Measurement of hydrogen diffusion in brine at high pressures and elevated temperatures: Implications for underground hydrogen storage

Hydrogen diffusion process critically affects the safety and efficiency of underground hydrogen storage (UHS) projects. Nevertheless, experimental measurements of hydrogen diffusivity under real reservoir conditions are scarce in the literature. Hence, this study aims to investigate hydrogen diffusion into brine at various reservoir conditions. To do so, five pressure decay tests were conducted to measure hydrogen diffusion into brine at high pressures and elevated temperatures using a visual head PVT cell. Later, a mathematical model was developed to analyze the recorded pressure decay data of the conducted tests. The developed model could estimate the hydrogen diffusion (D) coefficients in brine by optimizing an objective function. The results showed that the hydrogen diffusion is dependent on both system pressure and temperature, with the temperature having a more pronounced impact. The obtained hydrogen D coefficients are higher at elevated temperatures, the D value being 5.5 × 10⁻⁹ m²/s at 35 °C, increasing to 28 × 10⁻⁹ m²/s at 50 °C. Increasing the system pressure from 3.4 MPa to 10.4 MPa shows a reduction in the hydrogen D coefficient from 45 × 10⁻⁹ m²/s to 28 × 10⁻⁹ m²/s when the tests were conducted at 50 °C. The calculated dynamic changes in hydrogen concentration in brine showed that UHS in low-temperature formations can reduce the hydrogen losses by diffusion either into the brine-saturated cap rocks or residual brine in reservoirs. Reliable hydrogen diffusion coefficients, determined under specific reservoir conditions, need to be used in simulating field-scale hydrogen storage projects.