Single-well hot push-pull testing for efficient and reliable high temperature ATES operation

1 Single-well hot push-pull testing for efficient and reliable high temperature ATES operation Lioba Virchow1, Stefan Kranz1, Simona Regenspurg1, Guido Blöcher1, Ben Norden1, Liang Pei1, Cornelius O. Schwarze1, Elena Petrova1, Christian Cunow1, Lukas Knüttel1, Katrin Kieling1 1 GFZ Helmholtz Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany [email protected] Keywords: high temperature ATES, push-pull single well test, thermo-hydraulic, fluid rock interaction ABSTRACT Aquifer thermal energy storages (ATES) utilising low storage temperatures (30°C) have become a prevalent technology in recent years, demonstrating their benefits as an efficient and greenhouse gas-reducing technology. In urban areas with a high degree of heterogeneity in building structures, the requirements for heat supply vary considerably, particularly with regard to the temperature demand. Here, relatively high temperatures (up to 110°C) are still required. In existing district and local heating grids with multiple energy sources, large, high temperature heat storage systems enable balancing of the temporal variability between heat supply and demand. This allows for the additional potential of tapping into further efficient and renewable sources. In this context, high temperature ATES represents a promising heat storage technology, offering a compact urban footprint and substantial storage capacity. As with ATES in general, the availability of suitable local geological conditions is a requirement. In case of high temperature ATES, it is particularly important that they are suitable for temperatures above 70°C, as this results in a significant change in temperature compared to the natural aquifer temperature. The induced temperature change can have a variety of effects on the thermohydraulic storage behavior, the aquifer chemistry, microbiology, and the fluid-rock interactions. In order to investigate and understand these effects and to identify potential interactions, we have developed single-borehole tests that can be carried out on ATES research or pilot wells. Similar to the tracer push-pull tests known from hydrogeology, the fluid injection is performed at high temperatures resulting in hot push-pull test (HPPT). The test is carried out in several push and pull stages with gradually increasing temperatures until the target temperature is reached. For each push and pull phase, different conservative and reactive tracers are selected for thermohydraulic characterisation and reactive transport studies. Tracer concentrations are measured continuously to enable precise interpretation. A comprehensive fluid sampling and monitoring program is carried out during all test phases to detect changes in fluid composition and thus to draw conclusions on possible geochemical processes. Flow rates and downhole temperatures and pressures are recorded for thermal-hydraulic monitoring. The entire borehole is equipped with fibre optic cables that enable continuous temperature monitoring of the borehole over time and depth. These contributions present the entire monitoring and well testing concept and the results of an extensive well test on HT-ATES research well in Berlin-Adlershof, Germany, targeting a sandy aquifer at a depth range of 360 – 390 m below ground surface. The performed tests and results presented serve as basis for the construction of a HT-ATES that is to be integrated into an existing district heating network. 1. INTRODUCTION Low temperature ATES (up to 30 °C storage temperature) has already proven to be an efficient and climate-friendly technology (Fleuchaus et al., 2018). However, existing district heating networks in urban areas often require temperatures up to 110 °C. This study examines the storage of hot water and the consequences of the subsurface: High Temperature ATES (HT-ATES). Higher temperatures usually also increase the reactivity of the aquifer and can lead to increased changes in thermohydraulic storage behaviour, geochemistry and microbiology (Bonte et al., 2013; Lüders et al., 2020). HT-ATES implementation depends on suitable geological conditions and the system’s ability to handle injection temperatures above 70 °C. To assess these effects, we have developed single-well hydraulic test methods applicable to HT-ATES research at pilot sites (Blöcher et al., 2024). Our field characterization program combines conventional hydraulic tests including step rate and