Applied geoscience relies on robust structural models that are appropriately scaled and detailed to address specific challenges. However, the process of developing these models is influenced by human biases shaped by personal and professional experiences, area of expertise, cognition, and values. This diversity of approaches to geoscience interpretation, such as fracture characterisation, impacts the reliability of structural models, which are critical for geoenergy, resource and infrastructure applications. Ensuring robust interpretations is vital for improving safety, enhancing decision-making, and securing project success.

This study investigates the variability in fracture interpretations made by geoscientists analysing an aerial drone image of a fractured outcrop. We compare outputs such as fracture frequency, orientation & density, and network topology across participants to assess the variability in their observations and the uncertainty this develops.

Previous studies on 3D seismic data have shown that geoscientists’ experience and approach significantly impact structural models. Our research systematically assesses similar variability using remotely sensed outcrop data and shows that while our cohorts of geoscientists agree of the “big stuff”, there is less consensus when we examine the detail.

By illustrating these uncertainties, we can begin to inform improved interpretation workflows, team arrangements, assurance processes, and geoscience education and communication. Understanding biases in fracture interpretation is a critical step towards enhancing interpretational accuracy. Coupled with a clear idea of how “good” the structural model needs to be for the problem being solved and appropriate mitigation measures, (if necessary) this ensures better project outcomes and supports the development of reliable geoscience outputs across applications.

How to cite: Evans, L., Shipton, Z., Bond, C., Roberts, J., and Kim, N.: How Good is Your Fracture Model? Evaluating Human Biases and Uncertainty in Geoscientific Interpretations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16786, https://doi.org/10.5194/egusphere-egu25-16786, 2025.

Fracture network develops in response to deformation in competent rocks and are often related to fault related folding (Allmendinger, 1982; Watkins et al., 2018). The Baghewala structure in the Bikaner-Nagaur sub-basin is a fault related anti-form that encompasses the Marwar Supergroup. The structure has a spatial extent of ~12 km² and is bounded by a major ENE-WSW trending reverse fault. The Upper carbonate (UC) Formation of Marwar supergroup here carries dominantly the dolomites and is traversed by extensive fractures that are visible in cores and image logs. This is a zone of severe mud loss and drilling complications.  

We study the image logs from 6 wells to decipher the fracture orientation, understand the deformation mechanism and the implications for drilling and hydrocarbon production. The interpreted fractures from image logs are observed to be dominantly high-angle (60°-90°) extensional fractures oriented along ENE-WNW direction with respect to the sub-horizontal bedding. Additionally, it is seen that the fractures are confined to dolomitic part of UC Formation. Spatially the wells nearer to the fold crest and fault show higher fracture intensity compared to the peripheral wells. The transpressional tectonics after the deposition of the Marwar Super group induced ~NW-SE compression that led to the formation of fault related Baghewala fold. Consequently, the outer arc extension resulted in formation of the syn-post folding fracture network (Price 1966). The fractures thus can be inferred as late-stage high angle fractures due to its orientation (Basa et al., 2019; Ahmed and Bhattacharyya, 2021) and relative prominence (Ismat and Mitra, 2001). Dolomites accommodate deformation by forming fracture networks and fracture intensity is higher, proximal to the fault zone and fold structure (Ahmed and Bhattacharyya, 2021). Thus, lithology and structural position played a role in the partitioning of fractures vertically and spatially.

We see the horizontal wells that are oriented sub-parallel to the fracture network have significant history of mud loss within UC compared to the wells whose profile trends at high-angle to fracture orientation. The deviated wells oriented along the major fracture network will encounter weak planes leading to drilling complications compared to profiles that are oriented across. Therefore, understanding orientation of fracture networks has implications in designing deviation profile of the wells. In low permeability rocks like dolomite, fractures can affect fluid flow due to increase or decrease in permeability (Hanks et al., 2004). The extensional fractures developed in the Baghewala structure can lead to increase in permeability in the reservoir zone also i.e. the competent Jodhpur sandstone. We see enhanced production in one well that is closer to the fault zone which may be due to the increased permeability effected by the fractures, but the data is limited to conclusively prove this. This study will help to design the well position and trajectory not only to avoid mud loss and drilling complications but also to increase production by optimally placing the wells in proximity to the higher fracture intensity after arriving at a Discrete Fracture Network (DFN) model.

