Exploring Effective Diffusion Coefficients in Water-Saturated Reservoir Rocks via the Pressure Decay Technique: Implications for Underground Hydrogen Storage

Transport in Porous Media, vol. 152

Abstract

Abstract The assessment of gas diffusion in water-saturated rocks is essential for quantifying gas loss and determining the amount of gas that could trigger abiotic and biotic processes, potentially altering fluid and rock properties. Additionally, estimating diffusion coefficients is critical for evaluating the balance between hydrogen generation and dissipation in radioactive waste repositories. This investigation involved experimental determination of diffusion coefficients for various gases both in water and in water-saturated Bentheim, Oberkirchner, Grey Weser, and Red Weser sandstones. Experimental conditions included pressures ranging from 0.2 to 1.0 MPa, consistently maintained at a temperature of 35 °C. The diffusion coefficients of hydrogen, helium, and methane in water were determined to be 6.7·10–9, 9.6·10–9, and 2.8·10–9 m2/s, respectively, consistent with literature values obtained through gas concentration measurements without pressure gradients. However, the diffusivity of carbon dioxide and argon in water was measured at 10.9·10–9 and 44.6·10–9 m2/s, significantly exceeding their corresponding literature values by an order of magnitude. This discrepancy is attributed to the significant solubility of these gases in water, resulting in density-driven convection as the primary transport mechanism. Furthermore, the effective diffusion coefficients for hydrogen within the analyzed rock specimens varied from 0.8·10–9 to 2.9·10–9 m2/s, which are higher than those for methane and carbon dioxide, both ranging from 0.3·10–9 to 0.9·10–9 m2/s. This yielded diffusive tortuosity values ranging from 2.6 to 8.2. The observed effective diffusivity values were positively correlated with porosity, permeability, and mean pore size, while exhibiting a negative correlation with tortuosity. Given that the gas–liquid mass transfer coefficient is directly proportional to the effective gas diffusivity in water, the determined values for H2 are essential for studying the impact of pore characteristics on microbial activity.

Authors 6

1. Saeed Khajooie Aachen Institute of Applied Structural Geology Teaching and Research Unit Institute of Organic Biogeochemistry in Geo-Systems

   RWTH Aachen University

   Affiliation as printed

   Institute of Applied Structural Geology, Teaching and Research Unit, RWTH Aachen University, Aachen, Germany

   Institute of Organic Biogeochemistry in Geo-Systems, RWTH Aachen University, Aachen, Germany

   Institute of Applied Structural Geology, Teaching and Research Unit, RWTH Aachen University, Lochnerstrasse 4-20, 52056, Aachen, Germany

   Institute of Organic Biogeochemistry in Geo-Systems, RWTH Aachen University, Lochnerstr. 4-20, Haus B, 52056, Aachen, Germany

2. Garri Gaus corresponding Aachen Institute of Organic Biogeochemistry in Geo-Systems

   Fraunhofer Research Institution for Energy Infrastructures and Geotechnologies IEG · RWTH Aachen University

   Affiliation as printed

   Fraunhofer IEG – Fraunhofer Research Institution for Energy Infrastructures and Geothermal Systems, Aachen, Germany

   Institute of Organic Biogeochemistry in Geo-Systems, RWTH Aachen University, Aachen, Germany

   Institute of Organic Biogeochemistry in Geo-Systems, RWTH Aachen University, Lochnerstr. 4-20, Haus B, 52056, Aachen, Germany

3. Timo Seemann Aachen Institute of Engineering Geology and Hydrogeology

   RWTH Aachen University

   Affiliation as printed

   Institute of Engineering Geology and Hydrogeology, RWTH Aachen University, Aachen, Germany

   Institute of Engineering Geology and Hydrogeology, RWTH Aachen University, Lochnerstrasse 4–20, 52056, Aachen, Germany

4. Benedikt Ahrens

   Fraunhofer Research Institution for Energy Infrastructures and Geotechnologies IEG

   Affiliation as printed

   Fraunhofer IEG - Fraunhofer Research Institution for Energy Infrastructures and Geothermal Systems, Bochum, Germany

   Fraunhofer IEG - Fraunhofer Research Institution for Energy Infrastructures and Geothermal Systems, Am Hochschulcampus 1, 44801, Bochum, Germany

5. Hua Tian

   Research Institute of Petroleum Exploration and Development

   Affiliation as printed

   Research Institute of Petroleum Exploration and Development, PetroChina, Beijing, China

   Research Institute of Petroleum Exploration and Development, PetroChina, Beijing, 100083, China

6. Ralf Littke Aachen Institute of Organic Biogeochemistry in Geo-Systems

   RWTH Aachen University

   Affiliation as printed

   Institute of Organic Biogeochemistry in Geo-Systems, RWTH Aachen University, Aachen, Germany

   Institute of Organic Biogeochemistry in Geo-Systems, RWTH Aachen University, Lochnerstr. 4-20, Haus B, 52056, Aachen, Germany