In the pursuit of sustainable development and the mitigation of climate change, shallow geothermal energy has been widely recognized as a type of clean energy with great potential. Accurate estimation of thermal ground properties is needed to optimally apply shallow geothermal energy technologies, which are of growing importance for the heating and cooling sector. A special challenge is posed by the often significant heterogeneity and variability of the geological media at a site.

As an innovative investigation method, we focus on the actively heated fiber optics-based thermal response test (ATRT) and its application in a borehole in Changzhou, China. A copper mesh heated optical cable (CMHC), which both serves as a heating source and a temperature sensing cable, was applied in the borehole. By inducing the electric current to the cable at a relatively low power of 26 W/m, the in-situ heating process was recorded at high depth resolution. This information serves to infer the thermal conductivity distribution along the borehole. The presented field experience reveals that the temperature rise in the early phase of the test should not be used due to initial heat accumulation caused by the outer jacket of the CMHC. The comparison of these results with those of a conventional thermal response test (TRT) and a distributed thermal response test (DTRT) in the same borehole confirmed that the ATRT result is reliable (with a difference less than 5% and 1%, respectively). Most importantly, this novel method affords much less energy and testing time.

Additionally, to estimate the uncertainty and limits associated with the method, a 2D axisymmetric numerical model based on COMSOL Multiphysics® has been developed. The results indicate that an accurate calculated thermal conductivity requires heating duration to be in the range of 90~400 min considering test efficiency and cost. Our study promotes ATRT as an advanced geothermal field investigation method and it also extends the applicability of the thermal response test as a downhole tool for measurement of soil hydraulic properties.

How to cite: Gu, K., Zhang, B., Shi, B., Liu, C., Bayer, P., and Wei, Z.: Actively heated fiber optics based thermal response test, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4104, https://doi.org/10.5194/egusphere-egu21-4104, 2021.

            Our ability to characterize aquifers, predict contaminant transport and understand biogeochemical reactions occurring in the subsurface directly depends on our ability of characterizing the distribution of groundwater flow. In this context, recently-developed active-Distributed Temperature Sensing (DTS) experiments are particularly promising, offering the possibility to characterize groundwater flows resulting from heterogeneous flow fields. Here, based on theoretical developments and numerical simulations, we propose a general framework for estimating active-DTS measurements, which can be easily applied and takes into account the spatial distribution of the thermal conductivities of sediments.

            Two independent methods for interpreting active-DTS experiments are proposed to estimate both the porous media thermal conductivities and the groundwater fluxes in sediments. These methods rely on the interpretation of the temperature increase measured along a single heated fiber optic (FO) cable and consider heat transfer processes occurring both through the FO cable itself and through the porous media. In order to validate these interpretation methods with independent experimental data, active-DTS measurements were collected under different flow-conditions during laboratory tests in a sandbox. First, the combination of a numerical model with laboratory experiments allowed improving the understanding of the thermal processes controlling the temperature increase. Then, the two complementary and independent interpretation methods providing an estimate of both the thermal conductivity and the groundwater flux were fully validated and the excellent accuracy of groundwater flux estimates (< 5%) was demonstrated.

            Our results suggest that active-DTS experiments allow investigating groundwater fluxes over a large range spanning 1x10^-6 to 5x10^-2 m/s, depending on the duration of the experiment. The active-DTS method could thus be potentially applied to a very wide range of flow systems since groundwater fluxes can be investigated over more than three orders of magnitude. In the field, the reliable and direct estimation of the distribution of fluxes could replace the measurement of hydraulic conductivity, whose distribution and variability still remains difficult and time consuming to evaluate.

How to cite: Simon, N., Bour, O., Lavenant, N., Porel, G., Nauleau, B., Pouladi, B., Longuevergne, L., and Crave, A.: An experimentally-validated framework for interpreting active-DTS measurements conducted in fully saturated porous media, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15683, https://doi.org/10.5194/egusphere-egu21-15683, 2021.

