"The energy transition requires large-scale energy storage. Underground hydrogen storage in porous reservoirs is a relatively new concept, having been introduced only four or five years ago. Therefore, there are not many measurements for the interaction of hydrogen, brine and rock," explains Maartje Boon, SimTech Junior Professor for Advanced Methods in Porous Media at the Institute of Applied Mechanics (MIB) at the University of Stuttgart. "However, we know that we can store gases underground, as natural gas has been stored in porous reservoirs since the 1960s," she adds. CO2 has also been stored underground for more than 20 years to reduce its amount in the atmosphere. The CO2 is intended to remain in the ground permanently, while hydrogen as an energy source is only stored temporarily so that it can be extracted when needed, for example before winter.
Hydrogen (H) is the most abundant element in the universe and the lightest chemical element. In nature, it occurs mainly in a bonded form, for example in water (H₂O) and in organic compounds. Natural (white) hydrogen occurs in considerable quantities underground, but its formation and transport through the Earth's crust are still largely unexplored.
Due to the low density of hydrogen, storage requires a volume that far exceeds the capacity of above-ground facilities. Geological formations such as salt caverns, depleted hydrocarbon reservoirs and saline, groundwater-bearing rock layers (aquifers) are suitable.
Underground porous reservoirs often consist of layers of sandstone. Gases or liquids are then injected into the pores of the sandstone. However, since gases are often very light and rise to the top, there must be a seal above the sandstone layer. This can be another layer of rock that acts as a barrier, with very low permeability and porosity, i.e. as few voids as possible.
The reason why hydrogen is so interesting for the energy transition is because of its properties as an energy carrier. Hydrogen has a very high energy density, making it a very good energy storage medium. When hydrogen burns, it can be converted into energy without producing climate-changing gases, and hydrogen can be stored for a long time. “If we would inject hydrogen into a porous reservoir, we are interested in how much of the hydrogen - and therefore energy - we can get out of the reservoir again,” says Maartje Boon. She is therefore investigating how hydrogen behaves in underground reservoirs, how it moves, what factors influence its recovery and how it interacts with the environment. She then wants to use simulation models to find out whether underground storage is feasible and whether the safety of a reservoir can be guaranteed.
The wettability indicates how gases or liquids behave when they come in contact with a surface. If the system is hydrophylic, the liquid will spread; if the it is hydrophobic, the liquid will roll off. Wetting can be characterized by the contact angle of droplets.
In order for the simulation to produce meaningful results, the models require various input parameters. These must correctly describe the behavior of the hydrogen-brine-rock system and take into account the effects of the heterogeneity of the subsurface. Such input parameters include relative permeability, i.e. the relative permeability of the gas when it flows together with the brine through the rock, or the capillary pressure required to force the gas through a brine filled pore. There wasalso little reliable data on wettability in such a hydrogen-brine-rock system, up to a couple of years ago. "These parameters can be measured in the lab, and since a couple of years many researchers are working on this," says Boon.
Experimental work in the lab
Much of her work therefore takes place in the laboratory, where she and her team conduct experiments. "We then try to simulate these experiments on the computer and use them to derive the input parameters for our models," she explains. The Porous Media Lab at the University of Stuttgart offers the best conditions for this. Here, the experiments can be carried out under controlled conditions using specially designed setups. To determine wettability, for example, the contact angles of the droplets on the rock surfaces have to be measured.
The scientists in Maartje Boon's team have built a small cell that is about 15 cm high and has two windows. "This cell contains a substrate. It can withstand pressures of up to 500 bar and temperatures of up to 270 degrees Celsius," explains Maartje Boon. The experiments are performed at various pressures and temperatures. Inside the cell, bubbles and droplets are released on which the scientists carry out their measurements and investigate what happens when gas or liquid comes into contact with the surface of the substrate.
The substrate in the cell is made out of pure quartz. “Sandstone rock mostly consists of quartz. It doesn't have a porous surface. We start with the simplest system, which is just bubbles and droplets. We then move on to a more complicated system where we incorporate a porous structure and look at flow experiments," explains Maartje Boon. A built-in camera takes high-resolution images that show how these bubbles and droplets are in contact with the substrate. The scientists are interested in whether the quartz prefers contact with the salt solution, i.e. the brine, or with the gas.
Core flooding is an experimental method in geoscience where liquids and gases are forced through a rock core to study the flow properties and distribution in porous media. Parameters such as permeability, porosity and degree of saturation can be derived.
Initial basic findings
In a previous project carried out at TU Delft, Maartje Boon studied the interplay between gravitational, capillary and viscous forces. These occur when hydrogen is injected into or removed from a reservoir. In a so-called core flooding experiment, she performed the measurements on a water-soaked rock core made of Berea sandstone.
