Temperature and Water Saturation Influence on Effective Hydrogen-Methane Diffusion in Reservoir Rocks: Estimation of Hydrogen Diffusive Losses

Summary When hydrogen is stored underground in porous media, mixing of remaining hydrocarbon gas with hydrogen occurs. One of the major processes enhancing gas mixing is molecular diffusion, which depends on subsurface conditions (pressure and temperature), phase saturation, gas composition, and rock properties (porosity and tortuosity). Also, molecular diffusion of hydrogen through water-saturated caprock might lead to hydrogen losses during underground storage. Limited data is available describing the hydrogen-methane diffusion through reservoir rocks. Moreover, minor attention was given to the influence of temperature and water saturation on effective diffusion. To evaluate these effects, a series of experiments were carried out using an unsteady state method with fixed volumes. In this experimental setup, methane diffusing into a hydrogen-filled view cell is measured using infrared light spectroscopy. Diffusion through 100% water-saturated rock was measured using the pressure decay method. In this experiment, the partial pressure of the gas diffusing into saturated rock was measured and converted to concentration change. 3D numerical models were constructed to determine the diffusion coefficients by matching observed concentration changes. The measured bulk and effective hydrogen-methane diffusion coefficients within the temperature range of 28–100°C follow the kinetic gas theory. In both sets of experiments, the diffusion coefficient is proportional to temperature to the power of 3/2. Deviations from predicted diffusion coefficient values are smaller than 5%. Diffusion of gases (nitrogen, hydrogen, and methane) in water and formation brine was measured and found to be several orders of magnitude smaller than gas-gas diffusion. Hydrogen diffusion in 100% water-saturated Berea rock revealed a slow diffusion process. This indicates minor diffusive transport of hydrogen through water-saturated rocks (e.g., caprock shales). The results of effective hydrogen-methane diffusion measurements through partially saturated rock samples (40% and 60% water saturation) prove that water reduces effective pore space for gas mixing and, thus, the diffusion coefficient reduces with increasing water saturation. The data show that the effective tortuosity needs to increase with water saturation increase. Also, it was found that gas diffusivity is affected by saturation hysteresis. Several experiments were performed with a core sample saturated with the imbibition process revealing slower diffusion through a core plug in comparison to the drainage saturation process. Having obtained reliable hydrogen effective diffusion coefficients through saturated rock samples, the analytical equations and numerical models were developed to quantify diffusive hydrogen losses into the caprock. It is estimated that the amount of hydrogen losses due to diffusion accounts for 0.09% of cushion gas volume for case-study reservoirs during 30 years of underground hydrogen storage (UHS) operations. It was also shown that the introduction of hydrogen diffusion into the full-field simulation model might not be plausible due to the requirement of small grid sizes to minimize numerical dispersion effects. The study contributes a high-quality data set of effective diffusion coefficients with temperature, pressure, and water saturation. It is crucial to assess hydrogen losses through caprock and hydrogen-hydrocarbon gas mixing within the reservoir between high- and low-permeability layers. We also show an example of hydrogen loss assessment and point out the slow process of diffusion into caprock.