different scientific open-source and commercial black-box software packages are available for the simulation of fluid flow and transport processes in the geological subsurface. Unfortunately, most of these simulators are limited by tightly integrated chemical modules with insufficient capabilities or the general lack of flexible interfaces applicable for an efficient coupling of third-party chemical libraries. Furthermore, most available open-source numerical frameworks are too complex to be used for educating geosciences students in numerical modelling techniques beyond the general application of ready-for-use simulators to specific modelling challenges. Taking into consideration that the development of a critical perspective of an emerging modeller requires fundamental analysis and understanding of common numerical modelling approaches and pitfalls, scientific source codes written in lower-level programming languages (e.g., FORTRAN, C++ or C) are per se less comprehensible compared to higher-level language implementations (e.g., Python). Hereby, the general lack of proper source code documentation, observed in many scientific open-source numerical codes additionally reduces code readability, and thus hinders code further development by third parties.To overcome many of these limitations, the TRANsport Simulation Environment (TRANSE) has been developed based on the finite difference method. It allows for a highly flexible integration and coupling of arbitrary processes with thermodynamic and chemical libraries to consider chemical reactions and fluid equations of state. To date, TRANSE solves the pressure-based and density-driven formulation of the Darcy flow equation, coupled with the equations for transport of heat and chemical species on structured grids by simple explicit, weighted semi-implicit or fully-implicit numerical schemes, and is composed of less than 1,000 lines of Python code. A flux-corrected advection scheme can be employed in addition to pure upwinding to minimise numerical dispersion in transport problems dominated by high Péclet numbers.Just-in-time compilation by means of the Python Numba library results in computational times in the order of equivalent lower-level language implementations (e.g., FORTRAN, C or C++), while CPU-based parallelisation allows for the realisation of high spatial model discretisations. Chemical libraries coupled to TRANSE can be easily parallelised to increase the overall computational efficiency, whereby the latter is especially relevant as chemistry usually represents the main computational bottleneck in reactive transport simulations. Python’s numpy library is used to enable fast and efficient model parametrisation as well as simulation runtime control, whereby the Matplotlib library is employed for automated visualisation. More sophisticated visualisation and post-processing are achieved by using the EVTK library for exporting VTK-compatible data to the interactive software packages VisIt, Mayavi or Paraview. The present contribution demonstrates the basic validity of the code implementation by comparison against standard numerical model benchmarks for heat (1D heat diffusion) and fluid flow (Theis problem), advective transport (rotating cone test), density-driven fluid flow (Henry’s and Elder’s problems) as well as available density- and viscosity-driven hydrothermal convection in porous media. A fully coupled application example considering reactive transport of gaseous chemical species at high temperatures is presented.

In the present study, a pre-existing stoichiometric equilibrium model based on direct minimization of Gibbs free energy has been further developed and applied to estimate the equilibrium composition of synthesis gases produced by the gasification of carbon-rich feedstock (e.g., coal, municipal waste or biomass) in a fixed-bed reactor [1]. Our modeling approach is validated against thermodynamic models, laboratory gasification and demonstration-scale experiments reported in the literature. The simulated synthesis gas compositions have been found to be in good agreement under a wide range of different operating conditions. Consequently, the presented modeling approach enables an efficient quantification of synthesis gas compositions derived from feedstock gasification, considering varying feedstock and oxidizer compositions as well as pressures and temperatures. Furthermore, the developed model can be easily integrated with numerical flow and transport simulators to simulate reactive transport of a multi-componentgas phase.[1] Otto and Kempka, Synthesis gas composition prediction for underground coal gasification using a thermochemical equilibrium modeling approach, Energies (in review)

Mineral dissolution is a micro-scale process, which may significantly alter the microstructure of rocks, and consequently affect their effective mechanical behavior at the macro scale. Predicting changes in rock stiffness is of paramount importance within the context of risk assessment for most applications related to geological subsurface utilization, where reduction of mechanical parameters is of particular relevance to assess reservoir, caprock and fault integrity [1].In the present study, the effective elastic properties of typical reservoir rocks are determined based on micro-computer tomography (micro-CT) scans. The resulting three-dimensional rock geometry comprises a more realistic microstructure regarding the shapes of grains, cements and the overall porous network compared to available empirical approaches. Effective rock stiffness is calculated by a static finite element method, which imposes an uniform strain on the digital rock sample and calculates the resulting stresses. The effect of spatial cement distribution within the pore network is taken into account, considering passive pore filling as well as framework supporting cements. Rock stiffness increases due to the precipitation of pore-filling minerals. This quantitative approach substantially improves the accuracy in predicting elastic rock properties compared to general analytical methods, and further enables quantification of uncertainties related to spatial variations in mineral distribution.[1] Wetzel M., Kempka T., Kühn M. (2018): Quantifying Rock Weakening Due to Decreasing Calcite Mineral Content by Numerical Simulations. Materials, 11, 4, 542. DOI: http://doi.org/10.3390/ma11040542

Excess electricity produced from renewables can be converted into CH4 by consuming CO2 and H2 by means of the Power-to-Gas (PtG) technology [1]. Previous work indicates that subsurface storage of CO2 and CH4 can meet the projected energy storage requirements [1] [2]. However, gas mixing occurs if both gases are stored in the same reservoir [3], and energy is lost if CH4 is used as cushion gas when both gases are separately stored in different reservoirs [2]. Therefore, an innovative approach to overcome the limitation of aforementioned storage schemes is introduced in this study. For that purpose, the focus is on a double reservoir setting in one anticline system as it is commonly found in, e.g., the Northern German Basin. Here, the confining layer and preexisting or artificial hydraulic connections between the two reservoirs enable the operator to reduce energy losses and avoid gas mixing. We have elaborated a numerical multiphase flow model including the wellbore systems and reservoirs to study the fluid flow and beneficiary effects of pressure interaction between both reservoirs. Based on the geological and operational data of our regional showcase in Germany [4] [5], the energy storage efficiency is quantified, and the potential benefits of the proposed storage scheme are evaluated. It shows that the production of CH4 increases by 68% over twenty years of injection and production. Furthermore, the factors that affect storage efficiency are analyzed to provide information for the optimization of PtG-based subsurface energy storage systems. The simulation can be applied to different geological systems and for parameter sensitivity studies to reduce energy losses and improve storage efficiency. Keywords: Power-to-Gas; Subsurface gas storage; Carbon dioxide; Methane [1] Kühn M, Nakaten N, Streibel M, Kempka T. CO2 Geological storage and utilization for a carbon neutral “Power-to-gas-to-power” cycle to even out fluctuations of renewable energy provision. Energy Procedia. 2014; 63:8044-9.[2] Ma J, Li Q, Kühn M, Nakaten N. Power-to-gas based subsurface energy storage: A review. Renewable and Sustainable Energy Reviews. 2018; 97:478-96.[3] Ma J, Li Q, Kempka T, Kühn M. Hydromechanical response and impact of gas mixing behavior in subsurface CH4 storage with CO2-based cushion gas. Energy & Fuels, 2019; 33 (7), 6527-6541[4] Streibel M, Nakaten N, Kempka T, Kühn M. Analysis of an integrated carbon cycle for storage of renewables. Energy Procedia 40 (2013): 202-211.[5] Kühn M, Streibel M, Nakaten N, Kempka T. Integrated underground gas storage of CO2 and CH4 to decarbonise the “power-to-gas-to-gas-to-power” technology. Energy Procedia 59 (2014): 9-15.