Borehole thermal energy storage is a well-established technology for seasonal geological heat storage, where arrays of borehole heat exchangers (BHE) are installed in low permeability geological media dominated by conductive heat transfer. Increasing storage temperatures would increase storage capacities and rates and would thus allow for a better inclusion of BTES in the energy system. When using storage temperatures of up 90°C, however, highly permeable zones or intermediate layers may allow for thermally induced fluid migration and convective heat transport in the storage medium, which may increase heat losses from the storage and thus limit the thermal performance of the BTES system. Therefore, we present results from experimental work and subsequent numerical modelling aimed at quantifying thermally induced convection for a lab-scale BHE in a water saturated porous medium for a temperature range of 20°C to 70°C.

The experimental heat storage unit consists of a fully water saturated coarse sand within a cylindrical polypropylene barrel of 1.23 m height and 0.6 m radius and a vertical coaxial BHE, which is grouted by a thermally enhanced cement. The barrel is cooled from the outside using ventilators and laboratory air. A grid of 68 thermocouples is emplaced in the storage medium for monitoring the temperature distribution. For the stationary experiment, heat is transferred to the storage unit using a supply temperature of 70°C for 6 days until a steady state temperature distribution is achieved, followed by 3 days of heat recovery. The dynamic experiment begins with 3 days of heating with 70°C followed by 6 cycles of alternating heating at 70°C and cooling at approximately 18°C for 12 hours each.

The stationary experiment reveals a vertical temperature stratification, with temperatures increasing up to 48°C towards the top of the porous medium, as well as a horizontal temperature gradient along the top of the sand, while the lower part of the barrel and the outer wall remain at the laboratory temperature of approximately 18°C. This temperature distribution has stabilized after about 90 hours and represents a clear tilted thermal front, suggesting a significant contribution of induced thermal convection to the overall heat transport. The cyclic experiment shows a decrease of storage temperatures relative to the stationary experiment, with temperatures near to the BHE at the top of the porous lower by 2.5°C and 4.75°C, respectively, because the heating phase is not long enough to reach the stationary temperature distribution. This lower horizontal temperature gradient indicates a weakened thermal convection, however the thermal stratification is conserved. This shows that even under the cyclic loading conditions thermal convection may impair high temperature BTES operation and efficiency.

Numerical process simulation of coupled flow and heat transport accounting for variable density and the experimental boundary conditions reproduces the spatial and temporal temperature distribution of both experiments with good accuracy. This shows that induced thermal is causing the observed temperature distributions.

How to cite: Djotsa Nguimeya Ngninjio, V., Bo, W., Beyer, C., and Bauer, S.: Experimental and numerical investigation of thermal convection in a water saturated porous medium induced by heat exchangers in high temperature borehole thermal energy storage, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8071, https://doi.org/10.5194/egusphere-egu21-8071, 2021.

Heating and cooling is responsible for about 50% of the European total energy use. Therefore, renewable sources of heat are needed to reduce GHG emissions (e.g. solar, geothermal, waste-heat). Due to a temporal and spatial mismatch between availability and demand of heat, large scale heat storage facilities are needed. High Temperature Aquifer Thermal Energy Storage (HT-ATES) systems are one of the cheapest and most adequate ways to store large amounts of sensible heat. Regular/Low-T ATES systems are considered a proven technology with currently more than 3 000 systems operable world-wide. However, at higher storage temperatures (e.g. 40-100 °C) temperature dependent water properties (density, viscosity) more strongly affect physical processes, resulting in higher and unpredictable heat losses. While first applications and research on this subject started more than 50 years ago, many uncertainties still remain. In this research we study the (hydrogeological) storage conditions that affect the heat losses of HT-ATES systems. Numerical simulations of a wide range of storage conditions, are done to obtain generic insights in the performance of HT-ATES systems. These insights allow to identify which heat transport processes dominate in contribution to heat losses. Results show that conduction always contributes to heat losses for HT-ATES systems and relate to geometric storage conditions. While buoyancy flow (free convection) may also contribute considerable to heat losses under specific conditions.

How to cite: Beernink, S., Bloemendal, M., and Hartog, N.: The impact of storage conditions on heat losses of HT-ATES systems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5093, https://doi.org/10.5194/egusphere-egu21-5093, 2021.

