InnoGeoPot

With the constant increase in the number of installed borehole heat exchangers (BHE) systems in Europe, the need and utilization of thermal energy contained in deeper geological layers has been recognized. In selected urban and rural pilot areas in Croatia and Slovenia, the investigation of geothermal potentials at medium-depths, between 200 & 500 m, via BHE has not yet been carried out. The proposed research intends to establish different scale geological-hydrogeological-thermal models using data collected during drilling of deep wells in selected areas (geological and hydrogeological raw data) as well as monitoring real data from shallow geothermal borehole field in operation to determine heat rejection/extraction rates. Furthermore, the exploitation of shallow and medium-deep geothermal potential can be extended to the application of borehole thermal energy system (BTES), as well as the possibility of exploiting geothermal potentials by revitalizing abandoned wells via deep closed-loop BHE. The developed geological-hydrogeological-geothermal model, along with a decision-support system (DSS), is important as it will provide a better insight into the shallow and medium-deep geothermal potential of the studied pilot areas as well as defining areas where it is possible to use BTES. Also, the evaluation of the geothermal potential through the revitalization of abandoned wells will be considered. The research area in Croatia covers the wider area of the city of Zagreb, the city o structural framework for 3D modelling of the subsurface in transboundary pilot area between Croatia and Slovenia (GeoConnect3d , DARLINGe, GeoMol, TransEnergy), Ljubljana pilot area (GeoPLASMA-CE, MUSE) and Zagreb pilot area (MUSE).

Climate change requires a shift in energy demand away from fossil fuels. In the search for solutions, more frequent use of thermal energy from underground should be given greater consideration. Whether this is done on a small scale (residential unit) or on larger scales (district heating), a characterization of the regional and local subsurface and climate is necessary for the evaluation of subsurface thermal energy potentials. Expected project results is the provision of DSS, based on results from geological, hydraulic and thermal models, as application-oriented management tools which allow an evaluation of BHE in different settings and to derive thermal energy potential for selected areas. This will enhance the knowledge for exploiting geothermal resources. The developed tools provide specific information regarding the potential for future geothermal uses, but also help to enforce the assessment of mutual influence; questions regarding water protection and permit practice. The tools also enable planners to quantitatively integrate geothermal potentials into urban energy plans. Based on the geological, hydrogeological and geothermal 3D models and the heat potentials derived from them, future uses can be assessed in terms of groundwater availability and temperature spreads (difference between inlet and return temperatures). The expected results will provide the basis for how to realize innovative and efficient systems for the sustainable use of geothermal sources. The developed methodological approach and the created tools can, with site-specific adaptations, be applied to be transferred to other locations with similar settings.

One focus of this project is to create a basis for application-oriented procedures, with which it is possible to provide an assessment tool for the enforcement as well as to develop long-term and development strategies for the sustainable use of geothermal energy. The project outcome is to minimize knowledge gap in design issues of such projects which will result in increase of new shallow and medium-deep geothermal projects since the DSS tool will decrease uncertainty (for example, not knowing exact thermal heat extraction rates, borehole thermal interference, borehole thermal resistance etc.).

C1 Geological database & DSS (GeoZS lead)

Geological database

The development of a geological database will be based on the experience of the Slovenian partner. Geological Survey is the central institution responsible for collection, processing, storage and dissemination of geological data in Slovenia and is actively participating in development of EuroGeoSurveys’ European Geological Data Infrastructure. The development of a geological database involves the (A) compilation, review and evaluation of new measurements and existing documents on geology and hydrogeology as well as the (B) data management and visualization and the development of a GIS for the project.

The data survey would be focused on wells drilled within the Zagreb geothermal aquifer, which covers the City of Zagreb and its wider area as well as the transboundary area located mostly in Međimurje County. From the well database, located at Croatian Hydrocarbon Agency, geological data, thermogeological, technical and other data of importance for research, for depths of interest will be collected for the considered research area. Other input data will be collected from existing and disseminated research results pertaining to geology, hydrogeology and BHE utilization as well as results of the BHE monitoring research well and the monitoring of existing BHE fields in operation.