How to cite: Ahmed, F. and Rai, T.: Fracture network mapping using image logs from Baghewala structure, Bikaner Nagaur sub-basin: Implications for well profile and hydrocarbon production, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3144, https://doi.org/10.5194/egusphere-egu25-3144, 2025.

Faults are an important factor for geoenergy applications due to either their sealing or conducting properties or their mechanical behaviour. Consequently, (thermo-hydro-) mechanical numerical investigation of geoenergy applications often include faults in their modelled rock volume. It is often assumed, that faults can significantly alter the far-field stresses, impacting both magnitudes and the orientation. In contrast to the far-field, stress rotations in the vicinity of faults are clearly observed in numerous borehole stress analyses across the world.

(1) The impact of faults on the stress state at distances of several hundred meters to a few kilometres (far-field) is tested. Therefore, faults are modelled with different numerical representations, material properties, fault orientations w.r.t. the stress field, fault width, extent, and boundary conditions. The results show that the impact of faults on the far-field is negligible in terms of the principal stress magnitudes and orientations. Only in extreme cases, stress changes in the far-field (>1km) can be observed, but these are not significant considering the general uncertainties in stress field observations.

(2) Stress changes within the fault zone are investigated, too. Particularly, the material contrast between the intact rock and the damage zone and fault core is regarded. This contrast can be responsible for a dramatic change in the stress tensor, observed as a rotation of the principal stress axes. In general, the change in the stress field increases with increasing stiffness contrast. The orientation of the fault w.r.t. the background stress field and the relative stress magnitudes, particularly the differential stress, lead to further stress changes. A small angle between the fault and the maximum principal stress axis and a small differential stress promote stress changes.

The study indicates that the impact of faults on the stress field is mostly limited to the fault’s near-field. These models provide an upper limit of stress changes, as several factor which alter stress changes (joints, viscosity etc.) are not included. However, the stress changes depend on the acting processes and material properties. Furthermore, for models used for site investigation, the implementation method and the mesh resolution can play an important role. All these factors need to be considered when planning the setup of a model with faults and their implementation.

The work was partly funded by BGE SpannEnD 2.0 project, the Bavarian State Ministry of Education and Culture (Science and Arts) within the framework of the “Geothermal-Alliance Bavaria” (GAB), and the DFG (grant PHYSALIS 523456847).

How to cite: Ziegler, M., Reiter, K., Heidbach, O., Seithel, R., Rajabi, M., Niederhuber, T., Röckel, L., Müller, B., and Kohl, T.: Faults in geomechanical models – Necessary, nice, or nonsense?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8308, https://doi.org/10.5194/egusphere-egu25-8308, 2025.

Safe and controlled exploration of deep geothermal resources in future decades is a key pillar for successfully transitioning to a carbon-neutral economy. A largely untouched source of deep hot source rocks can be found in crystalline basement formations in many European regions. Hydraulic stimulations are required to harness this thermal energy. Past geothermal projects were not always publicly accepted due to unintended induced seismicity that accompanied the projects. Faults and entire fault systems are jointly responsible for these seismic events and must therefore be thoroughly understood before they may be stimulated, to minimize tremors and unintentional shaking.

The Bedretto Underground Laboratory for Geosciences and Geoenergies in Ticino, Switzerland allows for decameter scale stimulation experiments and access to deeply (> 1km) buried crystalline faults. A complex fault zone is hydraulically and petrophysically described as part of the FEAR project, using various field and laboratory techniques. Two sub-parallel boreholes obliquely intersecting the target fault are analyzed using geophysical image and sonic logs. Hydraulic tests on predefined, packered intervals in the form of pulse-, constant rate- and step-rate injection tests are implemented on field scale, deducing parameters such as hydraulic conductivity and injectivity. In addition, laboratory petrophysical experiments on samples retrieved from varying parts along the fault zone are performed to determine permeability under certain effective stresses, porosity, and p-wave velocity, among other properties. This allows for a cross-scale hydraulic and petrophysical comparison. Prior, structural geologists described and analyzed the target fault using core logging, outcrop, and fracture data. Correlations between structural, hydraulic, and petrophysical observations can be drawn.

How to cite: Schaber, T., Jalali, M., Ceccato, A., Zappone, A. S., Pozzi, G., Selvadurai, P., Spagnuolo, E., Gischig, V., Meier, M.-A., Hertrich, M., and Amann, F. and the FEAR Team: Multidisciplinary characterization of a complex fault zone in crystalline basement rock, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8404, https://doi.org/10.5194/egusphere-egu25-8404, 2025.