High-temperature aquifer thermal energy storage (HT-ATES) in the geological subsurface will affect the temperature distribution in and close to the storage site, with potential impacts on groundwater flow and biogeochemistry. Quantification of the subsurface space affected by a HT-ATES operation is thus required as one basis for urban subsurface space planning, which would allow to address potential competitive and conflicting uses of the urban subsurface. Therefore, this study shows a quantitative evaluation of induced thermal impacts and subsurface space required for a synthetic ATES operated at varying temperature levels.

A hypothetic seasonal HT-ATES operation is simulated using the coupled groundwater flow and heat transport code OpenGeoSys. A well doublet system consisting of fully screened “warm” and “cold” wells 500 m apart is used for the storage operation. A sandy aquifer typical for the North German Basin at a depth of 110 m and with a thickness of 20 m in between two confining impermeable layers is used as storage formation. Seasonal cyclic storage is simulated for 20 years, assuming charging and discharging for six months each. During charging, water with the aquifer background temperature of 13°C is extracted at the "cold" well, heated to 70°C and reinjected at the “warm” well using a pumping rate of 30 m³/h. During discharging, the stored hot water is retrieved at the "warm" well using the same pumping rate and reinjected at the “cold” well after heat extraction at aquifer background temperature.

The simulation results show that during a single storage cycle using a storage temperature of 70°C 7.51 GWh of thermal energy is injected, of which 4.79 GWh can be retrieved. This corresponds to a thermal recovery factor of 63.8% and thus an effective storage capacity of 0.43 kWh/m³/K can be deduced in relation to the heat capacity of the storage medium. For storage temperatures of 18°C, 30°C and 50°C, the effective storage capacity is 0.56 kWh/m³/K, 0.55 kWh/m³/K and 0.49 kWh/m³/K, respectively. By delineating the subsurface volume with a temperature increase larger than 1°C, the subsurface space used for and affected by the storage operation at the storage temperature of 70 °C is determined to be 10.56 million m³. In relation to the retrieved thermal energy, a subsurface volume of 2.2 m³ is thus required to retrieve one kWh of heat energy at 70 °C injection temperature. At lower temperatures of 18°C, 30°C and 50°C, the subsurface space required is 1.77 m³/kWh, 1.54 m³/kWh and 1.76 m³/kWh, respectively. The lower effective storage capacity and the relatively larger required space, which correspond to a lower thermal recovery factor, are caused by induced thermal convection and higher heat losses by conduction at higher temperatures.

How to cite: Wang, B., Delfs, J.-O., Beyer, C., and Bauer, S.: Numerical investigation of induced thermal impacts from high-temperature thermal energy storage in porous aquifers, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8302, https://doi.org/10.5194/egusphere-egu21-8302, 2021.

With the transition of the heating sector towards renewable energy sources technologies are needed to compensate for the seasonal mismatch between heat supply and demand. Aquifer thermal energy storage (ATES) is considered a promising candidate for that purpose. Especially high temperature ATES (HT-ATES) with temperatures up to 90 °C has the advantage of higher storage capacities and allows for the direct use of the stored heat without intermediate heat pumps. In order to improve the understanding of processes induced by HT-ATES and to validate numerical tools for the prediction of storage capacities, storage rates as well as thermal impacts, a heat injection field test with an injection temperature of 75 °C was conducted, densely monitored and numerically simulated. This work presents a sensitivity analysis of the governing processes and parameters, from which the parameters on which the simulation results are most dependent are derived and thus identified for future site characterization and monitoring studies.

The heat injection test took place at a shallow aquifer with a low natural groundwater flow velocity of 0.07 m/d. Hot water was injected at a borehole using flow rates of 14 l/min for 4.5 days and the resulting thermal plume was monitored by a dense arrangement of thermocouples. Previous to the experiment, the field site was thoroughly investigated for the thermal and hydraulic parameters by standard hydrogeological methods, such as pumping tests, hydraulic head measurements, Hydraulic Profiling Tool (HTP) employment, liner sampling and laboratory measurements.  A coupled heat transport and fluid flow model was set up and the heat injection test was simulated using high resolution numerical modelling of the coupled thermo-hydraulic processes using the OpenGeoSys (OGS) simulation code.

