"Porous media are materials that have holes that are connected to each other. For example, liquids or gases such as water or air can be transported through porous media," says Holger Steeb, Professor of Mechanics at the University of Stuttgart, defining the term porous media, which are the focus of his research. He and members of his team - Samaneh Vahid Dastjerdi and Matthias Ruf - are investigating the properties of porous media that engineers or geoscientists need when planning large-scale projects, such as an underground reservoir in a rock formation to store CO2 or hydrogen.
Such a reservoir can have an extension of several hundred to several thousand meters and is usually located at a depth of 500 meters to five kilometers. In order to be able to describe the behavior of the stored substances in the surrounding porous rock, the scientists do not investigate the material properties of the entire rock reservoir - which is hardly possible - but use representative rock samples.
The Porous Media Lab (PML) is an experimental platform that can be used by researchers at the University of Stuttgart and guest scientists interested in characterizing the coupled electro-thermo-hydro-chemo-mechanical properties of porous materials. The data obtained from the PML is then published on DaRUS, the data repository of the University of Stuttgart, where it is freely accessible.
These come from quarries or are recovered from deep boreholes and are typically several millimeters to several centimeters in size. "We then use an X-ray microscope to look at the flow processes in the rock pores, for example, and try to find out which physical phenomena are relevant for the flow process, so that we can then describe the pore space on the scale of the reservoir using suitable models," says Steeb.
The rock samples may contain, for example, very small cracks, i.e. openings in the range of a few micrometers. If these samples are then subjected to mechanical stress, the cracks deform and the material behavior changes. Depending on the type and direction of load, the cracks can open or close, effectively making the rock samples softer or stiffer. "And for us, this means that these small-scale cracks on the micrometer scale can also influence the actual material behavior on the kilometer scale," explains Holger Steeb.
Microfluidics for two-dimensional studies
Scientists at the Porous Media Lab at the University of Stuttgart are investigating these and other small-scale phenomena. Microfluidic studies are conducted to investigate the basic properties of porous media and the flow behavior of fluids, such as various liquids or gases, in the pore space of these materials. Optically transparent microfluidic models that mimic pore structures are used for this purpose. Due to the transparency of these models, traditional light microscopy can be used to directly observe how fluids spread in the quasi-2D pore structure.
Microfluidic studies in the Porous Media Lab
A transparent microfluidic model allows to observe how fluids spread in the pore space. Photos: Tabea Siegle
Close-up of a microfluidic model with a clearly recognizable pore structure.
The microfluidic model is placed on the stage of the open optical microscope. Fluids are supplied and discharged in a controlled manner via the tubes.
Using high-resolution X-ray computed tomography to make slow three-dimensional processes in the rock interior visible
For three-dimensional studies, postdoctoral researcher Matthias Ruf and a former colleague developed a modular high-resolution X-ray computed tomography system, or μXRCT system for short, and set it up at the PML. "With the high-resolution system, we can visualize the inside of the core sample - the pores and the surrounding rock - with a spatial resolution of less than ten micrometers," says Matthias Ruf. Ten micrometers (μm) is 0.01 mm. Sandstone, for example, is often found as a sedimentary reservoir rock in geothermal reservoirs.
"The pore space of a sandstone contains pores of around ten to perhaps 100 micrometers in size, which is 0.01 to 0.1 millimeters. These orders of magnitude can only be observed with sufficient precision using high-resolution X-ray computed tomography," Holger Steeb explains. He and his team have been designing and setting up the µXRCT system since 2015, as there were no devices on the market at the time that could also be used to conduct "in-situ" experiments during the actual scanning process. "We were aiming at not only observing the pore structure of these materials, but also performing physical flow experiments, for example on multiphase flow under the conditions that are also found in geothermic," says Holger Steeb.
Among these conditions are temperatures of up to 100 degrees and very high ambient stresses and fluid pressures of up to 50 MPa (megapascals) which are acting on the rock at deep layers. These conditions are simulated in the laboratory in special high-pressure triaxial cells. "We produce confining pressures up to 50 MPa.” The high-pressure triaxial cells are X-ray transparent, which means they allow the X-rays to pass through to the sample. This allows the scientists to get an idea of what is actually happening on site in a reservoir.
