POREMECH: Natural energy carriers and barrier rocks

Porous sedimentary rocks are of great importance for numerous technical applications like the efficient production of natural energy carriers (oil and gas), the utilization of geothermal energy, as well as the secondary storage of energy carriers (e.g., hydrogen) or climate active pollutants (CO2) in geological formations. The suitability of such geological production and storage targets for the aforementioned purposes is strongly controlled by the pore space characteristics of the respective rocks, delivering storage space for the liquid and gaseous fluids of interest. Furthermore, the natural barrier effect of fine- grained sedimentary rocks is controlled by their capillary behaviour as well. This project aims at a better understanding of the interdependency of different geological factors (e.g., the type of depositional environment and resulting sediment composition) and the preservation of primary accessible (effective) porosity within the sediment. Furthermore, secondary changes in pore space distribution during ongoing burial of the sediment in the subsurface will be covered. To do so, a broad array of modern analytical techniques will be applied. High-resolution imaging techniques such as transmission electron microscopy, scanning electron microscopy and micro computed tomography allow a spatially resolved characterization of pores, as well as the calculation of geometry factors, etc. even at smallest length scales (micrometers to nanometers). Highly resolved structural information can furthermore be derived from petrophysical techniques such as gas adsorption or small angle X-ray/neutron scattering tests. By the application of theoretical models to the acquired data, pore size distributions down to the nanometer- scale can be obtained. Furthermore, these techniques deliver knowledge on the relative adsorptive storage capacity for different gas molecules, an important information with respect to secondary storage purposes. Finally, the pore space development in sedimentary rocks is strongly controlled by the prevailing geomechanical stress regime, which gradually changes during deep burial within a sedimentary basin, but is also influenced by other geological factors such as tectonic reactivation or hydrocarbon generation. Therefore, micromechanical models will be established in order to test the sensitivity of pore space characteristics to changing effective stress conditions. These models will be based on empirical data collected in micromechanical tests (nano-indentation), which allow the determination of single-phase mechanical properties of rock minerals and organic constituents at sub- micrometer scale. In summary, this project should deliver a comprehensive understanding of the pore space evolution in sedimentary rocks as a response to a complex geological framework.

Fine-grained sedimentary rocks are essential to geoenergy applications, particularly as geological barriers for subsurface storage of energy carriers (e.g., hydrogen), carbon dioxide, or nuclear waste. The suitability of such rocks (e.g., mudstones) as "seals" is determined by geological processes that control their microstructural and -mechanical properties. POREMECH aimed at developing workflows to investigate these processes on a fundamental level, thereby answering many questions related to the energy transition. The five main aspects addressed are i) the relationship between changes in microstructure and seal capacity during sediment burial in the subsurface, ii) the micromechanical response to compaction and resulting fracturing properties, iii) the simulation of rock-fluid interactions in seal layers during geological storage of hydrogen, iv) the suitability of unmined coal seams to safely store CO2, and v) the hydrocarbon transport mechanisms in organic-rich source rocks (e.g., for shale oil/gas production), as well as their potential for new primary energy carriers (e.g., natural hydrogen formed at high temperature conditions). All these points have direct impact to the general public as the safe production and storage of energy carriers, as well as the role of "geological containers" in decarbonization efforts, are the central challenges of our global society. By the comprehensive characterization workflows developed in this project, important gaps could be filled in the prediction of fluid breakthrough through mudstone layers; mathematical models could be derived from the experimental data sets to describe the changes of seal capacity with burial compaction of the sediments. Induced chemical changes in the rocks during hydrogen storage could be quantified. While such changes, particularly carbonate dissolution, are evident, the overall suitability of seal rocks from select storage targets could be proven in pressure reactor tests that are able to mimic the in-situ subsurface conditions. The potential of deep coal seams to capture CO2 by adsorptive storage could be quantified and although coals will not represent a major global storage target compared to other alternatives like saline aquifers or depleted oil and gas fields, it was concluded that they could play a role as local storage targets. It could be proven that the investigated coals are efficient storage media for CO2 due to a combination of different adsorption processes including both physisorption and weak chemisorption. Furthermore, the project contributed to a better understanding of the role of water saturation in coal fracture networks to the resulting gas injection and extraction behavior at microscale. Finally, the results related to the generation characteristics of shale oil and gas improved the understanding of their production efficiency. The project team was part of key studies addressing post-mature hydrogen generation in source rock reservoirs, revealing a substantial hydrogen potential that may represent a new geogenic energy system.