Understanding and predicting the hydro-mechanical (HM) behavior of subsurface porous and fractured formations is key to a number of engineering applications, including fluid injection/extraction, construction/excavation, geo-energy production and deep geological disposal. The interaction between fluid pressure, deformations and stresses is particularly affected by the subsurface heterogeneity, which may lead to non-intuitive responses, such as effective stress reduction and pressure increase during fluid extraction. While the impact of large-scale heterogeneities is acknowledged in most studies and modeling efforts, the presence of heterogeneities at smaller scales cannot be included in reservoir-scale models and it must be encompassed into equivalent properties assigned to uniform materials.

In this work, we focus on the Biot effective stress coefficient, a central property determining the HM behavior of fluid-saturated geological media. When not simply assumed as equal to 1, this coefficient is estimated experimentally at the laboratory sample-scale or analytically through expressions valid for isotropic homogeneous materials. However, these approaches are not able to estimate a representative equivalent coefficient for fractured rocks, which are strongly anisotropic and prone to sample-size effects, with fracture lengths spanning several orders of magnitudes from millimeters up to hundreds of meters. By employing a theoretical framework to quantify an equivalent Biot coefficient for a fractured rock mass from the properties of both the porous intact rock and the discrete fracture network (DFN), it is possible to analyze the variability of this coefficient with the DFN properties and highlight the implications for the rock upscaled HM behavior, in the context of natural processes and engineering applications.

How to cite: De Simone, S.: The equivalent Biot coefficient reveals the effects of heterogeneity on the Hydro-Mechanical behavior of fractured rocks, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11332, https://doi.org/10.5194/egusphere-egu25-11332, 2025.

Seismological observations show that earthquakes produce significant changes in the elastic and transport properties of active faults, with co-seismic drops in seismic wave velocities consistently followed by a slow post-seismic recovery, over months to few years. Such variations occur in volumes up to several hundred metres thick that correlate well with the dimensions of fault damage zones. This suggests the existence of a damage-recovery cycle within active fault zones, with the recovery phase possibly driven by a range of fluid-assisted re-strengthening “healing” mechanisms in the fractured medium and/or stress relaxation. Understanding how and how rapidly fractured rocks seal, regain their stiffness, and drive fluid flow in fault zones is fundamental to comprehend the mechanics of the brittle crust and for geo-engineering applications such as geothermal energy, ore deposits, the deep disposal of radioactive waste and CO2 sequestration.

At the Department of Geosciences of the University of Padua, a “percolation cell” apparatus has recently been installed to study long-term fluid-rock interaction under hydrostatic conditions with a maximum confining and pore pressure of 100 MPa and a maximum temperature of 250°C. Such apparatus is equipped with two syringe pumps and a back-pressure regulator that allow to monitor permeability evolution through time and a set of high-temperature P- and S- ultrasonic transducers to track changes of rock elastic properties in-situ. In addition, the pore fluid inlet circuit can flow into a stirred autoclave to pump solutions with controlled chemistry up to 20 MPa pore pressure and 200°C temperature through an externally heated pipe. Such an experimental apparatus allows to study both diffusion- and advection-dominated regimes within conditions representative for the upper crust.

Together with the experimental setup, here we present some preliminary long-term percolation tests in which de-ionized water was flowed at 25°C through rock cylinders of micritic limestones with mated and non-mated single fractures under 20 MPa confining pressure and 5 MPa pore pressure. The temporal evolution of permeability and elastic properties were monitored together with the fluid-chemistry at the outlet. Mechano-chemical processes along the fractures were also investigated through X-ray microtomography and SEM analyses.

How to cite: Fondriest, M. and Akker, I. V.: An experimental apparatus to investigate fluid-assisted long-term recovery of fractured rocks, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12950, https://doi.org/10.5194/egusphere-egu25-12950, 2025.

Permeability in crystalline rocks, considered for use as geothermal reservoirs or deep geological repositories, is controlled by networks of well-connected fractures. Fracture connectivity depends on fracture orientation and density, which are influenced by tectonic and non-tectonic stresses, pre-existing foliations, and fracture zones. This study investigates fracture variability in the Devonian Wilsons Promontory granitic batholith of southeast Australia, which intruded Devonian metasediments that are unconformably overlain by Cretaceous rift-related sedimentary rocks.

