Conference Agenda

Recent investigations of the GeoLaB-1 wellbore and the recovered cores have significantly advanced subsurface characterization through a multi-scale core–log integration strategy. This approach connects decimeter-scale (dm) downhole logging data with centimeter-scale (cm) non-destructive measurements from the Multi-Sensor Core Logger (MSCL) and extends to micro-scale (µm) petrographic analyses.

The combined dataset enables a consistent and high-resolution characterization of the Tromm subsurface. It is supported by complementary mineralogical and geochemical analyses on selected core samples, including X-ray diffraction (XRD), whole-rock geochemistry obtained by X-ray fluorescence (XRF), and detailed thin section investigations using optical microscopy as well as in situ mineral geochemical analyses by electron microprobe. Together, these methods provide both bulk and mineral-specific information, allowing for a robust identification and classification of the encountered lithologies, including granitic and metamorphic units, as well as their textural and compositional variability.

This study focuses on establishing an integrated petrophysical and petrological framework in which mineralogy acts as the primary control linking log responses to core observations across scales. Particular emphasis lies on the distribution of Fe-bearing minerals and their alteration patterns, which are key controls on geophysical log signatures. By integrating observations from decimeter-scale logs to micrometer-scale thin section analyses, this approach enrich the quantitative interpretation of geophysical data in terms of mineralogy, texture, and associated rock properties. Our results provide crucial input for design of forthcoming geomechanical and flow-cell experiments using core material. Furthermore, improved constraints on alteration contribute to unravel the rock properties, which are critical for the development of the GeoLaB laboratory.

Geothermal energy has the potential to contribute to achieving sustainable development and energy security. Enhanced geothermal systems are a prime candidate for the needed heat source. Constraining in-situ stress is central to devising stimulation strategies and understanding induced seismicity. We present ultrasonic investigations of 6 m of granite core from borehole SB1.1 in the Bedretto Underground Laboratory with an overburden of about 1 km. The cores represent borehole sections that were hydraulically fracked. The laboratory measurements were conducted at 10 cm intervals along the core length and at different angles in 30ᵒ increments around the core circumference. The velocities of ultrasonic P-waves and S-waves, with two different polarisations, were analysed to evaluate elastic heterogeneity and anisotropy. For travel-time determination, we explored two automatic picking algorithms, 1) a combination of kurtosis and AIC (kurt+AIC), and 2) STA/LTA, besides manual picking. Overall, STA/LTA worked better than kurt+AIC for picking P-waves arrivals; the automatic analysis of S-waves is generally difficult due to the influence of noise and converted waves. The results show systematic angular and positional variabilities, indicating velocity anisotropy of the granite. The maximum and minimum velocities occur at similar angles for most core pieces. We discuss the relation of anisotropy to magmatic fabric and microcracking, possibly due to stress relaxation. Furthermore, we correlate the determined velocity heterogeneity to observed fracture traces on the borehole wall caused by the hydraulic fracturing operations, to examine the roles of stress heterogeneity and material heterogeneity in the fracturing process and the implications for stress determination.

This study investigates seismic anisotropy in hard crystalline basement rocks, including granite, granodiorite, and gneiss, which are commonly encountered in Enhanced Geothermal Systems (EGS). Understanding anisotropy is essential for evaluating reservoir quality, fracture geometry, and fluid flow behavior in deep geothermal reservoirs. Ultrasonic pulse transmission experiments were performed on rock core samples under confining pressures up to 400 MPa and temperatures up to 400°C to simulate in-situ conditions at depths of 3–8 km. Compressional (P-wave) and shear (S-wave) velocities were measured in multiple directions to determine three-dimensional elastic velocity tensors and quantify anisotropic behavior.

The study analyzed anisotropy parameters, including Thomsen parameters for transversely isotropic and orthorhombic media, as functions of stress, temperature, microcrack density, mineral alignment, and foliation structure. Microstructural investigations using scanning electron microscopy and thin-section analysis were conducted to relate mineral composition and fabric to macroscopic elastic anisotropy. The results reveal that seismic anisotropy is controlled by two dominant mechanisms: stress-sensitive microcrack anisotropy, which is significant at low pressures and decreases with crack closure under higher stress, and intrinsic mineral fabric anisotropy, which remains persistent at high pressures due to crystallographic preferred orientations and tectonic history. Measured P-wave anisotropy ranges from 2% to 18%, while S-wave splitting reaches up to 8%.

