Conference Agenda

Understanding how thermally induced stress changes modify rock properties is a key challenge in the characterisation of geothermal reservoirs. Within the ‘TRIGGER’ project (‘formation of fractures and changes in permeability in geothermal reservoirs caused by thermally induced stress changes’), we combine laboratory measurements with numerical simulations to better understand small-scale changes in geothermal reservoirs under temperature change. We present a database of porosity and thermal property measurements of representative reservoir rock samples from the Upper-Rhine Graben. We also present preliminary Discrete Element Method (DEM) simulations of fracture initiation and propagation created using ESyS-Particle, informed by the parameters measured in our laboratory. The DEM approach is used to explore how fracture development and permeability evolution relate to the initial material properties and the imposed thermal conditions. This work ultimately contributes to advancing the development of more robust geothermal reservoir models.

Reliable characterization of pore space and pore connectivity in geological materials is essential for evaluating their suitability for geothermal energy production. In geothermal reservoirs, pore connectivity directly controls permeability, fluid circulation, and heat extraction efficiency. Thermal stimulation during geothermal operations can significantly modify the pore network by inducing microcracks or altering existing fractures, thereby affecting reservoir performance and long-term stability. Understanding these thermally induced changes is therefore critical for optimizing geothermal production and predicting reservoir behaviour.

Conventional petrophysical techniques, such as mercury intrusion porosimetry (MIP) and gas pycnometry, provide estimates of porosity and pore-size distributions but offer only limited insight into pore connectivity. To overcome these limitations, Alloy Intrusion Porosimetry (AIP) combined with Broad Ion Beam–Scanning Electron Microscopy (BIB-SEM) enables direct three-dimensional visualization of connected pore networks at nanometre resolution. In this approach, a low-melting-point alloy is pressure-injected into hydraulically connected pores, solidified, and subsequently imaged using high-resolution SEM techniques. The method allows quantitative analysis of pore connectivity, tortuosity, and fracture propagation pathways.

Furthermore, the integration of AIP-derived connectivity data with mineralogical and lithofacies information obtained through Energy-Dispersive Spectroscopy (EDS) and automated mineral-phase characterization facilitates upscaling from pore-scale observations to reservoir-scale properties.

We present first results from the TRIGGER project (funded by BMWE), in which this workflow is applied to investigate thermally induced microstructural changes in geothermal reservoir rocks, mainly from the German Rotliegend formation, and their impact on permeability evolution.

Mica minerals are integral components of granitic geothermal reservoirs, influencing fluid-rock interactions and impacting reservoir permeability, mechanical integrity, and geochemical processes. Recent research indicates that the dissolution of biotite, a common iron-rich mica, can be accelerated in the presence of specific ligands under geothermal conditions. This study examines the dissolution behavior of biotite under simulated geothermal temperatures, utilizing the biodegradable chelating agent N,N-bis(carboxymethyl)-L-glutamic acid tetrasodium salt (GLDA-Na₄) and acidified with HNO₃. Batch experiments conducted at 100–150 °C and pH 4 demonstrated sustained iron release, while transmission electron microscopy with energy-dispersive X-ray spectroscopy (TEM-EDX) revealed sodium penetration into delamination structures and surface complexation of GLDA-Na₄ onto the biotite. Observed structural changes include channel development, exfoliation, bending, and cracking of the mineral's surfaces. Existing internal layering and nanoscale inclusions (goethite, apatite) within the biotite structure could also facilitate deeper penetration of the chelating agent and localized chemical alteration. These findings suggest that the chelation-assisted dissolution of biotite in granitic reservoirs is governed by inherent structural weaknesses and mineralogical variations, enhancing the understanding of mica dissolution mechanisms during fluid-rock interaction. These results support the development of chemical stimulation technologies for Enhanced Geothermal Systems (EGS).

