Conference Agenda

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Session Overview
EGW - Exploration of Geothermal Reservoirs
Friday, 24/Sept/2021:
9:45am - 11:00am

9:45am - 9:57am

Insights from surface analogues of the Odenwald into the structural architecture of crystalline units in the Northern Upper Rhine Graben

Claire Bossennec1, Matthis Frey1, Lukas Seib1, Jeroen van der Vaart1, Kristian Bär1, Ingo Sass1,2

1Technical University of Darmstadt, Institute of Applied Geosciences, Department of Geothermal Science and Technology, Schnittspahnstraße 9, 64287 Darmstadt, Germany; 2Darmstadt Graduate School of Excellence Energy Science and Engineering, Otto-Berndt-Straße 3, 64287 Darmstadt, Germany

The Upper Rhine Graben (URG) is a target area for deep geothermal and heat storage projects, as petrophysical and hydraulic properties of the faulted crystalline basement rocks, and the temperature field comprise a high geothermal potential (Soultz-sous-Forêts, Landau, Insheim, Rittershoffen). However, there is a lack of knowledge on the multi-scale structural architecture of such rock units in fault zones. Therefore, a multi-scale structural analysis is performed on surface analogues to improve the conceptual crystalline reservoir model accuracy. The surface analogues selected are located in the Odenwald Massif, the largest outcropping section of the Mid German Crystalline High. Regional-scale lineament analysis and LIDAR and GIS interpretation of the fracture network on 21 profiles in 11 outcrops were analysed to quantify statistical parameters describing the fracture and fault network. The variability of crystalline rock lithologies (granite, granodiorite, ‘Flasergranitoid’, amphibolite and gabbro) and fault directions sampled allows for the construction of an extensive structural dataset with fracture network geometry, dimension, and connectivity features. Four significant lineament strikes dominate the structural trend of the NURG, being N000-N015°E, N050-N075°E, N100-N115°E and N150-N165°E. In the Odenwald itself, lineaments striking N100-N115°E and N055-N070°E are in a predominant proportion, compared to the N000-N015°E and N150-N165°E striking trends. Fracture length distribution follows a power law with an exponent varying from -2.2 to -1.8, depending on the background lithology. The connectivity of the fracture network is heterogeneous, with varying configurations (no fractal organisation), due to a fault control at hectometric scale and clustering marked by secondary faults. At the outcrop scale, this pattern is strongly enhanced in the vicinity of weathered fractures or fault corridors. These properties distribution can be implemented into sub-surface semi-artificial discrete fracture network models to quantify the flow properties of fractured reservoir rocks.

9:57am - 10:09am

Structural and Geophysical Characterisation of the Crystalline Basement in the Northern Upper Rhine Graben

Matthis Frey1, Claire Bossennec1, Lukas Seib1, Kristian Bär1, Ingo Sass1,2

1Technical University of Darmstadt, Institute of Applied Geosciences, Department of Geothermal Science and Technology, Germany; 2Darmstadt Graduate School of Excellence Energy Science and Engineering, Germany

In the northern Upper Rhine Graben (URG), the crystalline basement constitutes an attractive target for deep geothermal exploitation due to the favourable reservoir temperatures and abundance of natural fractures and large-scale faults. Consequently, especially the upper, hydrothermally altered part is already successfully used for heat and power generation at several locations (Insheim, Landau, Rittershoffen and Soultz-sous-Forêts). Nevertheless, because of the small number of very deep boreholes drilled into the crystalline basement, little is known about its structure and composition. An interdisciplinary multi-scale approach was applied to gain new insights into the properties of the crystalline crust. By building on existing geological models of the URG, a detailed 3D model of the crystalline basement was developed. Additional information was provided by high-resolution gravity and magnetic data, which served as input for a stochastic joint inversion. Inverted density and susceptibility models allowed to identify the predominant rock types below the sedimentary cover. The Tromm granite in the southern Odenwald was chosen as an outcrop analogue to further analyse the hydraulic properties of the crystalline reservoir. By examining the lineaments on the regional scale and the fractures in a total of 5 outcrops, statistical parameters describing the fracture network were extracted. These were then used to create discrete fracture network (DFN) models, in order to calculate the equivalent porous media permeabilities of the bedrock at reservoir depth. In addition, gravity and radon measurements were carried out, which enabled more precise localisation of naturally permeable fault zones. The combination of structural geological and geophysical methods results in a more advanced characterisation of the crystalline basement, that can in future studies be used for more realistic potential assessments and a reduction of exploration risks for geothermal projects.