production tests for key hydraulic properties such as transmissivity, hydraulic permeability, productivity index and skin effect and further push-pull tests for dispersivity and ambient groundwater flow with thermal methods to determine thermal parameters such as bulk volumetric heat capacity of the target formation, porosity dependent heat retardation and expected heat recovery efficiency. Virchow et al. 2 At the same time, geochemical parameters provide a baseline for the groundwater composition and any changes that occur upon heating, including mineral reactions such as calcite precipitation or dissolution and mobilisation of potential contaminants. In addition, changes in microbial community composition favouring thermophilic taxa, their metabolic activities and the potential for biofilm formation or biomineralisation can be detected during HPPT. 2. STUDY SITE Within the GeoFern research project (Geothermal District Heat Supply in Berlin), a HT-ATES exploration well Gt BTrKoe 1/2021 was drilled in Berlin-Adlershof, Germany in 2021. As Berlin's drinking water supply is largely provided by freshwater aquifers located up to 140 m below ground surface (m bgs), the target horizons for a HT-ATES lie below the freshwater-saltwater barrier. This Tertiary Rupelian clay is located between 99 and 203 m bgs in the study area (Norden et al., 2023). Due to problems with the cementation, a sidetrack was drilled in 2024 (Gt BTrKoe 1a/2024) as part of the EU research project PUSH-IT (Piloting Underground Storage of Heat In geoThermal reservoirs) to a final depth of 410 m bgs, with the filter screen set between 371 and 389 m bgs. The filtered aquifer is a weakly consolidated Jurassic Hettangian sandstone (Saadat et al., 2022). During filter development, saline water containing 39 g/L of total dissolved solids (NaCl dominated) was extracted. Initial hydraulic analysis indicates a productivity index up to 1.2 l/s/bar and hydraulic permeability of 1.5*10-12 m2 (Schwarze et al., in prep). 3. METHOD DEVELOPMENT A comprehensive testing and monitoring program has been developed to characterize the aquifer for the use of a HT-ATES: Single Well Hot Push Pull Tests (HPPT). The tests will comprise up to six push and pull cycles with alternating tracers including subsurface and surface monitoring. Testing is scheduled for May to July 2025. 3.1 Setup The test setup consists of various monitoring components as well as components for fluid handling and fluid conditioning. The borehole and the wellhead are designed in such a way that both production and injection are possible. To pump the groundwater, a submersible pump is installed at a depth of approx. 90 m bgs and equipped with a pressure and temperature sensor. In addition, the borehole is equipped with fibre optical cables to measure temperature and acoustics along the borehole. The measurements in the surface fluid system include temperatures, pressures, flow rates, thermophysical and chemical fluid parameters as well as tracer concentrations. During injection, the fluid is heated by means of a heat exchanger connected to a nearby district heating system. For fluid storage, four 70 m³ fluid containers will be arranged at the site which are flushed and pressurized with nitrogen to minimize fluid-air contact during test operation (Figure 1). Figure 1: HPPT setup Berlin Virchow et al. 3 3.2 Test design The push-pull tests consist of up to six cycles. First, 240 m3 of formation water is extracted in a five-stage production step rate test and stored in the four nitrogen flushed tanks at the drilling site. After the shut-in and resting period of 24 hours, the water is heated to 90°C, mixed with a tracer via a dosing pump and then injected into the aquifer. After another 24-hour resting phase, further 240 m3 of water is extracted and stored. This cycle is repeated several more times, each time using different tracers. With each cycle, the temperature of the aquifer continues to rise. After the last injection, the aquifer is left to rest for 35 days to allow microbial community to adapt to the temperature. Finally, 480 m3 formation water is extracted and disposed. 3.3 Thermal modeling The thermal impact of the HPPT on the target aquifer and on the surroundings of the borehole was simulated. Parameterization of the model was based on the geological and hydraulic data obtained during the drilling and filter development. According to the model, the aquifer within a 5 m radius of the well is thermally affected after the sixth injection of 90°C hot formation water (Figure 2). Figure 2: Results of thermal modelling of hot push-pull tests. Thermal radial extension within the target aquifer is up to 5 m according to the model. 