Geologic CH4 storage with CO2 cushion gas: using dynamic binary diffusion coefficients instead of a global constant in numerical simulations is more precise and results in lower mixing losses

Natural gas storage is currently considered as one key pillar of the EU strategy to ensure security of energy supply. In view of CH₄ storage, Oldenburg [1] demonstrated in a theoretical study that using CO₂ as cushion gas instead of CH₄ has the advantage of increasing natural gas storage capacities of geologic reservoirs by up to 30 %. This is due to CO₂ undergoing a significant density change when its’ critical pressure is exceeded during CH₄ injection. Kühn et al. [2] investigated a comparable scenario with CH₄ gas storage in a closed cycle with CO₂ in one reservoir to temporarily store and reuse wind and solar energy. However, the potential qualitative degradation of the stored CH₄ due to mixing with the CO₂ cushion gas has not yet been sufficiently addressed in terms of the impact of the modeller’s choice of diffusion coefficients. Hence, the present study focuses on a quantitative assessment of the mixing behaviour of CH₄ and CO₂ under consideration of dynamic binary diffusion coefficients in a reference numerical simulation benchmark [1,3]. The TRANSPORTSE numerical simulator [4,5] is used with dedicated measures to mitigate the initially high numerical dispersion, introduced by the benchmark’s relatively coarse grid discretisation. The simulation results show that the mixing region is substantially reduced if dynamic binary diffusion coefficients are applied instead of a global constant for both gas components. Consequently, it is demonstrated that previous numerical assessments of natural gas storage with a carbon dioxide cushion gas overestimate the simulated CH₄-CO₂-mixing area, and thus the calculated mixing losses. Hence, combined gas storage of CH₄ and CO₂ is more efficient than expected so far.

[1] Oldenburg, C. M. (2003) Carbon Dioxide as Cushion Gas for Natural Gas Storage. Energy Fuels 17(1), 240−246. https://doi.org/10.1021/ef020162b

[2] Kühn, M., Nakaten, N. C., Streibel, M., Kempka, T. (2014): CO2 Geological Storage and Utilization for a Carbon Neutral “Power-to-gas-to-power” Cycle to Even Out Fluctuations of Renewable Energy Provision. Energy Procedia, 63, 8044-8049. https://doi.org/10.1016/j.egypro.2014.11.841

[3] Ma, J., Li, Q., Kempka, T., Kühn, M. (2019) Hydromechanical Response and Impact of Gas Mixing Behavior in Subsurface CH4 Storage with CO2-Based Cushion Gas Energy & Fuels 33 (7), 6527-6541. https://doi.org/10.1021/acs.energyfuels.9b00518

[4] Kempka, T. (2020) Verification of a Python-based TRANsport Simulation Environment for density-driven fluid flow and coupled transport of heat and chemical species. Advances in Geosciences, 54, 67-77. https://doi.org/10.5194/adgeo-54-67-2020

[5] Kempka, T., Steding, S., Kühn, M. (2022) Verification of TRANSPORT Simulation Environment coupling with PHREEQC for reactive transport modelling. Advances in Geosciences, 58, 19-29. https://doi.org/10.5194/adgeo-58-19-2022