10:09am - 10:21am

Exploration of the geologic and hydrogeologic conditions for a medium deep borehole high-temperature thermal energy storage system at TU Darmstadt, Germany

Lukas Seib, Bastian Welsch, Matthis Frey, Claire Bossennec, Kristian Bär, Ingo Sass

Technische Universität Darmstadt, Germany

Solutions for seasonal energy storage systems are an essential component for the reliable use of fluctuating renewable energy sources and to bridge the gap between abundant heat availability from renewable sources in summer and an increased heat demand in winter. As a part of the research project ’solar crystalline borehole thermal energy storage system‘ – ‘SKEWS’, a field-scale demonstrator for a medium deep borehole thermal energy storage (BTES) system with a maximum depth of 750 m is to be built at Campus Lichtwiese of the Technische Universität Darmstadt, Germany, to demonstrate this innovative technology. In this first demonstration phase, the storage array consists of four coaxial BHEs (Borehole Heat Exchangers), which tap into a crystalline reservoir rock underneath a thin sedimentary cover. Prior to project launch a numerical model of the storage system was to be built to investigate the storages behavior under the local geological and hydrogeological conditions. In the first stage, the geological context in the surroundings of the project location was investigated using archive drilling data and groundwater measurements. The data obtained facilitated the development of a geological model concept. It suggests the assumption that the uppermost part of the intended storage domain is crosscut by a normal fault, which displaces the Permian rocks east of Darmstadt against granodioritic rocks of the Odenwald crystalline complex. The simplified geological model was implemented in a 3D-finite-element numerical model to simulate the thermal effect of the storage system operation on the surrounding subsurface. The model for the four planned BHEs did not show the formation of any significant heat plumes by groundwater flow with only a minor increase in groundwater temperature.

Additionally, the numerical model was used to estimate the effect of the potentially highly permeable fault zone on the planned storage site. For this purpose, a storage operation over a time span of 30 years was simulated for different parametrizations of the fault zone and the storage system. The simulations reveal a limited but visible removal of heat from the storage region with increasing groundwater flow in the fault zone. However, since the section of the BTES system affected by the fault is very small in comparison to the system’s total depth, only a minor impairment of the storage efficiency could be observed in the worst case.

10:21am - 10:33am

Gravity survey in delineating geologic features of interest for deep geothermal use at Campus North of KIT.

Maximiliano Pavez1, Natalia Cornejo1,2, Florian Bauer1, Eva Schill1,2

1Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal; 2Technical University of Darmstadt, Institute of Applied Geosciences

The seasonal mismatch of the thermal energy demand can be addressed by thermal energy storage systems of high capacity (e.g. Lee 2013). In this scenario, High-temperatures aquifer thermal energy storage systems (HT-ATES), which commonly supply domestic needs could expand to meet heating or industrial processes demands by storing excess heat.

DeepStor is a planned scientific infrastructure that address the demonstration the concept of HT-ATES in former hydrocarbon reservoirs of deep sedimentary rocks. Specifically, the concept development of the use of deep geothermal energy at Campus North (CN) of KIT. The latter is located on the largest known thermal anomaly in Germany (up to 140°C at 2 km depth), and in the central part of the Upper Rhine Graben (URG).

In order to improve the understanding of the depleted oil reservoir conditions, a gravity survey is being carried out to support the modeling of geological structures. The gravity data in and around the CN is being acquired in an area of ~10 km2 using a CG-6 Autograv Gravity Meter (Scintrex Ltd) has a measurement range of over 8000 mGals and a resolution of 0.0001 mGal. This enable to study in both detailed local and large scale regional structures.

Previous works in the URG have shown that temperatures above 100°C located in the central part of the graben superpose with areas of low values of Bouguer anomaly (Baillieux et al., 2013). On the other hand, the gravity observations on the URG have been interpreted in terms of subsurface density variation due to lithological heterogeneities.

The results of the new gravity data at a local scale will improve the understanding of the local lithological heterogeneities and fracture porosity, giving feedback for the improvement of the new geological model in this area.

10:33am - 10:45am

Transport mechanisms of hydrothermal convection in faulted sandstone reservoir ----- Implications for kilometer-scale thermal anomalies in Piesberg quarry

Guoqiang Yan, Robert Egert, Maziar Gholami Korzani, Thomas Kohl

Karlsruhe Institute of Technology (KIT), Germany

A transient 3D model obtained from the Piesberg quarry as an illustrative example is based on idealized structural models that characterize all geological features during Late Jurassic rifting (162 Ma) to infer possible transport mechanisms of fluids leading to the formation of kilometer-scale thermal anomaly. Three-dimensional numerical simulations on hydrothermal convection systems in the faulted sedimentary basins are investigated with the aim to assess the lateral heating capacity of hydrothermal convection systems in faults, using realistic rock properties (widths, inclinations, anisotropic permeabilities, etc.), fault dimensions, and fault intersections patterns and using variable parameter suites to assess the effect of lateral heating derived from hydrothermal convection systems. Three kinds of transport mechanisms of hydrothermal convection have been inferred and the effects of geological conditions on the transport mechanisms of hydrothermal convection in faulted sandstone reservoirs have been discovered. Furthermore, this study demonstrates that the local thermal anomalies are presumably provoked by circulating hydrothermal fluids along the fault damage zone of a large NNW-SSE striking fault, laterally heating up the entire exposed sandstone reservoir. Results suggest that this thermal event was reached prior to peak subsidence during Late Jurassic rifting (162 Ma). Owing to the idealized nature of the presented models, the numerical results and the associated analytical solution can be applied to petroleum and geothermal system models to avoid overestimating burial depth and reservoir quality, etc.