In Slovenia, publicly available data on boreholes can be assessed through web viewer (https://e-vrtina.si/). The pilot areas have been investigated in more detail in scope of different research projects. The proposed data base will be built on results of previous projects which provided geological and structural framework for 3D modelling of the subsurface in transboundary pilot area between Croatia and Slovenia (GeoConnect3d , DARLINGe, GeoMol, TransEnergy), Ljubljana pilot area (GeoPLASMA-CE, MUSE) and Zagreb pilot area (MUSE).

Development DSS - Tool for managing subsurface thermal energy resources

The developed database (WP II) organizes the different geological, climate and BHE geometry properties and the resulting DSS will be a tool with public access on demand, which will help with design of appropriate heat exchanger system in selected areas of research. The resulting tool (referred to as a prototype in the technical jargon of the enforcement directive) can be used locally and to evaluate future GES. The regional-scale models (WP III) will provide the geological, hydrogeological and geothermal framework or constraints used in borehole heat transfer modelling (WP II). DSS will utilise borehole heat transfer model to simulate effects of different BHE designs on the performance of GES. Based on user defined priorities (e.g., minimization of initial investment costs, electric energy consumption) objective functions will be defined. Together with identified constrains (related to legal framework, technical limits, geological settings) they will be implemented in optimisation procedure based on evolutionary algorithm. The result of the optimisation will be site specific optimal BHE design.

C2 Field research & Borehole heat transfer model (UNIZG – RGNF lead)

Field research

To carry out the proposed research monitoring systems will be implemented on existing BHE systems in urban and transboundary pilot areas. Monitoring of existing systems with multiple drilled boreholes will provide temperature profiles of geothermal source temperature on an annual basis, during both heating and cooling season. For selected pilot sites in Slovenia and Croatia, thermal response test will allow to define thermogeological and hydrogeological input parameters for the model applications. Obtaining real operation data of GES and annual BHE temperature profiles is crucial to validate the developed models for the implementation for deeper geothermal systems. BHE monitoring involves the analysis of hydraulic parameters (e.g. pressure drop) of commonly used BHE settings (single U (1U), double U (2U), coaxial (CX)), since the effectiveness of the system is dependent on the electric energy used to circulate the working fluid within the pipes and thermal output. When, from the hydraulic viewpoint, optimal settings for the BHE are chosen, the SCOP will be determined and validated with real data obtained from the pilot sites, which provides better values ensuring a more efficient system.

Selected BHE will be equipped with monitoring systems, such as temperature loggers within the BHE manhole gathering system. This will ensure the control of temperature behavior of the BHE, which will be used for the setup of the base models and during the process of model validation. Continuous monitoring of air temperature will also be carried out to determine required heating and cooling demands for the chosen location. With TRT measurements heat extraction rates for the modelling approaches will be determined. Monitoring results will serve as input parameters for the different modelling approaches and for model confirmation. Also, the TRT data from the monitoring well will be compared to data collected from similar, already existing BHE in different settings and different operation modes, depths and BHE pipe geometry.

The Slovenian partner will perform operational monitoring on selected BHE system in the transboundary pilot area and measurements of thermal properties of the main lithological units in all pilot areas. Rock samples will be collected in the field (outcrops) and from available borehole cores. The measurements of thermal conductivity and thermal diffusivity on rock samples will be performed in the laboratory using a Thermal Conductivity Scanner. The measurements of the thermal parameters of the sediments will be performed in the field and in the laboratory using the KD2 Pro thermal properties analyzer. Altogether 300 measurements of rocks and sediments thermal properties are planned. Incorporating site specific measurements will assure more reliable modelling of geothermal condition in the pilot areas (WP III).

Borehole heat transfer model

The efficiency of the GSHP systems is dependent on the chosen geometry settings of the BHE. When designing a BHE it is important to ensure that the heat transfer is as high as possible, i.e. the thermal borehole resistance is as low as possible. This can be realized by ensuring turbulent fluid flow in the BHE pipes, i.e. ensuring high enough Reynolds number. Also, the pressure drop in the BHE can be of a significant influence when choosing between BHE systems. With larger depth, the pressure drop is higher, therefore the electricity consumption of the circulating pump is higher, which results in lower SCOP of the GSHP systems. Thus, the evaluation of the system BHE performance will depend on heat extraction rates and electricity consumption of the circulating pump. The model will be based on sensitivity analysis of different parameters

