Research Project B05

The aim of this project is to characterize fractures and fracture networks based on a combination of physical experiments, image-based methods, and numerical simulations. The focus of all investigations lies on the fundamental understanding of hydromechanical coupling effects of (deformable) fluid-saturated fractures embedded in a deformable porous material. Project B05 aims at providing the scientific community with detailed information and further insight into i) the development and evolution of preferential flow paths in fracture networks, ii) the role of asperities in the non-linear effective hydromechanical coupling between fractures, iii) the possibility of back-analyze  the fracture properties in the case of non-linear hydromechanical coupling, iv) the role of leak-off in crystalline rocks, and v) whether the same concepts can be applied on fractures saturated with more than one fluid phase.

Carrara marble & segmentation

In order to characterize sophisticated topology of the fracture network, μXRCT image data of a thermally treated Carrara marble core sample was analyzed. Three conventional and two machine learning segmentation methods were compared, with U-net proving most effective (Figure 1).

A Digital Rock Physics (DRP) workflow was employed to perform XRCT-based Stokes flow simulations on large volumes (up to 1000³ voxels), utilizing a high-performance solver to complement experimental approaches, cf. Krach et al. (2025) .

Additionally, both numerical and experimental investigations of hydro-mechanical coupling in fractures revealed a non-linear flux–pressure relationship under harmonic stimulation. The presence of overtones in both approaches indicates non-linear system behavior, likely resulting from Hertzian contact between fracture asperities during closure, cf. Schmidt et al. (2023) .

Krach, D., Ruf, M., & Steeb, H. (2025). POREMAPS: A finite difference based Porous Media Anisotropic Permeability Solver for Stokes flow. InterPore Journal, 2(1), IPJ260225–7. doi: 10.69631/ipj.v2i1nr39

Schmidt, P., Steeb, H. & Renner, J. Diagnosing Hydro-Mechanical Effects in Subsurface Fluid Flow Through Fractures. Pure Appl. Geophys. 180, 2841–2860 (2023). doi: 10.1007/s00024-023-03304-z

Additionally, high-pressure triaxial experiments on thermally treated Carrara marble core samples assessed permeability and fracture stiffness. Fracture roughness and asperity effects were investigated through combined numerical and experimental approaches. In close collaboration with Z02 a new user-friendly high pressure test rig for maco-cores up to 50 mm in diameter was developed (Figure 4).

In FP 2, this project focuses on developing PDMS-based microfluidic cells to analyze single fractures, generate datasets across varying Reynolds numbers, and evaluate μPIV data from fractured porous models. In collaboration with Projects B02 and Z02, we successfully simulated and visualized flow interactions between free flow and porous media in shallow microfluidic chips (Figure 2).

The project also involved high-resolution μXRCT imaging of fractured rock cores to visualize fractures, morphological changes, and multiphase flow, with contrast agents enhancing segmentation. Together with Project Z02 and the group of Mercator Fellow V. Niasar (University of Manchester, UK), in-situ μXRCT was successfully employed to track fracture morphology changes induced by CO₂ dissolution in two different carbonate rock types (Figure 3), cf. Shokri et al. (2024) .

Javad Shokri, Matthias Ruf, Dongwon Lee, Saleh Mohammadrezaei, Holger Steeb, Vahid Niasar. Exploring Carbonate Rock Dissolution Dynamics and the Influence of Rock Mineralogy in CO₂ Injection, Environmental Science & Technology, 58(6), 2728-2738. 2024. doi:  10.1021/acs.est.3c06758

Additionally, high-pressure triaxial experiments on thermally treated Carrara marble core samples assessed permeability and fracture stiffness. Fracture roughness and asperity effects were investigated through combined numerical and experimental approaches. In close collaboration with Z02 a new user-friendly high pressure test rig for maco-cores up to 50 mm in diameter was developed (Figure 4).

In FP 3, this project aims to characterize fractures and fracture networks through physical experiments, image-based techniques, and numerical simulations, with a focus on hydromechanical coupling in fluid-saturated, deformable fractures within porous materials.

In particular, we aim to investigate fractures in bio-concrete core samples—optimized and studied extensively in Project C04 —alongside sedimentary rocks such as Berea, and Bentheim (Anröchter) sandstone. Although both materials consist of cemented grains with similar pore morphologies, they differ significantly in their mechanical and poroelastic properties, particularly in fracture initiation and failure behavior.

To predict the long-term performance of bio-concrete, we will analyze the evolution of micro- and macro-fractures under cyclic fatigue loading, applying controlled static uniaxial and triaxial stresses conditions, combined with harmonic uniaxial loads. A key tool is the open μXRCT system developed by the project leader’s group, which enables high-resolution (~1.5 μm) imaging under different triaxial stress conditions using an X-ray transparent triaxial cell. While macro-fractures will be clearly visualized, micro-fractures will approach the resolution limit, leading to some uncertainty. To complement this, together with Z02 highly sensitive Dynamic Mechanical Analysis (DMA) tests will be included in-situ.

The results will advance understanding of the mechanical behavior of both rock and bio-concrete over their service life and help improve image-based characterization techniques through advanced hydromechanical experiments. Ultimately, the experimental findings will inform and expand the hybrid-dimensional modeling approaches developed in FP 1 and FP 2.

Contributing to the Salt Vision topic of the overall SFB, this project investigates how precipitation and dissolution processes alter fracture roughness and aperture through chemically induced changes. Building on experimental data from Project Z02 and prior work in FP 2 with Mercator Fellow V. Niasar (University of Manchester, UK), we aim to visualize pore-scale salt precipitation during CO₂ injection under realistic subsurface conditions. These time-resolved (4D) observations will support hybrid-dimensional modeling in FP 3 and contribute to refining our effective stress concept for fractures containing precipitated crystals.

University of Manchester, UK

With Mercator Fellow Vahid Joekar Niasar (University of Manchester, UK) we have been collaborating on calcite dissolution in fractured carbonate rocks under CO₂ injection using lab-based in-situ µXRCT. This has led to a successful beamtime proposal at Diamond Light Source (DLS), UK for 4D µXRCT imaging of salt precipitation and permeability impairment

University of Arizona, USA

Through Z02 and Mercator Vahid Joekar Niasar (University of Manchester, UK) and external partner Bo Guo (University of Arizona, USA), we contribute to the Salt Vision topic.

Université de Lausanne, Switzerland

We collaborate with our external partners Dr. B. Quintal and Dr. L. Pimienta on DMA-based material characterization of fracture states.

Technical University of Crete, Greece

We collaborate with our external partner, Assoc. Prof. Aandreas Yiotis, on pore-scale modeling of multiphase flow. In this model, we aim to investigate the dynamics of two immiscible fluids and reproduce the evolution of flow in porous media under varying Capillary numbers. By uniquely integrating a flow controller with the model, we are able to mimic the injection conditions used in the corresponding experiments.