Outcrop analogues allow 2D and 3D observation of fracture networks at scales from centimeters to hundreds of meters, complementing sub-meter-scale borehole data and regional lineament mapping. Additionally, digital outcrop models from uncrewed aerial vehicle (UAV) surveys enable fracture characterization in outcrops that are difficult to physically access, such as the granites in the study area. In this study, over 2,500 fractures were mapped and characterized from a UAV-derived point cloud. Most fractures strike NNW-SSE to N-S; they are are interpreted as extensional and to have formed coevally with NNW-SSE striking joints in outcropping Cretaceous rocks during regional uplift under NNW-SSE horizontal compression. Domains characterized by distinct fracture patterns are separated by meter-scale fracture zones, suggesting structural segmentation within the granite.

Future work will investigate the geometry and origin of these domain-bounding fracture zones and their links to mechanical heterogeneities in the granite. These insights will inform discrete fracture network (DFN) and hydrological models of granite reservoirs and repositories for spent nuclear waste. They will also support comparisons of brittle deformation in granitic versus siliciclastic rocks under shared tectonic regimes, relevant to energy projects involving multi-level, multi-lithology reservoirs.

How to cite: Samsu, A., Gasche, G., and Cruden, A. R.: Fracture Network Variability in Granite: Insights from Wilsons Promontory, Southeast Australia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13175, https://doi.org/10.5194/egusphere-egu25-13175, 2025.

Discrete Fracture Network (DFN) models have a widespread use when predicting hydraulic properties of fractured rock masses with different numerical and (semi-)analytical methods. However, recent advances in the way fracture network parameters are characterized in the field or in geophysical datasets are not completely reflected in input options of DFN simulators. For instance, to our knowledge no 3D DFN stochastic simulator is able to generate fracture networks with realistic topological relationships, and fracture spatial distributions different from a completely random Poisson distribution cannot be generated (so clustered or regular distributions cannot be modelled). This means that stochastic fracture networks cannot show realistic connectivity, with a strong impact on our possibility to model hydraulic properties.

Here we report on a comparative experiment where we have (i) reconstructed a 3D deterministic fracture network, based on rich outcrop data (Cretaceous platform limestones from Cava Pontrelli, Puglia, Italy), and stochastic DFNs with the same statistical parameters, and then (ii) we have modelled hydraulic properties with different semi-analytical (e.g. Oda method) and numerical methods (e.g. finite volumes implemented in DFNWorks).

Our preliminary results suggest that more advanced numerical methods are more sensitive to the quality of input data than simple semi-analytical methods. This is explained by the fact that for instance the Oda method simply ignores topology, connectivity, fracture height/length ratio and other important parameters.

How to cite: Favaro, S., Facci, M., Casiraghi, S., Mittempergher, S., and Bistacchi, A.: Comparing the impact of deterministic and stochastic fracture networks on modelling hydraulic properties, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16743, https://doi.org/10.5194/egusphere-egu25-16743, 2025.

Switzerland aims to reach net-zero CO₂ emissions by 2050 (BAFU, 2021). Implementation of geological storage of CO₂ and geothermal heat mining to help achieve this aim requires reservoir properties to be assessed. The structural and permeability architectures of the target reservoirs are essential input for numerical models used to assess storage and production potential, minimize fluid injection and extraction uncertainties, and reduce exploration risks. The so-called Muschelkalk aquifer (Triassic) including the Schinznach Formation is regarded as one of the key potential aquifers for gas storage and hydrothermal geothermal systems in Switzerland (Chevalier et al., 2010). While the matrix permeability of the Schinznach Formation is relatively well known (Diamond et al., 2019), magnitude and distribution of its fracture permeability and structural controls on these fractures are poorly understood.

This study aims to assess the style and intensity of natural fracture networks in the Muschelkalk aquifer at sub-seismic scale and explain their regional variability. Outcrop analogues in the Wutach Gorge of southern Germany are used to improve understanding of lateral and vertical variability of fracture networks, including how they are influenced by regional structures. The Wutach Gorge is within the Tabular Jura and provides cliff exposures along 5–12 km-long E–W and N–S transects, aligning with and crossing fault strands of the Freiburg–Bonndorf–Bodensee Fault zone.

Insights from field observations contribute to ongoing work that supports the proposed pilot CO₂ injection test into the Schinznach Formation via an existing exploration borehole at Trüllikon in northern canton Zurich. A feasibility study (Diamond et al., 2023) assessed the reservoir properties at Trüllikon by building discrete fracture network models and computing their permeabilities from a combination of rock-matrix properties, vertical drill hole fracture logs, a horizontal fracture log from another nearby drill hole, and results from hydraulic tests. This multidisciplinary approach should provide a more robust basis for exploration for CO₂ storage sites and geothermal energy.