The comparison of measured and simulated temperature breakthrough curves showed a good correspondence, indicating the capability of the model to predict the general thermal behaviour of the heat injection test. The accuracy was higher for larger distances to the injection well and at the longer time scale, while the largest deviations occurred close to the injection well and shortly after the injection. The model was then used to estimate the sensitivity of the simulated temperature distribution on thermal and hydraulic aquifer parameters, which were varied according to the span of measurements. The thermal plume development is most sensitive on the hydraulic conductivity, since this parameter influences the intensity of buoyancy driven flow and was measured in the large range 3.00E-05 to 7.15 E-04 m/s. The dispersivity and the anisotropy in hydraulic conductivity effect the same process and show a significant impact on the result as well, together with the thermal conductivity. The sensitivity of the simulated temperature distribution on the groundwater flow velocity and the specific heat capacity is a little lower compared to the previously mentioned parameters, while the result is insensitive to the specific storage. It is shown, that a heat injection test in combination with numerical simulations is suitable for identifying parameter sensitivities also on small scales, thus showing the investigation needs for HT-ATES projects.

How to cite: Heldt, S., Wang, B., and Bauer, S.: A High Temperature Heat Injection Test – Numerical Modelling and Sensitivity Analysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9823, https://doi.org/10.5194/egusphere-egu21-9823, 2021.

Aquifer Thermal Energy Storage (ATES) systems combined with a heat pump stores and thereby reduces energy use for space heating and cooling of buildings. In most countries, the temperature of the stored heat is limited to maximum 25-30°C for such systems. However, when heat is available at higher temperatures (e.g. waste heat, solar heat), it is more efficient to store higher temperatures because that improves heat pump performance or may even makes it abundant. As a result there is a large potential for additional energy savings by transforming ‘regular’ low-temperature ATES systems to a HT-ATES. Such a transformation is tested for a greenhouse in the Netherlands. This greenhouse has a LT-ATES system operational since 2012. From 2015 onwards the storage temperature increased and currently heat is stored in the warm well at temperatures up to 40°C. In this HT-ATES transformation pilot, water quality parameters are closely monitored as well as temperature distribution in the subsurface (using DTS). Together with the operators, the results from the ATES monitoring are used to continuously improve system performance. Numerical groundwater and heat flow simulations of actual and expected well pumping data are used to evaluate how well operation can be optimized. Results show that due to the extra heat harvested and higher warm well temperature CO₂ emissions are reduced by 70%, due to larger contribution of heat delivery by the ATES and a more efficient heat pump due to the higher warm well temperature. Groundwater infiltration temperature peaks are up to 40°C during the hottest summer days, while the average warm well temperatures increased mildly by 6°C to about 21°C. Groundwater monitoring results therefore only showed limited water quality changes. The changes that were identified are predominantly contributed to mixing processes, as the ATES system is installed in 2 different aquifers. 

How to cite: Bloemendal, M., Beernink, S., Hockin, A., van Bel, N., and Hartog, N.: Optimizing ATES performance by increasing warm well temperature and harvesting waste/solar heat, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3481, https://doi.org/10.5194/egusphere-egu21-3481, 2021.

In the attempt to reduce the CO2 emissions and dependence on fossil fuels geothermal energy started to receive increased scientific interest. With the development of the Enhanced Geothermal System (EGS) technology, extensive geothermal energy applications have become feasible. However, the geothermal reservoirs are usually situated several kilometers below the ground whichmeans the experiments within the geothermal reservoir are difficult to be implemented. Therefore, the models capable of simulating thermohydraulic (TH) effects were the common approaches to analyzing geothermal reservoir efficiency. To simulate fluid migration and heat propagation within the fractured geothermal reservoir in EGS, discrete fracture models (DFMs) of the TH processes  were widely used. However, the heterogeneity of the fracture apertures is most of the times ignored in these models. In this work, considering the aperture heterogeneity, a DFM of the TH processes was established. It is assumed the apertures follow a normal distribution. The outlet temperature and energy production rate are employed to evaluate the efficiency of the geothermal reservoir. The results of the simulation show that the heterogeneity of the aperture strongly affects the performance of the geothermal reservoir. At the end of simulation, the variation in outlet temperature decreased by approximately 20% and the average produced energy had a reduction of over 26%. Furthermore, the average produced energy has an inversely proportional relationship with the aperture heterogeneity. Finally, several statistical realizations of the fracture network were generated to test and verify if the influence from aperture heterogeneity are generally valid. 