The scientists wanted to understand how the hydrogen flows in the sandstone core and how it behaves during injection. This is crucial for the safety and efficiency of underground hydrogen storage in porous, water-saturated geological formations. X-ray computed tomography was used to visualize the distribution of hydrogen.
It was shown that complex displacement processes occur in the underground hydrogen storage due to the interaction of gravitational, capillary and viscous forces in combination with dissolution processes. Small high water saturation fingers can form, which develop into channels through which water flows, displacing the hydrogen. The hydrogen can be trapped outside the channels and is thus lost as an energy source. The results have contributed significantly to the fundamental understanding of how hydrogen and water behave when transported through porous media.
Heterogeneous rock layers with different flow behavior
Building on this, her current research aims to understand how differences in rock structure affect the flow of hydrogen when it is injected into a reservoir. "Now we want to find the parameters that capture the effects of the heterogeneity of the different layers in the subsurface on the flow and gas trapping," she adds. For this reason, the next step is to conduct experiments in the Porous Media Lab with different heterogeneity structures.
These heterogeneities exist in the reservoir at many different length scales, from the small pore structure at the micrometer scale to the kilometer field scale. All have a significant impact on the flow behavior of hydrogen through the rock. By combining these results with numerical and analytical modeling tools, Maartje Boon hopes to find effective parameters that correctly capture the behavior in the reservoir.
Microbes also get involved
There is another factor that complicates the calculations: hydrogen can be used by certain microbes in the reservoir for their metabolism. They eat the hydrogen to meet their energy needs. That part is lost for energy production. "But not only that. When the microbes eat hydrogen, they also grow a lot. And when microbes grow strongly, they form biofilms that can block the pore space in the reservoir," explains Maartje Boon. This not only makes it more difficult to inject the hydrogen, but also has a major impact on how it can be removed from the reservoir.
The results of another study by Maartje Boon showed that on a Bentheimer sandstone with a rough surface, the biofilm had mainly settled in the cracks and the contact surface with the hydrogen remained unchanged. On a quartz stone with a smooth surface, however, the biofilm covered the entire surface, resulting in a change in wettability. Therefore, the effects of microbial activity on wettability during underground hydrogen storage can only be determined if the conditions of the reservoir are exactly reproduced. In addition, the hydrogen extracted from the reservoir must be of a certain purity so that it can be reused directly. The purity can be greatly reduced by microbes, but also by mixing with other gases already present in the reservoir.
All of these issues need to be studied to determine whether it is technically feasible and reasonable to store energy in the form of hydrogen in underground porous reservoirs - especially when a large portion of the hydrogen could be lost in the process. "Because the idea is so new, there are still a lot of unanswered questions, and that's what makes it so appealing to work on this research topic. There are so many things that have not yet been studied, and there are many new things, such as microbes, that can be studied further," concludes Maartje Boon.
Manuela Mild | SimTech Science Communication
Read more
Boon, M., Rademaker, T., Winardhi, C.W. et al. Multiscale experimental study of H2/brine multiphase flow in porous rock characterizing relative permeability hysteresis, hydrogen dissolution, and Ostwald ripening. Sci Rep 14, 30170 (2024). https://doi.org/10.1038/s41598-024-81720-4
Boon, M., Buntic, I., Ahmed, K. et al. Microbial induced wettability alteration with implications for Underground Hydrogen Storage. Sci Rep 14, 8248 (2024). https://doi.org/10.1038/s41598-024-58951-6
Boon, M., & Hajibeygi, H. (2022). Experimental characterization of H2/water multiphase flow in heterogeneous sandstone rock at the core scale relevant for underground hydrogen storage (UHS). Scientific Reports, 12(1), Article 1. https://doi.org/10.1038/s41598-022-18759-8
About the scientist
Maartje Boon has been a junior professor for “Advanced Methods in Porous Media” at the Institute of Applied Mechanics (MIB) and the Stuttgart Center for Simulation Science (SC SimTech) at the University of Stuttgart since September 2023. The Dutchwoman received her master's degree in hydrogeology from the University of Utrecht and her doctorate in the field of geological storage of CO₂ from Imperial College London.
She continued her research at Stanford University. Before coming to Stuttgart, she also spent two years as a postdoctoral research fellow at TU Delft. There she worked on underground hydrogen storage in porous media and the characterization of hydrogen transport from the pore to the field scale.
In Stuttgart, she is now working in the SimTech Cluster of Excellence (link) and the SFB 1313 on modeling the subsurface in hydrogen reservoirs. She is fascinated by the fact that in her research she can work on real, highly topical problems and make a difference. She also loves working in an international environment with colleagues from different countries. She herself has been to Russia for her Master's thesis, to England for her PhD, and to the United States as a postdoctoral research fellow.