The model will include parametrization of heating and cooling loads, based on typical space of housing and commercial build-ings in the research area. Different heating/cooling demands will be used to evaluate the number of required BHE for each iteration of the BHE type, depth and pipe geometry settings in accordance to energy demand. Furthermore, working fluid properties will be considered by means of changing the ratio of glycol-water mixtures. Working fluid flow rate will be set in such a manner that the flow is preferably in turbulent regime. Once the fluid flow is determined for the specific case, pressure drop will be calculated. With the pressure drop values the consumption of circulating pump is determined. Heat extraction rates will be determined for each case and sensitivity analysis carried out to evaluate shallow and medium-deep geothermal potential within the area of interest.

C3 Regional-scale modelling (UNIBAS lead)

In some cases, especially in alluvial planes during drilling shallow BHE will pass through aquifers of various thickness. Most numerical models currently consider only heat conduction as a primary heat transfer process from the source rock to the working fluid. However, in cases where an aquifer is present convective heat transport processes related to groundwater flow, which depends on aquifer thickness and fluid flow velocity in the porous medium, must be considered, as it will have an important influence on BHE performance.

By means of regional-scale geological, hydraulic and thermal models which consider different regional geological settings, a tool will be developed with which different scenarios for the optimization of geothermal use (additional BHE, different locations and operation) will be approached. The regional-scale 3D models allow to calculate energy potentials and to quantify and visualize changes of the groundwater circulation and the groundwater supply of the different aquifer systems by additional BHE.

Development of a structural geological 3D model: (a) Delineation of model-boundaries for selected locations; (b) Preparation of relevant geological data which are necessary to build the geological 3D models, including data evaluation and conversion of existing data; (c) 3D geological modeling and preparation of model geometries and relevant geological structures (lithological units and fracture systems); complex geological structures are summarized with respect to hydrogeological properties, or homogeneously considered by means of the continuum approach.

In the next step the geological structures are integrated to numerical hydraulic and thermal regional-scale 3D models (THM) to simulate the dynamics of regional groundwater circulation and the calculation of water and heat balances. The development of the THM include: (a) A qualitative verification of model geometries (iterative approach); (b) The setup of computational mesh; parameterization of geological units; setting up hydraulic and thermal boundary conditions; (c) The calibration and validation of the THM; (c) Sensitivity analysis: calculation of the influence of different parameters (e.g. permeabilites of different geological units and regional groundwater recharge processes); (d) Model calculations/scenarios; (e) Visualization of 3D groundwater circulation at regional scale; derivation of flow cells (convergence of flow paths in the subsurface; localities of increased groundwater yield and thermal potentials); groundwater and thermal balances over defined cross-sections or balance areas and estimation of thermal utilization potential; (f) preparation of GIS maps (geothermal potential, pressure distribution aquifer, etc.; Link to DSS). Existing information from wells, pumping tests as well as hydrochemical and physical groundwater data (WP I & II) will be used to calibrate and validate the models.

Further development of the geological 3D model for the spatial representation of the geological structures in the context of the respective hydraulic properties (lithological units and fracture systems) will also involve investigations on fracture systems. Even if fracture systems are crucial for groundwater circulation, an exact mapping of the real fracture system cannot be done and often a generalized schematic or at best statistical view of fractures is implemented. This approach can be justified when considering sufficiently large scales (e.g. regional catchments and the scale usually approached for subsurface management). Like this the integral influence of fracture systems on groundwater circulation can be represented.

UNIZG-RNGF:
prof. Tomislav Kurevija, PhD, project lead for UNIZG-RGNF
Marija Macenić, PhD, associate
Ass. Prof. Luka Perković, PhD, associate
prof. Kristijan Posavec, PhD, associate

UNIBAS:
PD Dr. Jannis Epting, project lead for UNIBAS
Stefan Scheidler Dipl. Hyd., associate
Horst Dresmann, PhD, associate
Michel Alain Walde, associate

GeoZS:
Mitja Janža, PhD, project lead for GeoZS
Nina Rman, PhD, associate
Luka Serianz, PhD, associate
Simona Adrinek, associate