How to cite: Zhou, D., Tatomir, A., and Sauter, M.: Numerical investigation of fracture aperture heterogeneity on performance of geothermal reservoir in EGS, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6601, https://doi.org/10.5194/egusphere-egu21-6601, 2021.

Under geologically beneficial conditions, geothermal facilities are often rated as efficient, as well as clean and climate-neutral energy technologies. In fact, for supporting a good environmental performance of a technology, the total environmental impact caused by all associated material and energy consumption needs to be examined. Life cycle assessment (LCA) according to ISO standards 14040 and 14044 considers not only operation, but also the construction and decommissioning phases while addressing different environmental impact categories. Therefore, LCA-based environmental evaluation has been proposed in several previous studies. A review of the state-of the art in this field shows that some critical system parameters are often disregarded. Furthermore, many existing studies are solely based on theoretical datasets without validation to specific application cases.

Our work addresses these two shortcomings by performing a comprehensive LCA using operational data of the binary, two-stage ORC, Kirchstockach power plant in the Southern German Molasse Basin. Given its technical specifications, a representative base case scenario provides an excellent reference for benchmarking against other power plants. Environmental impacts of different technical modifications are assessed in terms of global warming potential, non-renewable energy consumption, aquatic acidification and eutrophication. Using scenario analyses, we consider the influence of emerging key factors, such as refrigerant leakage, focusing on various system components. Firstly, we identify reinforcing effects due to interrelationships between these system parameters, e.g. when using environmentally friendly ORC refrigerants. Secondly, uncertainty analyses provide insights into potential measures for ecological system improvements by using different materials and methods in the construction and operation phases. For comparison and benchmarking purposes, conventional power generation resources and comparable studies in the field of binary geothermal systems, enhanced geothermal systems, and flash systems are included. Besides the general positive ranking of the Kirchstockach power plant environmental performance, our multi-objective study ultimately reveals not only key performance factors, but it also underlines the overall relevance of case studies to validate generic and global assumptions.

How to cite: Bott, C., Menberg, K., Heberle, F., Brüggemann, D., and Bayer, P.: Life cycle assessment of geothermal power generation in the Southern German Molasse Basin – The binary plant Kirchstockach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9842, https://doi.org/10.5194/egusphere-egu21-9842, 2021.

Efficient construction and operation of borehole heat exchangers (BHEs) are essential in its contribution to the energy transition. In practice, implementation of BHE at larger scale requires low construction costs and high production rates. This requires small diameter drillings to reduce drilling and backfilling material costs, in which achieving a proper backfilling is a challenge. At present, there is an urgent need to improve the available techniques with more effectively and efficiently backfill methods for BHEs. In current Dutch practice, sealing (to prevent short-circuit flow between penetrated aquifers) is achieved by using either clay or grouts as backfilling materials, both have their pro’s and con’s. In optimisation of applying backfilling materials and methods, the filter cake, formed during the drilling procedure, also has a sealing capacity and is overlooked in addressing the sealing of the borehole.

In this study the effect of filter cake formation on sealing capacity in unconsolidated sediments is quantified. Filter cake formation in unconsolidated porous formations (aquifers) is a complex process, which is affected by pressure differences between the borehole and the aquifer, aquifer characteristics (e.g. grain size distribution, porosity and permeability) and drilling mud/fluid properties.

A laboratory configuration is designed to stimulate different scenarios during the construction of a BHE. Consequently, the effectiveness, in terms of hydraulic conductivity, of the formed filter cake is determined by falling head tests.

Uniform aquifers with the smallest grain size tested (D50 = 0.22 mm) show a two order of magnitude reduction in hydraulic conductivity, as a direct result of filter cake formation. In contrast, filter cake formation is absent in uniform more coarse sands (D50 ≥ 0.65 mm). This demonstrates that filter cake deposition is highly variable with the grain size of the aquifer penetrated. Moreover, the experiments performed indicate that the deposition of a filter cake is not limited by additive concentrations in the drilling fluid or the duration of drilling fluid exposure to the formation.

