(25d) Solubility of Gases in Brine for Underground Gas Storage Application: Experimental Measurements and Modeling

The conversion of electricity surplus into H₂ by electrolysis of H₂O is one of the best solutions for the storage of intermittent renewable energies. Methanation is a catalytic reaction to produce CH₄ from a mixture of CO₂ and H₂, and energy supply. This technology known as "power-to-gas" can be integrated into an EMO (Electrolysis-Methanation-Oxycombustion) unit for energy recovery while reducing CO₂ emissions by keeping it in a closed loop [1-3]. Due to the temporal differences between the production and use of gases (different possible scenarios), this EMO unit requires the temporary storage of large quantities of gases involved in the process such as CH₄, H₂, O₂ and CO₂. This is just one example of several industrial applications that require large quantities of gases; other examples include power generation, chemical, petrochemical and pharmaceutical industries, etc. The most suitable technique for storing these large amounts of gases is reversible storage in salt caverns (Advantages: high volume, high pressure, good salt tightness, and safe storage, etc.). This technique is considered as the second most commonly used form of storage in the world for several types of fluids (natural gas, hydrogen, compressed air, ethylene, etc.) [4]. The presence of electrolytes (NaCl, KCl, MgCl₂, etc.) dissolved in the residual water of the cavern has a non-negligible impact on the thermodynamic behavior of the stored gas (salting-out effect and reduction of water content).

The design and optimization of the storage facility, as well as the monitoring of temperature, pressure and gas quantity in the caverns, require the knowledge of phase diagrams (operating condition mainly with CO₂) and the use of a very accurate thermodynamic model under the operating conditions of storage [5] and transport (hydrate risk in the case of CH₄ and CO₂). The development of thermodynamic models for such systems (gas-water-salt) requires the availability of reliable experimental solubility data.

Several experimental techniques have been used to study the phase behavior of gas-water-salt systems. Herein we compare the results obtained using two different techniques: the first one is based on the static-analytic method (phase sampling and GC analysis) [5] and the second one is the one presented by Ahmadi and Chapoy 2018 [6] (liquid phase sampling, gas/brine separation, brine gravimetric analysis and gas volumetric analysis with gasometer). Experimental results of CO₂-H₂O-NaCl system obtained with both methods and literature data of gas-water and gas-brine systems are modeled with recently developed models based on asymmetric (gamma-phi) and symmetric (phi-phi) approach: a modified version of the Soreide and Whitson "m-SW" model, the electrolyte Peng-Robinson Cubic Plus Association "e-PR-CPA" and a geochemical model [5, 7]. An excellent description of the data can be obtained with the three models over a wide range of state conditions.

References

1. Bailera, M., et al., Decision-making methodology for managing photovoltaic surplus electricity through Power to Gas: Combined heat and power in urban buildings. Applied energy, 2018. 228: p. 1032-1045.
2. ANR. FLUIDSTORY project : Massive and reversible underground storage of fluids (O2, CO2, CH4) for energy storage and recovery. 2016-2020; Available from: http://www.agence-nationale-recherche.fr/Project-ANR-15-CE06-0015.
3. Kezibri, N. and C. Bouallou, Conceptual design and modelling of an industrial scale power to gas-oxy-combustion power plant. International Journal of Hydrogen Energy, 2017. 42(30): p. 19411-19419.
4. LAHAIE, F., P. GOMBERT, and C. BOUDET, "Le stockage souterrain dans le contexte de la transition énergétique" Maîtrise des risques et impacts. 2016.
5. Chabab, S., et al., Thermodynamic study of the CO2 – H2O – NaCl system: Measurements of CO2 solubility and modeling of phase equilibria using Soreide and Whitson, electrolyte CPA and SIT models. International Journal of Greenhouse Gas Control, in press 2019.
6. Ahmadi, P. and A. Chapoy, CO2 solubility in formation water under sequestration conditions. Fluid Phase Equilibria, 2018. 463: p. 80-90.
7. Corvisier, J. Modeling water-gas-rock interactions using CHESS/HYTEC. in Goldschmidt Conference, Florence–Italy. 2013.