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 19^th 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.

Literature

Kühn, 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.2016.10.034

Kühn, 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-56-107-2021

Kühn, 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-45-235-2018

Kühn, 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.2017.08.196

Präg, 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-54-165-2020

Somogyvári, 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

How to cite: Kühn, M. and Kempka, T.: Geographic Information System (GIS) as a basis for the next generation of hydrogeological models to manage the geothermal area Waiwera (New Zealand), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5583, https://doi.org/10.5194/egusphere-egu22-5583, 2022.

For geothermal reservoirs operated by production/re-injection wells, thermal lifetime is usually defined in terms of a temperature drop threshold, and estimated as a function of fluid turnover time and heat exchange surface-area-per-volume,

T_heat  =  R · T_fluid  +  D · σ² · T_fluid²  ,

with T_fluid supposed to be measurable by means of a tracer test; 
         σ is rather difficult to infer from tracer signals alone.

For ‘aquifer’-like reservoirs, the linear term prevails:

       R > (>>) 1 ,    D · σ² · T_fluid << 1

For fracture-dominated (‘petrothermal’) reservoirs, the quadratic term prevails:

       R ≈ 1 ,           D · σ² · T_fluid >> 1

Deriving T_fluid from artificial-tracer signals looks 'model-independent', but is subject to large-time extrapolation uncertainty (which 'restores' model-dependence).

Unlike thermal forecasting, tracer-based prognosis of solute co-production (more precisely, of its lower-bound level, assuming conservative transport by fluid turnover only, non-'replenished' from adjacent rocks) isn't impeded by large-time extrapolation uncertainty, nor by reservoir model/parameter ambiguity, since mass output prediction as a function of time,

M_out (t)   =   (C_ini – C_resid)  [ VOL_out(t) – ∫_o^t ∫_o^t' Q(t’) Q(t’’) g(t’’) dt’’ dt’  ]

requires just knowledge of conservative-tracer fluxes within the forecasting time horizon.

Once a tracer test was conducted in accordance with the rules of the art [usually including observance of flux-type B.C. for tracer input and fluid sampling, cf. Kreft_and_Zuber_1978], the reservoir can be treated like a 'black box' with 'response function' (Green’s kernel surrogate) g.

This approach is adequate for (conservative) solute co-production, but not for heat transport.

Tracer test results from a particular Upper-Jurassic (Malm) carbonate aquifer near Munich illustrate the issue with T_heat  as a 'function' of T_fluid. Tracer signals available to date yield T_fluid in the range of months (still subject to extrapolation uncertainty), and are compatible with both fracture-dominated and ‘aquifer’-like representations of reservoir structure; ‘compatible’ σ values span four(!) magnitude orders.

By contrast, tracer signals from a fractured-porous reservoir, Eastern side of the Upper Rhine rift could be used to predict 'geothermal lithium' output (and its gradual depletion in reservoir fluid turnover), irrespective of reservoir model availability/parametrization. The non-trivial challenge, however, is to foresee and quantify overall WRI effects of ‘spent fluid’ re-injection, the latter being depleted of its particular micro-constituent (albeit at trace levels only), but likely acidized / 'unreliably' buffered at major-ion levels. WRI cannot be told from conservative-tracer signals; hydrogeochemical modeling (Kölbel_et_al._2020, Maier_et_al._2021) becomes indispensable.

We gratefully acknowledge long-term support from Germany’s Federal Ministry for Economic Affairs and Energy (BMWi) within applied research projects “LOGRO”, “TRENDS”, “UnLimiteD”, funded under grant nos.  0325111B, 0325515, 03EE4023E (www.geothermal-lithium.org, https://sites.google.com/site/goetracerhydro/researchprojects, https://sites.google.com/view/bmwi-0325515-trends).

