Veranstaltungsprogramm

The North German Basin is a large intracratonic basin that is characterized by several sub-basins, thick sedimentary deposits and numerous salt bodies. Such passive geothermal play systems typically host low to medium enthalpy resources and are determined by conductive heat transfer. Geothermal resource utilization is therefore challenging and the selection of suitable sites is key in order to minimize the exploration and development risk. These challenges could be overcome by harnessing local temperature anomalies caused by the presence of salt in the subsurface. Rock salt has a significantly higher thermal conductivity than typical sedimentary rocks. It is, therefore, very efficient in conducting heat to the surface from depth, causing positive anomalies above the salt and negative anomalies below its base. Potential target zones in the North German Basin focus on the Mesozoic aquifers in the overburden of the thick Zechstein salt deposits, where increased temperatures of at least 5°C have been recorded. Understanding these thermal anomalies is vital for geothermal exploration in terms of drilling depth and costs. However, studies devoted to the analysis of the temperature field around salt domes are either limited to simple geometric bodies and boundary conditions, assume constant thermal conductivities, or are reduced to two-dimensional solutions.

Here we use the finite difference method to model the steady-state conductive temperature perturbations caused by the presence of salt in the basin. Unlike conventional models that rely on constant thermal conductivities, this model determines geothermal conductivity as a function of lithology, temperature, and depth. Our results demonstrate that the thermal effect of salt on its surrounding is controlled by two main factors: the contrast in thermal conductivity between salt and sediment and the shape and size of the salt structure. The highest positive temperature anomalies are recorded in the overburden of thick, near-surface salt deposits in porous, insulating sedimentary rocks. The ability of the salt to transport heat from the base to the top is shown to depend on the contrast in thermal conductivity and, hence, sedimentary infill and depth. Modelling the temperature field around various salt bodies, such as layers, pillows and domes shows, moreover, that both the size and shape are decisive. For compact salt bodies, such as domes or diapirs, the vertical extent of the salt structure is a direct measure of the temperature anomaly. For salt structures with extremely low aspect ratios or complex morphologies, this relationship is altered and detailed analyses are needed to determine its extent.

Applying this method to local sites and regional models in the North German Basin demonstrates that temperature anomalies of up to 20 ℃ are feasible. By comparison, to achieve the same temperature in an undisturbed, homogeneous geothermal gradient, one would have to drill 500m to 600m deeper. The exploitation of the geothermal energy above the salt could therefore be favourable, not only due to reduced drilling depths and costs but also because of the reduced risk of finding a highly compacted reservoir at depth.

Mesozoic sandstone aquifers offer significant potential to provide green and sustainable geothermal heat as well as large-scale storage of heat. The estimation of both potentials, in particular the ability of the underground to store heat, for the North German Basin remains a challenging task: based on sparse and partly uncertain subsurface data, the structural and depositional architecture of the underground needs to be integrated in adequate 3D subsurface models. The BGR TUNB project recently published a basin-wide structural interpretation of depth horizons of the main stratigraphic units, which were used in combination with temperature data from 3D models and GeotIS. Based on a reservoir facies identification using well logs and by providing technical boundary conditions we calculated the geothermal heat in place and the potential for heat storage for virtual well doublet systems. The results of the analysis show large potential for both geothermal heating and aquifer thermal energy storage in geologically favorable regions and in many areas of high demand (i.e. high population density, high industrial heat demand). Given the uncertainties in the input data, the applied methods and combined data are most powerful in identifying promising regions for economic subsurface utilization and should be combined with detailed geological analysis beforehand to decrease exploration risk.

Geothermal energy will play a major role for the heat supply in the forthcoming energy transition. Therefore, the Lower Carboniferous (Dinantian) carbonates have been investigated by the Interreg-funded project ‘Roll-out of Deep Geothermal Energy in North-West Europe’ (DGE-ROLLOUT).

In Dinantian times, a tropical, shallow-water platform developed in the south of the Laurussian shelf around the London-Brabant massif, forming thick limestone deposits, which were often affected by karstification and dolomitisation. These altered platform carbonates of the so-called Kohlenkalk Group provide ideal aquifers for hydrothermal energy production.

Each of the partner countries involved in this project constructed a virtual 3D model of this reservoir to determine the depth, thickness, and geothermal potential in their respective region. The models of Northern France, Belgium, the Netherlands, and North Rhine-Westphalia have been merged to a combined model for northwest Europe.

This contribution presents the results of the 3D mapping campaign in North Rhine-Westphalia, which covers not only the platform carbonates of the Kohlenkalk Group as a geothermal reservoir, but also the deep basinal facies of the Kulm basin and its possible potential. Well data and outcrop analogues were used to create a facies map, which is used for preliminary estimations of the reservoir qualities.

The 3D model further provides information about the depth, thickness and structure of the Dinantian strata. It is the basis for temperature estimates in the deep subsurface. Temperatures above 100 °C are expected in the southwestern part of the Lower Rhine Embayment (Roer valley graben/Wurm syncline) and throughout the Münsterland basin. Here, the additional implementation of hydrothermal power plants for electrical power generation could be considered.

