Reactive Transport Modelling of H2 Storage in Depleted Gas Fields: An Approach to Implement Biogeochemical Reactions in a Compositional Reservoir Simulator
Authors: Elgendy Ahmed M.S. , Pizzolato Alberto , Maniglio Marco , Geloni Claudio , Panfili Paola , Topini Caterina 
Published: June 2023

Abstract
The demand for large scale energy storage has been increasing for the integration of highly fluctuating energy production from renewables. Depleted gas fields are among the most suitable candidates for underground hydrogen storage, with well-known high-quality reservoir petrophysical characteristics, huge storage capacities and good sealing. However, biogeochemical interactions of hydrogen with rock-brine-resident gas could lead to hydrogen degradation as it is a favoured substrate for many anaerobic microorganisms. Thus, reservoir-scale predictive tools able to simulate these complex and tightly coupled physical, chemical, and biological phenomena are necessary for better investment decisions.
A novel approach to model underground hydrogen storage biogeochemical reactions in a commercial compositional reservoir simulator is presented, tested, and analyzed. The significance of this work is the inclusion of bacterial exponential growth and decay in the numerical models which is essential for a more realistic prediction of hydrogen behaviour in subsurface. This has been embedded in a well-known reservoir simulation tool, GEM unconventional and compositional reservoir simulator, frequently used in the oil and gas industry for subsurface 3D problems. First, a conceptual biogeochemical model was conceived, and the underlying reactions were identified. The reaction mechanisms allow to consider the tight coupling between biochemical and geochemical processes. Then, a set of numerical cases, based on the conceptual biogeochemical model, were simulated in batch mode using two software: PHREEQC geochemical code and GEM reservoir simulator. The cases follow a step increase in the model complexity by adding bacterial growth and decay. GEM does not support the Monod kinetics which describes the microorganism's growth; thus, a tuning of the Arrhenius equation parameters was performed to match the Monod formula over the substrate(s) concentrations of interest. Finally, the Arrhenius formulation was further customized to include bacterial exponential growth and decay by an adequate bacterial stoichiometry implementation in which the bacteria was defined as molar aqueous component.
The numerical simulations proved that a properly tuned Arrhenius kinetic model may reproduce the Monod dynamics with acceptable accuracy. In addition, for the most complete and complex case (D), GEM results show a good benchmark with PHREEQC ones, attesting the fact that a properly customized Arrhenius model, integrating the kinetics of both substrates and bacteria, and being modelled with a single (or two if decay is also considered) stoichiometric reaction, is able to appropriately capture underground hydrogen storage biogeochemical reactivity. In the cases considered, results show that the geochemistry has a limited impact on the biochemical process. However, the impact depends on pure geochemical limiting factors, i.e., presence of free protons. The study recommends that the estimation of kinetic parameters of biological processes (e.g., Methanogenesis) should be prioritized in future experimental campaigns to better understand their influence on underground hydrogen storage.