The µXRCT data provides them with a three-dimensional image of the rock's microstructure. "But we can do even more. Not only can we evaluate the three-dimensional data sets and view the images, we can also use this information to perform numerical simulations directly on the image data, for example multiphase flow simulations, which allow us to see what happens in the pores of the real rock, even under conditions that cannot be captured experimentally," Holger Steeb explains.
A journey through sandstone from top to bottom: Three-dimensional image of the microstructure of fractured sandstone
A particle accelerator for fast three-dimensional processes
However, when it comes to flow and transport processes that take place quickly, so-called transient processes, a different method is required to capture these using 3D imaging. "If we are interested in a two-phase flow process, for example if we want to know how water displaces oil in the pore space, then this is a transient, time-dependent process," says Steeb. "You have to imagine it like a movie. And we also want to produce these movies in three dimensions."
Transient processes are phenomena that take place very quickly and occur only occasionally and irregularly.
A special X-radiation, known as synchrotron radiation, is needed to record these rapid processes. A synchrotron is a particle accelerator that generates X-rays with a very high power and brilliance. "This means that a scan that would sometimes take several hours in the Porous Media Lab only takes a few seconds there," says Matthias Ruf. "3D imaging in the synchrotron is many times faster. This means we can also visualize and observe faster processes."
These particle accelerators can only be found in large-scale research facilities such as the German Electron Synchrotron DESY in Hamburg, the Diamond Light Source in the UK, and the Swiss Light Source at the Paul Scherrer Institute in Villigen, Switzerland. As a scientist, you can apply for beam time at these major international research institutions and carry out your project once it has been approved. The Stuttgart scientists have often traveled there as a team, with the complete experimental setup such as samples and triaxial cells in their luggage, and conducted their experiments on site in the laboratories.
In the synchrotron, electrons are accelerated to almost the speed of light in a ring-shaped vacuum tube. "By deflecting the electrons to keep them on a circular path, X-rays are generated that can be used to examine samples and the processes taking place in them," Holger Steeb explains. For example, a three-dimensional data set is created every second. The huge data sets, over hundreds of terabytes, are then analyzed by the scientists.
Inside the Swiss Light Source at the Paul Scherrer Institute.
Image: (c) Matthias Ruf
The Stuttgart scientists in the control room of the TOMCAT beamline at the Swiss Light Source of the Paul Scherrer Institute.
Image: (c) Paul Voland (MIB)
The scientists are examining their results from the synchrotron measurements at the TOMCAT beamline of the Swiss Light Source of the Paul Scherrer Institute.
Image: (c) Paul Voland (MIB)
Breakthrough in understanding complex flow and transport phenomena
And of course, this also requires special numerical methods. "We are, so to speak, at the interface between experiment, modeling, and simulation. The insights we gain from our experiments, both the imaging and the classical physical results, are incorporated into the models. This improves the models and leads to more accurate predictions," says Holger Steeb. This basic research on porous media is primarily relevant for the scientific community in order to improve models.
A hysteresis effect is an effect that does not occur immediately but with a delay, and depends not only on the input variable but also on the initial state of a system.
"Multiphase flow in porous media contain a variety of phenomena that are not sufficiently understood, such as the mixing of two fluid phases or hysteresis effects in cyclic displacement processes. This is a shortcoming and has a corresponding impact on the quality of predictions," explains Holger Steeb. With the data obtained from experiments in the synchrotron, Holger Steeb and his team of scientists were able to achieve a breakthrough in understanding complex flow and transport phenomena in porous media and uncover misunderstandings in modeling approaches. The integration of the findings from the experiments into improved mathematical models led to accurate simulations.
In their research, Holger Steeb and his team also benefit from the environment at the University of Stuttgart. "Stuttgart has a special position internationally, particularly in the field of porous media. We have a very strong and very large research environment: There are not many locations in the world where so many scientists are working in this relatively specialized field," says Holger Steeb. They also come from very different disciplines. "For example, we have the specialty that mathematicians work closely with physicists and engineers, such as in the Collaborative Research Center SFB 1313. They are, in turn, supported by computer scientists who are using special visualization methods to make phenomena visible that are usually not so easy to see. So that's a Stuttgart specialty."