The Geothermal Laboratory in the Crystalline Basement (GeoLaB) is an underground research facility to investigate the sustainable production of geothermal energy from crystalline basement rocks. This study assesses the potential environmental impacts of fluid flow through these rocks, with a focus on element mobilisation and the underlying fluid-rock interactions that are important for the permitting process and long-term energy generation.
For this purpose, an apparatus was developed for long-term flow-through experiments. Seven-day tests were conducted at temperatures of 50 °C, 100 °C and 150 °C on mechanically fractured Tromm granite samples with pegmatitic structures from the Streitsdöll quarry (Odenwald, Germany). Groundwater sourced from neighbouring springs was collected and pumped through the samples within the apparatus. The influents and effluents were analysed for hydrochemical components using ion chromatography (IC) and inductively coupled plasma mass spectrometry (ICP-MS). The rock samples were characterised chemically using X-ray fluorescence (XRF). Results indicate no temperature dependence on element behaviour. Temperature only affected calcium dissolution. Magnesium, strontium and bromine displayed a tendency to adsorb onto the rock matrix, whereas potassium, fluorine, chromium, manganese, nickel, copper, arsenic, rubidium, molybdenum, silver, tin, caesium and lead were dissolved.
Numerical modelling using PhreeqC failed to accurately predict these dissolution behaviours, suggesting that processes such as surface complexation play a significant role. The findings suggest that fluid-rock interactions are unlikely to result in groundwater contamination that exceeds the limits defined by the German Drinking Water Ordinance. This indicates the viability of producing geothermal energy in crystalline basement rocks.

Reactive transport is essential in many geological and geo-engineered fluid-solid processes such as natural rock alteration, CO₂ sequestration, or energy recovery from Enhanced Geothermal Systems (EGS). In EGS, chemical stimulation through reactive fluid flow leads to dynamic changes of the reservoir mineralogy and petrophysical properties. Generation of porosity and permeability maintenance are crucial for EGS, as they enable efficient fluid flow and, consequently, heat transport. However, the parameters governing the efficiency of chemical stimulation of low-permeable rocks are incompletely understood and experimental studies are limited.

To simulate reactive transport processes in crystalline reservoirs and to investigate in situ petrophysical property changes, batch and flow-through experiments were conducted on a laboratory scale. Therefore, granite was stimulated with modified regular mud acid (RMA) under conditions relevant to chemical stimulation of EGS.

After reaction with the reactive fluids, we characterized and quantified chemical, mineralogical, and microstructural changes of granite by integrating X-ray powder diffraction (XRD), scanning electron microscopy (SEM), electron probe micro analyses (EPMA), optical surface profilometry, and X-ray micro-computed tomography (µCT). Furthermore, continuous monitoring of permeability and effluent fluid chemistry enables assessment of stimulation efficiency and short-term petrophysical evolution.

Our results demonstrate, in both batch and flow-through experiments, a significant increase of porosity and permeability and the creation of flow paths, driven by coupled mineral dissolution and precipitation of denser secondary phases. These key findings on the laboratory scale underscore the potential of chemical stimulation to induce changes in petrophysical properties that are beneficial for the exploitation of crystalline geothermal reservoirs.

Carbonate reservoirs host some of the world’s most productive hydrothermal geothermal systems. Current exploration in North Rhine-Westphalia targets Devonian and Carboniferous carbonates of the Rhenohercynian Zone. These rocks underwent Variscan deformation, burial, dolomitisation, fracturing, and karstification, resulting in strong reservoir heterogeneity. These deformation related and diagenetically overprinted changes exert a primary control on fluid flow and geothermal reservoir performance. Permeability is governed by matrix pore networks, fracture and fault related pathways, and karst related conduit systems, with karstification strongly enhancing secondary porosity and hydraulic connectivity.

This study presents laboratory analyses of Devonian and Carboniferous carbonates with emphasis on diagenetically overprinted lithologies. Porosity reaches up to 22 % in karstified samples, while permeability ranges between approximately 10⁻¹⁵ and 10⁻¹⁷ m². Even at plug scale, karstified units outperform non karstified samples by up to four orders of magnitude in hydraulic conductivity. μCT imaging shows that solution enlarged interconnected pore systems form dominant fluid pathways. The results demonstrate that karstification is a first order control on reservoir quality and fluid flow in Paleozoic carbonates. It further highlights the strong scale dependence of petrophysical measurements in heterogeneous carbonate systems, as plug scale analyses cannot fully resolve fracture and karst network connectivity. Reliable reservoir assessment therefore requires integration of laboratory, structural, hydrogeological, and geophysical data. Overall, the results underline the considerable geothermal potential of karstified Paleozoic carbonates in North Rhine-Westphalia.