REFERENCES 

Chevalier, G., Diamond, L. W., & Leu, W. (2010). Potential for deep geological sequestration of CO2 in Switzerland: a first appraisal. Swiss Journal of Geosciences, 103, 427-455.

Diamond, L. W., Alt-Epping, P., Brett, A.C., Aschwanden, L. and Wanner, C. (2023) Geochemical–hydrogeological study of a proposed CO2 injection pilot at Trüllikon, Switzerland. Report 2023-7 submitted to the Swiss Geological Survey (swisstopo). Rock Water Interaction, University of Bern, 87 pp. https://doi.org/10.5281/zenodo.10938102

Diamond L.W., Aschwanden, L., Adams, A., and Egli, D. (2019) Revised potential of the Upper Muschelkalk Formation (Central Swiss Plateau) for CO2 storage and geothermal electricity. Slides of an oral presentation at the SCCER-SoE Annual Conference at EPFL-Lausanne, 4th Sept. 2019. 13 pp. http://static.seismo.ethz.ch/sccer-soe/Annual_Conference_2019/AC19_S3a_08_Diamond.pdf

BAFU (2021) Switzerland Long-Term Climate Strategy. 4 pp. https://www.bafu.admin.ch/dam/bafu/en/dokumente/klima/fachinfo-daten/langfristige-klimastrategie-der-schweiz.pdf.download.pdf/Switzerland's%20Long-Term%20Climate%20Strategy.pdf

How to cite: Brett, A. C., Caldeira, J., Samsu, A., Diamond, L. W., and Madritsch, H.: Fracture networks in the Muschelkalk aquifer of the external northen foreland of the Central Alps (CH/DE); implications for permeability, CO2 storage and geothermal potential, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8885, https://doi.org/10.5194/egusphere-egu25-8885, 2025.

Abstract: Fracture propagation dynamics in complex geological formations is crucial for understanding cracking mechanism of deep rock and facilitating subsurface reconstruction and resource extraction. However, predominant mechanism-driven numerical model exist several inherent limitations: 1) Over-reliance on empirical formulas and simplified hydraulic fracture propagation model with multi-assumptions restrict its capacity for effectively characterizing the multi-physics coupling in 3-D space, thereby reducing the accuracy of fracture morphology. 2) Computational schemes such as finite element method (FEM) or discrete element method (DEM) involving extensive repetitive calculations, are resource-intensive and exhibit poor temporal efficiency, posing a challenge to engineering requirements. Therefore, a data-model-interactive neural proxy model combining the prior-knowledge from mechanism models and fitting efficiency of deep neural networks, is put forward to depict the fracture propagation dynamics in complex geological formation. Initially, a numerical model for fracture propagation is developed by implementing the 3-D discrete lattice method alongside the elastic-plastic constitutive equation. The coupling of rock deformation and fluid flow is iteratively processed in a stepwise manner to generate a sequence of fracture morphology evolution over time. These mechanism data will provide training samples for the subsequent neural proxy model. Secondly, the efficacy of the neural proxy model is contingent upon the richness and diversity of features presented in the training dataset, necessitating a close approximation of all conceivable scenarios. In light of the irregular spatial distribution of data resulting from the complex geological formation with strong heterogeneity, the Latin Hypercube sampling method is employed to ensure a uniform selection of all conditions, mitigating the potential data imbalance. Furthermore, the integration of numerical results with empirical measurements is employed to train the developed deep-neural networks, fitting high-dimensional mapping relationships among formation physical parameters, engineering parameters, and fracture morphology. Finally, the efficiency and the accuracy of the proposed method are verified by multi-level comparison experiments between real data and simulation results. Our research provides reliable technical support for rapid evaluation of formation fracturing potential in field and guidance of development process.

Keywords: Fracture propagation, Neural proxy model, Deep learning, Numerical simulation, Deep-formation

How to cite: Zhang, F., Tang, J., Fan, Y., Yang, J., Li, J., Chen, W., Wang, H., and Jia, Y.: Fracture Propagation Dynamics Predicted by Data-model-interactive Neural Proxy Model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14719, https://doi.org/10.5194/egusphere-egu25-14719, 2025.