This preliminary study creates the foundation for further research, since the experiments demonstrate the potential of filter cakes to significantly contribute to the sealing capacity within a borehole.

How to cite: Nijhof, R., van Lopik, J., and Bloemendal, M.: The effect of filter cake deposition on the hydraulic conductivity of boreholes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5456, https://doi.org/10.5194/egusphere-egu21-5456, 2021.

Neither regional development, construction projects nor infrastructure development – structural planning does not fully consider energy supply in Austria (yet). The project “Spatial Energy Planning for Heat Transition” is part of the research initiative “Green Energy Lab”, which has a project life-time from June 2018 to May 2021. It aims to provide a sound basis for the integration of heat in private and public planning processes and for the implementation of the energy infrastructure of the future together with energy providers.

Three Austrian states (Vienna, Styria and Salzburg), their capital cities and pilot-municipalities of all scales work together to provide all information necessary for the implementation of spatial heat-planning – as role model for Austria and other European countries. The GIS-based web-tool “heat-atlas” will provide this harmonized data and serve an information platform for project developers as well as for regional planning, fostering a sustainable use of all available sustainable energy resources and infrastructures to their full extent. The system of the information platform is arbitrarily scalable and is aimed to be expanded to other interested regions of Austria on demand.

One part of this “heat-atlas” is about shallow geothermal energy and covers vertical closed loop and open loop systems. The Geological Survey of Austria developed new methods to estimate capacity and energy resources as well as to show possible limitations of shallow geothermal energy use on property level. The resource calculations combine location-specific parameters such as thermal conductivity, underground temperature and groundwater availability with system-specific parameters such as mode of operation, operational hours, geometry and threshold values demanded by official regulations.

The method provides not only information about the maximum amount of energy available on the property, but also about the cover ratio of the demand. So called level-1 maps show the resources for standardized well-doublets and borehole heat exchangers independently of the property. The calculations for level-2 maps consider site-specific properties such as heating and cooling demand, operational hours and size of the property. This enables the estimation of the overall energy resources and the cover ratio of the property.

The results are shown as maps and as location specific query, which gives a concise summary of all relevant information for one location in form of an automatically generated report. More information about the project is available at http://www.waermeplanung.at/.

How to cite: Steiner, C., Goetzl, G., Fuchsluger, M., and Rehbogen, A.: Harmonized web-based information systems for shallow geothermal energy use in Austria, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4644, https://doi.org/10.5194/egusphere-egu21-4644, 2021.

In urban areas where the shallow subsurface is used for thermal energy storage (TES), interactions between the introduced heat and groundwater pollution caused by toxic organic contaminants can be expected. Temperature elevations may affect the transfer of these volatile organic compounds (VOCs) from the groundwater to the unsaturated zone, creating a redistribution or release of the contaminants in/from the subsurface environment. Such effects are particularly important considering the intersection of the unsaturated zone with the land surface and the remediation capacity of polluted aquifers. In this work, a non-isothermal multi-component two-phase flow model was developed to investigate the thermally induced volatilization and migration of the VOCs in contaminated aquifers. The numerical model, which is implemented in the open source framework OpenGeoSys-6, is able to simulate temperature-dependent mass and heat transfer processes in partially-saturated soils while allowing for phase change. Verification of the model against various benchmark problems and experimental data showed good accuracy. Simulation results revealed that a temperature-driven migration of dissolved trichloroethylene (TCE) from the groundwater to the drier regions of the unsaturated zone can be observed in general. A temperature increase of 20 K around the borehole led to a maximum decline of the total TCE concentration by 63% assuming zero TCE concentration at the soil surface. In addition, the TCE concentration distribution varied considerably with the depth-dependent water saturation. Further investigations were carried out to study the effects of different parameters, e.g. groundwater velocity, contaminant type and boundary conditions. Based on our analysis, the planning of subsurface TES systems can be optimized to account for the possible interactions with pre-existing groundwater contamination.

References:

Kolditz, O., Bauer, S., Bilke, L., Böttcher, N., Delfs, J. O., Fischer, T., ... & Zehner, B. (2012). OpenGeoSys: an open-source initiative for numerical simulation of thermo-hydro-mechanical/chemical (THM/C) processes in porous media. Environmental Earth Sciences, 67(2), 589-599.

