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

The assessment of gas difusion 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 fuid and rock properties. Additionally, estimating difusion coef cients is critical for evaluating the balance between hydrogen generation and dissipation in radioactive waste repositories. This investigation involved experimental determination of difusion coefcients 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 difusion coefcients 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 val ues obtained through gas concentration measurements without pressure gradients. How ever, the difusivity of carbon dioxide and argon in water was measured at 10.9·10-9 and 44.6·10-9 m2/s, signifcantly exceeding their corresponding literature values by an order of magnitude. This discrepancy is attributed to the signifcant solubility of these gases in water, resulting in density-driven convection as the primary transport mechanism. Further more, the efective difusion coefcients 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 car bon dioxide, both ranging from 0.3·10-9 to 0.9·10-9 m2/s. This yielded difusive tortuosity values ranging from 2.6 to 8.2. The observed efective difusivity values were positively correlated with porosity, permeability, and mean pore size, while exhibiting a negative cor relation with tortuosity. Given that the gas–liquid mass transfer coefcient is directly pro portional to the efective gas difusivity in water, the determined values for H2 are essential for studying the impact of pore characteristics on microbial activity.