The global energy demand is continuously rising and requires a significant transition to reduce greenhouse gases (GHG), reduce the ecological footprint of fossil resources and foster the expansion of renewable energies. The current European energy mix requires a meaningful change to reach the agreed 2 °C target and become a vastly net-zero economy by 2050.
The concept of large-scale underground energy storage, so-called “Geo-Batteries”, could significantly increase the stored energy volume, enable seasonal on and offloading, and balance the increased demand in winter. During hydrogen storage operations in sedimentary rocks, a side-effect of microbial methanation was detected, which led to the concept of underground energy conversion. Hereby, the injected hydrogen (H2) and carbon dioxide (CO2) is converted to methane (CH4) by hydrogenotrophic methanogens. The production of methane by microorganisms from carbon dioxide and hydrogen solves two environmental problems. It captures CO2 cutting down greenhouse gas emissions by forming a sustainable carbon cycle. Additionally, retrieved hydrogen and methane can be used for electricity and heat generation. Pilot projects investigate the controlling parameters to increase significant conversion rates and resolve the impact of biomass accumulation on storage capacity and operational conditions.
Experiments on the pore scale are necessary to describe the behavior of microorganisms and the accompanying chemical reactions. The presence and growth of microorganisms can significantly change a porous medium’s hydraulic properties. This thesis uses a “Digital Twin” approach to resolve the biomass-induced effects on the porous medium by direct numerical simulations. The task is to develop a numerical model to simulate flow in a micro model occupied by microorganisms and verify the porosity-permeability relationship with experimental results. For this, an archean culture, Methanobacterium formicicum (M. formicicum), was used and inoculated in the micro model (MM). The workflow included an improvement of a previously developed and deployed numerical twin. The MM curvature and bifurcating inflow and outflow channels were analyzed to estimate their impact on the solution. Those additional features are necessary to resolve the porosity-permeability development and extract meaningful velocity and stress fields.
The temporal velocity and stress distributions play a significant role in biofilm reconfiguration, preferential flow path formation, and intra-biomass permeability.
The velocity and stress fields were analyzed using histograms and the results revealed that the net accumulation of biomass decreased the hydraulic properties following a power law. As a result, narrower pore channels led to increased velocities and shear stresses in the domain. Moreover, the width of the histograms increased over time and a bimodal distribution of the velocity and stress peaks was detected. This refers to the assumed intra-biomass permeability and is first noticed with the onset of biomass accumulation.
This master thesis attempts to contribute to a continuing understanding of the underground hydrogen storage (UHS) concept. The primary objective of this thesis was to enhance the modeling workflow necessary to simulate velocity and stress fields, which are significant for biomass characterization in porous media.