Kölbel L, Kölbel T, Maier U, Sauter M, Schäfer T, Wiegand B (2020) Water-rock interactions in the Bruchsal geothermal system by U-Th series radionuclides. GeoThermalEnergy, 8:24

Kreft A, Zuber A (1978) On the physical meaning of the dispersion equation and its solutions for different IBC. Chem Eng Sci, 33:1471–1480

Maier U, Tatomir A, Sauter M (2021) Hydrogeochemical modeling of mineral alterations following CO₂ injection. Appl Geochem, 136:10515

How to cite: Ghergut, H. B. J., Wiegand, B., Wagner, B., and Sauter, M.: Do tracer tests enable model-independent predictions of georeservoir output? two examples from Southern Germany, involving thermal drawdown and solute co-production, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6133, https://doi.org/10.5194/egusphere-egu22-6133, 2022.

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.

References

Akcil, 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.

How to cite: Schnepper, T., Kühn, M., and Kempka, T.: Hydrogeochemical impact assessment of pumped hydro power storage in open-pit lignite mines, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-259, https://doi.org/10.5194/egusphere-egu22-259, 2022.

Geothermal energy stored in the 1~2 km depth, which has advantages in terms of its continuity and sustainability, has attracted more attention in recent decades in the building heating industry. To meet the carbon-neutral prospect in the building sector, a new-type deep borehole heat exchanger (DBHE) is proposed to extract geothermal energy and has been utilized in Europe and northern China. The DBHE is typically drilled to more than 2000 m depth, and it is usually coupled with a heat pump to supply heat to the buildings. In this work, a dual-continuum finite element method was implemented in the open-source software OpenGeoSys to mimic the heat transfer process between the DBHE and the surrounding subsurface. After validating against in-situ experimental data of the pilot DBHE heating project in Xi’an, a heat pump model was also included in the OpenGeoSys model so that the entire heating system can be simulated over the long-term operation. Results show that the circulation temperatures of the DBHE have a decreasing trend during the long-term operation. Through the energy analysis, the amount of heat extracted by the DBHE was found to be mainly supplied by the energy stored within the surrounding soil. With different drilling depths of the DBHE, long-term simulation results illustrate that the heat extraction rate increases with deeper depth. After considering the electricity consumption of the heat pump and circulation pump, the Levelized cost of heating (LCOH) of the DBHE heating system was evaluated over its life-span cycle, and the optimal drilling depth of the DBHE was found to be 2600 m based on the specific geological properties of Weihe Basin, Xi’an. The proposing evaluation method provides a reference for decision-makers when designing the DBHE heating system.

How to cite: Cai, W., Wang, F., Chen, C., Wang, Z., Jiang, J., Kolditz, O., and Shao, H.: Life-span economic and environmental analysis of deep borehole heat exchanger coupled geothermal heat pump heating system with different drilling depths, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5401, https://doi.org/10.5194/egusphere-egu22-5401, 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.

How to cite: Chabab, E., Kühn, M., and Kempka, T.: Upconing of deep saline waters via Quaternary erosion windows considering varying hydrogeological boundary conditions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2734, https://doi.org/10.5194/egusphere-egu22-2734, 2022.

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/)

How to cite: Steding, S., Kühn, M., and Kempka, T.: Verification of TRANsport Simulation Environment coupling with PHREEQC for reactive transport modelling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5367, https://doi.org/10.5194/egusphere-egu22-5367, 2022.

Many countries worldwide have committed to mitigating global climate change by switching to renewable energy generation, leading to continuously increasing shares of renewable energy sources in the energy system. However, one major drawback is the strongly fluctuating nature of those energy sources. Power to gas technology to generate hydrogen and subsurface hydrogen storage is one of the options to balance this fluctuating availability. Depending on the specific development of the energy system, different scales of storage are needed with different scales of impacts on the subsurface that may arise. This study, therefore, quantifies the induced hydraulic effects of hydrogen storage in porous formations, accounting for four possible future energy system scenarios and based on an existing geological storage structure. The aim is to identify and quantify the large-scale and long-term hydraulic effects of the storage operations and to estimate the affected subsurface spaces using numerical simulation models.

The storage structure used was identified in a storage potential study and is located in the North German Basin at a depth of about 1000 m and consists of a Rhaetian sandstone formation. Formation permeability and porosity are derived from regional depth correlations, while boundary conditions are applied considering the local geological settings. For the storage operation, energy and mass balanced load profiles are derived from the four considered scenarios, with charging rates during times of surplus power varying from 1.9 GW to 6.4 GW and discharging rates during withdrawal from 4.7 GW to 15.9 GW, depending on the respective energy system scenario.

