Model development for microbial growth in underground hydrogen storage reservoirs

The topic of underground hydrogen storage brings microbial processes into the focus. Underground porous structures provide excellent living conditions for microorganisms because their habitat is protected but still allows a supply of substrates and nutrients. As a consequence, microorganisms exist and make a substantial contribution to many subsurface porous media systems, as e.g. oil and gas reservoirs and underground gas storages. Microbial effects can be negative, as e.g. decreased well injectivity and losses of the stored energy, however, they can also be positive as in microbial enhanced oil recovery (MEOR) and underground methanation reactors (UMR). To control the microbiological effects it is important to understand their interactions with flow and transport processes wherefore numerical modeling is a strong tool. In this work a new mathematical model was developed which takes into account the situation where the growth of the microbial population is not only limited by the substrate supply. An additional limiting factor has been introduced that allows growth to approach a maximum microbial density. This effect is related to observations in laboratory growth experiments. The bio-reactive transport model is composed out of compositional balance equations which describe the advective and diffusive transport for a two-phase system on Darcy scale. An additional balance equation is required for the microbial population, which is described using a substrate-limited growth model based on the Monod equation. The model which couples microbial growth to flow and transport processes was numerically implemented in the open-source porous media simulator DuMu x . An example simulation is shown for an underground hydrogen storage where methanogenic microorganisms are active. The simulation results indicate that in large parts of the storage the substrates (H 2 and CO 2 ) do not become limiting for the microbial growth. As a consequence, the microbial population in these areas grows to a maximum density, leading to a continuous conversion of hydrogen and carbon dioxide into methane. This results in a strong coupling with the flow and transport processes.