3.4 Hydraulic and geochemical monitoring The water pressure level in the well is measured by a pressure sensor. During the push-phases, tracers are added to the injected hot water to detect and quantify aquifer properties. When extracting the water during the pull-phase, the gained tracer concentration breakthrough curves give information about transport parameters like flow velocity, dispersion/dispersities and matrix porosities as well as reaction parameters. The shape of the curves and the mass recoveries of the tracers can be used to characterize multiple reaction within the aquifer like mixing, sorption, ion-exchange and degradation. Virchow et al. 4 The tracers are added as an impulse to the water flow via a dosing pump and then mixed within a mixing tube. After injecting the tracer, a formation water chaser (once the borehole volume) is injected without added tracer to reduce wellbore storage effects. During injection and extraction, the tracer concentrations is measured continuously (every 20 seconds) in real time by a flow-through field fluorometer. To monitor further geochemical parameters, a fluid monitoring system is installed in a bypass of the flow. It measures physico-chemical on-site parameters in real time and in flow during extraction and injection of groundwater. The measuring unit consists of pH, oxidation-reduction potential, specific electrical conductivity, dissolved oxygen, temperature, turbidity, density and flow rate, as well as a visual monitoring tube and multiple sampling ports. For the initial characterization of the formation water, water samples are taken and analyzed for major ions, trace elements and isotopes. Samples are also be analyzed for organic matter and microorganisms. Initial sampling of the water provides a baseline of the formation water with natural variations. Further sampling focuses on analyzing potential mobilization candidates like trace metals, as well as scaling and corrosion parameters, and potential changes in microorganisms with temperature variation. Real-time gas monitoring records the differences in gas composition throughout the HPPT. 4. CONCLUSIONS As part of a HT-ATES study, a test program has been developed to assess the suitability of aquifers for HTATES operation. In several push-pull cycles, water is extracted, heated, mixed with tracer, injected and extracted again. Multiple physico-chemical-microbial parameters are measured during injection and extraction. The results are used to analyse transport characteristics and reactive (chemical, microbiological and thermal) behaviour of the aquifer to assess its HTATES potential, and to provide a baseline to detect possible changes during the HT-ATES operation, which will be constructed in 2025 and connected to the district heating network by the end of 2027 (Reallabor GeoSpeicher Berlin - Integrating a high-temperature aquifer thermal energy storage into a district heating network). REFERENCES Blöcher, G., Regenspurg, S., Kranz, S., Lipus, M., Pei, L., Norden, B., Reinsch, T., Henninges, J., Siemon, R., Orenczuk, D., Zeilfelder, S., Scheytt, T., & Saadat, A. (2024). Best practices for characterization of High Temperature-Aquifer Thermal Energy Storage (HT-ATES) potential using well tests in Berlin (Germany) as an example. Geothermics, 116. https://doi.org/10.1016/j.geothermics.2023.102830 Bonte, M., Roling, W. F., Zaura, E., van der Wielen, P. W., Stuyfzand, P. J., & van Breukelen, B. M. 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The explorational drilling Gt BTrKoe 1/2021 in Berlin Adlershof, Germany - Field Data and Measurements (https://doi.org/10.5880/GFZ.4.8.2023.006 Saadat, A., Kranz, S., Norden, B., Blöcher, G., Virchow, L., Piekarski, J.-K., & Regenspurg, S. (2022). Geothermal district heating in Berlin: An ATES field study in Berlin-Adlershof. European Geothermal Congress 2022, Berlin. Schwarze, C. O. P., Liang, Virchow, L., Petrova, E., Norden, B., Regenspurg, S., Kieling, K., Blöcher, G., & Kranz, S. (in prep). Characterization of the hydraulic properties of a planned High Temperature Aquifer Thermal Energy Storage (HT-ATES) system during well development. Advances in Geosciences. Acknowledgements The PUSH-IT project is funded by the European Commission, Research and Innovation programme Horizon Europe, Project-ID 101096566.