Simulation results show that up to 21 storage wells and 2.8 billion cubic meters of storage gas in place volume are required to support the required energy output for all scenarios considered. Scenario analysis shows that significant pressure responses at the well bottom hole are thus induced, which are limited to a geomechanically allowable range of 80 bar to 130 bar. Due to the high withdrawal rates required, storage design is mainly influenced by the lower pressure limit. In the far field, pressure responses of more than 3 bars and 5 bars are found within horizontal distances of up to 7.5 km and 5 km, respectively. The vertical pressure impact is much lower at 5 m and 20 m, respectively. This can be recalculated as a total impacted volume by 3 bars and 5 bars from 1.25×10⁹ m³ - 4.63 ×10⁹ m³ to 0.57×10⁹ m³ - 1.10×10⁹ m³, depending on the scenario, respectively. This study thus shows that for grid-scale energy storage subsurface space on the order of tens of millions of m³ for the hydrogen gas phase will be required, while a much larger volume of 4.6×10⁹ m³ will be affected by pressure changes of 3 bar or more. On the other hand, at least from an energetic point of view the storage structure investigated is sufficient to accommodate the national storage demand. The study results and the approach presented can thus contribute during site selection and storage facility planning to characterize subsurface and energy system requirements.

How to cite: Gasanzade, F. and Bauer, S.: Hydraulic effects of porous media hydrogen storage for different future energy supply systems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5778, https://doi.org/10.5194/egusphere-egu22-5778, 2022.

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 <10 m) and deep aquifers (mainly boreholes 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.

How to cite: Manu, E., De Lucia, M., Schleicher, A. M., Kempka, T., and Kühn, M.: Geochemical evolution and reaction mechanisms controlling groundwater chemistry in the Pra Basin (Ghana), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-427, https://doi.org/10.5194/egusphere-egu22-427, 2022.

The performance of a geothermal plant is controlled by the permeabilities in  the geothermal reservoir. Hydrothermal systems often consist of porous sand- or limestone with high salt content. Most important minerals in this context are halite and calcite. During the operation of the geothermal plant the circulating fluid changes locally the chemical equilibrium which leads to changes of the permeability by dissolution and precipitation processes. To investigate these processes we have set up a numerical code on the basis of a general C++ Library, which is developed and maintained at LIAG for the solution of mathematical models of coupled thermal, hydraulic and chemical processes. The code concept is introduced and first numerical studies of the dissolution, transport and precipitation of halite and calcite in a geothermal doublet system are presented.

How to cite: Wuttke, M. W.: Simulation of Permeability Changes by Reactive Transport in a Geothermal Doublet, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9543, https://doi.org/10.5194/egusphere-egu22-9543, 2022.

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.

References

Hedayatzadeh, 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.html

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

Tranter, M. et al. (2022) Environmental hazard quantification toolkit based on modular numerical simulations, https://meetingorganizer.copernicus.org/EGU22/EGU22-10115.html

How to cite: Kempka, T., Steding, S., Tranter, M., Otto, C., Gorka, T., Hámor-Vidó, M., Basa, W., Kapusta, K., and Kalmár, I.: Probability of contaminant migration from abandoned in-situ coal conversion reactors, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11204, https://doi.org/10.5194/egusphere-egu22-11204, 2022.