How to cite: Meng, B., Beyer, C., Kolditz, O., and Shao, H.: Enhanced volatilization and redistribution of volatile organic compounds (VOCs) in contaminated aquifers subject to borehole thermal energy storage:  Model development and applications, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7689, https://doi.org/10.5194/egusphere-egu21-7689, 2021.

The increased use of the urban subsurface for multiple purposes, such as anthropogenic infrastructures and geothermal energy applications, leads to an urgent need for large-scale sophisticated modelling approaches for coupled mass and heat transfer. However, such models are subject to large uncertainties in model parameters, the physical model itself and in available measured data, which is often rare. Thus, the robustness and reliability of the computer model and its outcomes largely depend on successful parameter estimation and model calibration, which are often hampered by the computational burden of large-scale coupled models.

To tackle this problem, we present a novel Bayesian approach for parameter estimation, which allows to account for different sources of uncertainty, is capable of dealing with sparse field data and makes optimal use of the output data from computationally expensive numerical model runs. This is achieved by combining output data from different models that represent the same physical problem, but at different levels of fidelity, e.g. reflected by different spatial resolution, i.e. different model discretization. Our framework combines information from a few parametric model outputs from a physically accurate, but expensive, high-fidelity computer model, with a larger number of evaluations from a less expensive and less accurate low-fidelity model. This enables us to include accurate information about the model output at sparse points in the parameter space, as well as dense samples across the entire parameter space, albeit with a lower physical accuracy.

We first apply the multi-fidelity approach to a simple 1D analytical heat transfer model, and secondly on a semi-3D coupled mass and heat transport numerical model, and estimate the unknown model parameters. By using synthetic data generated with known parameter values, we are able to test the reliability of the new method, as well as the improved performance over a single-fidelity approach, under different framework settings. Overall, the results from the analytical and numerical model show that combining 50 runs of the low resolution model with data from only 10 runs of a higher resolution model significantly improves the posterior distribution results, both in terms of agreement with the true parameter values and the confidence interval around this value. The next steps for further testing of the method are employing real data from field measurements and adding statistical formulations for model calibration and prediction based on the inferred posterior distributions of the estimated parameters.

How to cite: Menberg, K., Bidarmaghz, A., Gregory, A., Choudhary, R., and Girolami, M.: Multi-fidelity approach to Bayesian parameter estimation in subsurface heat and fluid transport models , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10001, https://doi.org/10.5194/egusphere-egu21-10001, 2021.

Heat flow in the fore-arc, Northeast Japan shows characteristic highs and lows in the seaward and landward regions of the trench axis, respectively, compared to 50 mW/m² that is constrained from the corresponding half-space cooling model (135 Ma). For example, the high average of 70 mW/m² at the 150-km seaward region from the trench was observed while the low average of 30 mW/m² at the 50-km landward region was. To explain the differences between the constraints and observations of the heat flow, previous studies suggested that the high heat flow in the seaward region results from the reactivated hydrothermal circulations in the oceanic crust of the Pacific plate along the developed fractures by the flexural bending prior to subduction. The low heat flow is thought to result from thermal blanket effect of the accretionary prism that overlies the cooled subducting slab by the hydrothermal circulations. To understand heat transfer in the landward region of the trench, a series of two-dimensional numerical models are constructed by considering hydrothermal circulations in the kinematically thickening accretionary prism that overlies the converging oceanic crust of the Pacific plate where hydrothermal circulations developed prior to subduction. The model calculations demonstrate no meaningful hydrothermal circulations when the reasonable bulk permeability of the accretionary prism(<10^-14m²) is used; the thermal blanket effect significantly hinders the heat transfer, yielding only the heat flow of 10 mW/m² in the landward region, much lower than the average of 30 mW/m². This indicates that other mechanisms such as the expelled pore fluid by compaction of the accretionary prism play important roles in the heat transfer across the accretionary prism.

How to cite: Han, D. and Lee, C.: Roles of Hydrothermal Circulations on Heat Flow in the Fore-arc, Northeast Japan Subduction Zone, A Numerical Modeling Study., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1958, https://doi.org/10.5194/egusphere-egu21-1958, 2021.