Experimental and numerical investigation of microbial growth in two-phase saturated porous media at the pore-scale

Efficient long-term storage of energy is of crucial importance for an economy which is completely based on renewable energies. Subsurface storage of green hydrogen could contribute substantially to reaching this goal. However, the injection of hydrogen into the subsurface could lead to an increased activity of microorganisms which results in gas conversion and an increase in biomass. In this work, the growth of methanogenic microorganisms was studied by a combined experimental and numerical modeling approach. For the experiments, artificial porous structures between two glass plates, referred to as glass-silicon-glass micromodels, were used. These transparent quasi-two-dimensional micromodels allow the direct observation of microbial processes by microscopic analysis. Experiments were performed under static and dynamic conditions to get a detailed insight into the temporal and spatial dynamics of the microorganisms. The experiments were accompanied by two-dimensional reactive transport modeling to further improve the understanding of microbial dynamics. The model takes into account gas and water as phases and the diffusive transport of the substrate inside both phases. A Monod model is used for describing the growth of microbes inside a partially saturated porous medium. The experimental and simulated data are in very good agreement. It has been shown that during the static experiments, nutrient-limited growth inside the liquid phase of the porous medium takes place. However, during dynamic experiments with a re-supply of nutrients, the microbial density quickly reaches a maximum near the gas/liquid interface. Growth is continuous but much slower further away from this interface. The study shows new substantial findings which can serve as a basis for developing improved models on the continuum scale and can be used to optimize the management of long-term storage systems in deep reservoirs.