Upper Jurassic Malm carbonates represent the most important deep geothermal reservoir in the Molasse Basin in southern Germany and neighbouring countries. To fully utilize its geothermal energy potential, a quantified regional scale model is necessary to predict future exploration targets. In framework of the EU funded GeoERA-HotLime Project (Diepolder et al. 2020) the volumetric ‘Heat in Place’ (HIP) method (Muffler & Cataldi, 1978) was applied deterministically after Limberger et al. (2018) to calculate the geothermal energy potential of the deep Malm reservoir. The geometry data was based on a joint structural 3D model of the top Malm surface, including its thickness and temperature data from the GeotIS platform (www.geotis.de, Veldkamp et al. 2021). Petrological parameters like bulk porosity, density and specific heat capacity of rock and fluid were retrieved from regional studies (e.g. Clauser et al. 2006, Bohnsack et al. 2020) and scaled proportionally. The calculations resulted in a regional HIP distribution map of the study area, including an overall amount of 2,35 * 10⁶ PJ (Petajoule) of stored energy in the reservoir.

As several input parameters like the bulk porosity that were used for the deterministic HIP model in GeoERA-Hotlime can vary dramatically across the reservoir, a probabilistic modelling approach was also utilized by testing the MATLAB-based 3DHIP Calculator tool (Piris et al. 2019). The resulting probability maps (P10, P50 and P90) of HIP validate the deterministic model results. In additional simulations, the amount of recoverable heat in the reservoir was predicted by including a recovery factor (estimated percentage of recoverable energy in a specific reservoir volume) and the estimated lifetime of an average geothermal plant. This study highlights the importance for regional reservoir models and their potential to support future geothermal exploration projects.

References:

Bohnsack, D., Potten, M., Pfrang, D., Wolpert, P., & Zosseder, K. (2020): Porosity–permeability relationship derived from Upper Jurassic carbonate rock cores to assess the regional hydraulic matrix properties of the Malm reservoir in the South German Molasse Basin. – In: Geothermal Energy 8 (2020): 1-47.

Clauser, C., Koch, A., Hartmann, A., Rath, V., Mottaghy, D., Pechnig, R. (2006): Erstellung statistisch abgesicherter thermischer und hydraulischer Gesteinseigenschaften für den flachen und tiefen Untergrund in Deutschland Phase 1—Westliche Molasse und nördlich angrenzendes Süddeutsches Schichtstufenland. BMU Projekt, FKZ 0329985 (Final Report)

Diepolder, G.W. & HotLime Team (2020): HotLime – Mapping and Assessment of Geothermal Plays in Deep Carbonate Rocks – summary of mapping and generic characteristics of eleven case studies. – 10 pp, https://geoera.eu/wp-content/uploads/2020/02/HotLime-Midterm-Summary-Report.pdf

Veldkamp, H. (ed.) & HotLime Team (2021): Report on play and prospect evaluation in HotLime’s case study areas. - HotLime Deliverable 3.1: 92 pp. (in prep.).

Limberger, J., Boxem, T., Pluymaekers, M., Bruhn, D., Manzella, A., Calcagno, P., ... & van Wees, J. D. (2018): Geothermal energy in deep aquifers: A global assessment of the resource base for direct heat utilization. – In: Renewable and Sustainable Energy Reviews 82 (2018): 961-975.

Muffler, P., and Cataldi, R. (1978): Methods for regional assessment of geothermal resources. – In: Geothermics 7.2-4 (1978): 53-89.

Piris, G., Herms, I., Griera, A., Gómez-Rivas, E., Colomer, M. (2020): 3DHIP-Calculator (v1.0) [Software]. ICGC, UAB. CC-BY 4.0

The geothermal anomalies of the Upper Rhine are a great opportunity for geothermal heat exploitation and power generation. The high geothermal gradient, permeable fault zones and reservoir formation and central location in Europe make it a prime site. To facilitate the extraction of these resources, many models, like Hessen 3D (1.0 and 2.0) and GeORG have been created and published to map the resource potential. While these models give great insight, they cannot be taken as an exact representation of the subsurface. Errors from measurement, calculations, modelling and interpolation create uncertainties in a model. In the scope of the EU-NW-Interreg project DGE-Rollout (NWE 892) the influences of these uncertainties are further explored.

This presentation builds upon previous studies in the northern Upper Rhine Graben (nURG). By combining uncertainties that come from errors in the geophysical, borehole and laboratory data acquisition, a new uncertainty model is computed. It uses the uncertainty constraints provided by both structural geological inputs as well as intrinsic rock properties. A custom-made stochastic workflow based on a Monte Carlo simulation provides a multitude of equal valid models. Combining these outcomes, quantitative uncertainty models are created, which describe the uncertainty as a function of location and depth for parameters critical to geothermal exploration. With this uncertainty, the validity of models and the modelling techniques can be explored. As the horizons can have uncertainties of several hundreds of meters in the deeper parts and the properties have uncertainties in the range of tens of percentages, these maps are invaluable for minimizing geological and financial risk, and therefore decision making in unlocking the geothermal potential that the Upper Rhine Graben provides.