The findings provide valuable constraints for seismic interpretation, borehole logging, fracture characterization, hydraulic stimulation, and thermo-mechanical reservoir modeling in crystalline basement EGS reservoirs.

Faults and fractures exert a fundamental control on fluid flow in carbonate reservoirs, where matrix permeability is inherently low and secondary porosity dominates. In geothermal systems, the hydraulic behavior of such discontinuities can ultimately determine project viability.

The geometry and permeability of fracture networks are strongly conditioned by the prevailing stress field, reflecting both regional tectonic forces and localized geological processes. The orientation and magnitude of the principal stress axes govern whether fractures remain open and transmissive or close under compressive loading and thereby directly control the effective permeability of the fractured rock volume.

Here we investigate the role of stress magnitude and orientation on the permeability of Devonian carbonate rocks, a potential reservoir for deep geothermal energy exploitation in the Rhine-Ruhr region. We construct Discrete Fracture Network (DFN) models of the Devonian Massenkalk using the dfnWorks framework and structural data acquired in the field. Incorporating stress orientations derived from new focal mechanism solutions, we calculate stress- and depth-dependent permeability anisotropy within the Massenkalk. Our results indicate that permeability may be significantly overestimated when stress effects are neglected in DFN modeling. N–S and NNE–SSW fracture sets are found to be favorably oriented for opening and shear under the current stress regime, suggesting enhanced horizontal permeability in the north–south direction. In contrast, E–W and ENE–WSW fractures are likely to remain closed under the present-day stress field, highlighting the importance of stress-informed modeling for geothermal reservoir assessment in the region.

Fluctuating renewable energy sources, e.g. wind or solar, require efficient energy storage systems to minimise energy losses during periods of overproduction. Underground thermal energy storage in geological reservoirs is a promising method, due to the abundance of potential storage locations and the suitable thermal properties of rock. Additionally, geothermal energy reservoirs could be thermally ‘enhanced’ using the same approach.

To evaluate reservoir stability, the storage material, rock, must be analysed for the changing reservoir conditions. For this purpose, rock samples from the Upper-Rhine Graben were characterised after exposure to various maximum temperatures, and number of cycles. Basic physical, mechanical, hydraulic, and thermal properties were determined before and after cyclic thermal treatment.

With increasing max. temperature effective porosity of a sandstone increased, whereas density and P-wave velocity decreased. Uniaxial compressive strength showed variations only within sample variability. P-wave velocity also decreased, when the number of cycles was increased from 1 to 49 at a max. temperature of 200 °C. This implies a time-dependent development of microcracks even at lower temperatures, which was further investigated in a comparative microstructural analysis of thermally treated and untreated samples.

Extracting geothermal resources is often negatively affected by induced seismicity, resulting from anthropogenic perturbations of pressure, temperature, and stress in the subsurface. Numerical models frequently predict significant cooling-induced stress changes during long-term cold-water injection. However, observations from mature hydrothermal systems, such as the Paris Basin, suggest that seismic activity is often less than expected. This discrepancy indicates that stress-relaxation mechanisms in geothermal reservoirs remain insufficiently understood. Here, we hypothesize that temperature-dependent rock properties, such as permeability, thermal conductivity, specific heat capacity, and elastic modulus, play a fundamental role in controlling the evolution of thermally induced stresses. Experimental evidence supports the notion that these properties can significantly vary with temperature, thereby affecting the activated thermo-hydro-mechanical (THM) processes.

The COOLSTRESS project investigates coupled THM processes governing stress evolution during cold-water injection. We employ generic two-dimensional finite-element models to simulate pore-pressure diffusion and cooling-front propagation over operational timescales. The resulting stress changes are evaluated in terms of Coulomb failure stress and translated into induced seismic hazard. Sensitivity analyses are performed to quantify the relative importance of individual parameters and their interactions. This project advances the mechanistic understanding of thermally induced seismicity in hydrothermal reservoirs. It contributes to improved workflows for seismic hazard assessment and reservoir management strategies. Ultimately, it provides a physically consistent framework for predicting long-term reservoir behavior and supports the sustainable development of geothermal energy systems.