Permeability is a key parameter for characterizing fluid flow in digital rocks. It depends on pore geometry and topology and can be computed numerically via solving the Stokes equation. However, the associated computational effort can be enormous for large-scale models, even using the efficient Lattice-Boltzmann method. In this study, an efficient method is developed for computing the equivalent permeability of digital rocks by simplifying the Stokes equation to Darcy equation. The method is based on the idea that a 3D digital core can be approximated by the combination of multiple 2D slices/layers, and the property of each layer is governed by the Stokes equation. Specifically, to mimic the 2D fine-scale velocity solved from the Stokes equation, a local permeability is assigned according to the velocity for each voxel. In addition, the nearest distance from each voxel to the solid wall is used to approximate the 2D fine-scale velocity, without the need of solving the Stokes equation in 2D for each layer. By this means, the 3D Stokes equation can be simplified to multiple 2D cases that provide the local permeability distribution for the 3D Darcy equation, and thus the computational cost can be significantly reduced. Case studies have been conducted on various samples in different scales. The results demonstrate that the computed 3D permeabilities using finite difference method (based on the Darcy equation) agree well with those using Lattice-Boltzmann method (based on the Stokes equation), and a speedup factor of about O(10) is achieved. The method can be applied to both sandstone and carbonate rocks for fast estimation of block permeability.

How to cite: Liao, Q., Li, G., Huang, Z., Sheng, M., Li, J., and Zhang, D.: Fast digital rock upscaling: from Stokes equation to Darcy equation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4925, https://doi.org/10.5194/egusphere-egu22-4925, 2022.

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

How to cite: Wetzel, M., Kempka, T., and Kühn, M.: Hysteresis in permeability evolution of a virtual sandstone simulated by mineral precipitation and dissolution, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4676, https://doi.org/10.5194/egusphere-egu22-4676, 2022.

How to cite: Kala, S., Devaraju, J., and Rasheed, M. A.: Assessment of brittleness and hydrocarbon potential of deep Permian shales in Krishna Godavari Basin, India, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-275, https://doi.org/10.5194/egusphere-egu22-275, 2022.

One unconventional coal extraction method is in-situ coal conversion. In this process, the coal is gasified in an underground reactor. A crucial issue that should be considered in this approach is the assessment of short and long-term environmental hazards and risks to human health and the environment resulting from potential surface subsidence and fault activation in the presence of complex geological conditions. This research aimed to assess potential environmental impacts associated with a commercial-scale application of in-situ coal conversion in a high coal production area in the Maza-Varalja region in Hungary. The Maza-Varalja region is an environmentally protected forested area. In this research, the numerical modeling of surface subsidence and fault activation was implemented using the finite-volume numerical modeling software FLAC3D. A predictive three-dimensional geomechanical model has been developed using site-specific geological data. The material model is Mohr-Coulomb elastic-plastic. Zero thickness interfaces were employed to address the fault behavior with friction and cohesive characteristics. The in-situ stress was determined by geostatic loading and a horizontal stress factor. The boundary conditions were zero-displacement and positioned sufficiently far from the coal seams to not artificially influence the stress field. The initialized zone stresses were considered using the density of the zones above them and gravity. The horizontal to vertical stresses ratio was determined to be one. A series of sensitivity studies were conducted to address the importance of geologic parameters that have an impact on surface subsidence, fault activation, and pollutant migration to the surface. In order to achieve this, seven variables, including the unit weight, Young’s modulus, Poisson’s ratio, friction angle, cohesion, fault friction, and excavation sequence, were considered. Sixty-six simulations were undertaken and analyzed. The simulation results demonstrate that surface subsidence is affected by the average Young’s modulus of the geological strata and the fault activation to the friction angle of the faults. Also, shallower seams are more likely to produce surface subsidence, while as excavation depth increases, surface subsidence decreases. The model's results have been used to develop an Environmental Hazard and Risk Management toolkit (Tranter et al., 2022) for planning and decision-making processes during in-situ coal conversion to ensure that environmental risks and mitigation actions are clearly quantified and outcomes forecasted. 

How to cite: Hedayatzadeh, M., Sarhosis, V., Gorka, T., Hámor-Vidó, M., and Kalmár, I.: Ground subsidence and fault reactivation during in-situ coal conversion assessed by numerical simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11736, https://doi.org/10.5194/egusphere-egu22-11736, 2022.

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.

How to cite: Tranter, M., Steding, S., Otto, C., Pyrgaki, K., Hedayatzadeh, M., Sarhosis, V., Koukouzas, N., Louloudis, G., Roumpos, C., and Kempka, T.: Environmental hazard quantification toolkit based on modular numerical simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10115, https://doi.org/10.5194/egusphere-egu22-10115, 2022.