Manuela Mild | SimTech Science Communication
Read more
Shokri, J., Ruf, M., Lee, D., Mohammadrezaei, S., Steeb, H., & Niasar, V. (2024). Exploring carbonate rock dissolution dynamics and the influence of rock mineralogy in CO2 injection. Environmental Science & Technology, 58(6), 2728–2738. https://doi.org/10.1021/acs.est.3c06758
Taghizadeh, K., Ruf, M., Luding, S., & Steeb, H. (2023). X-ray 3D imaging-based microunderstanding of granular mixtures: Stiffness enhancement by adding small fractions of soft particles. Proceedings of the National Academy of Sciences (PNAS), 120(26), e2219999120. https://doi.org/10.1073/pnas.2219999120
Vahid Dastjerdi, S., Karadimitriou, N. K., Hassanizadeh, S., & Steeb, H. (2023). Experimental evaluation of fluid connectivity in two-phase flow in porous media. Advances in Water Resources, 172, 104378. https://doi.org/10.1016/j.advwatres.2023.104378
Vahid Dastjerdi, S., Karadimitriou, N. K., Hassanizadeh, S., & Steeb, H. (2022). Experimental evaluation of fluid connectivity in two‐phase flow in porous media during drainage. Water Resources Research, 58(11), e2022WR033451. https://doi.org/10.1029/2022WR033451
Chen, Y., Steeb, H., Erfani, H., Karadimitriou, N. K., Walczak, M. S., Ruf, M., Lee, D., An, S., Hasan, S., Connolley, T., Vo, N. T., & Niasar, V. (2021.) Nonuniqueness of hydrodynamic dispersion revealed using fast 4D synchrotron x-ray imaging. Science Advances, 7(52), eabj0960. https://doi.org/10.1126/sciadv.abj0960
Hasan, S., Niasar, V., Karadimitriou, N. K., Godinho, J. R. A., Vo, Nghia T., An, S., Rabbani, A., & Steeb, H. (2020). Direct characterization of solute transport in unsaturated porous media using fast X-ray synchrotron microtomography. Proceedings of the National Academy of Sciences (PNAS), 117(38), 23443-23449. https://doi.org/10.1073/pnas.2011716117
About the scientists
Samaneh Vahid Dastjerdi is working as a postdoctoral researcher in the CRC 1313 at the University of Stuttgart. She studied civil engineering at the Isfahan University of Technology and obtained a Master of Business Administration at the University of Tehran as well as a Master's degree in "Water Resources Engineering and Management" at the University of Stuttgart. In 2024, she completed her doctoral degree studies at the Institute of Applied Mechanics under the supervision of Holger Steeb in the SimTech project 1-4 (II). In her dissertation "Image-based characterization of multiphase flow in porous media", she dealt with the characterization of multiphase flow in porous structures by means of microfluidic studies.
Matthias Ruf is a postdoctoral researcher in the CRC 1313 at the University of Stuttgart. He has a Bachelor's degree in aircraft construction and economics and a Master's degree in mechanical engineering. In 2023, he completed his doctoral degree studies at the Institute of Applied Mechanics (MIB) at the University of Stuttgart under the supervision of Holger Steeb. Within the framework of his dissertation "Experimental multi-scale characterization using micro X-ray computed tomography", he developed a modular, open μCT system that can be used for a wide range of multi-physical, multi-scale problems and serves as an open experimental platform in the Porous Media Lab.
Holger Steeb is a professor at the Institute of Applied Mechanics (MIB) at the University of Stuttgart. He is also Director of the Stuttgart Center for Simulation Science (SC SimTech) and spokesman for the Collaborative Research Center (SFB) 1313 "Interface-Driven Multi-Field Processes in Porous Media - Flow, Transport and Deformation". His research focuses on porous media and functional materials. He founded the Porous Media Lab (PML) in 2015 with the idea of setting up a "shared lab" for researching porous media at the University of Stuttgart. In addition to Holger Steeb's team, numerous other scientists from other institutes as well as guest scientists from Germany and abroad are using the PML for their own research. The experimental setups are designed by the researchers themselves, often in cooperation with Holger Steeb and his team. In this way, the PML is constantly evolving and guarantees up-to-date research.
