Influence of Water Saturation, Temperature and Pressure on the Effective Hydrogen-Methane Gas Diffusion in Reservoir Rocks
Authors: Arekhov V. , Zhainakov T. , Clemens T. , Wegner J. 
Published: June 2024

Abstract
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. The magnitude of diffusion mixing depends on subsurface conditions (pressure and temperature), phase saturation, gas composition as well as rock properties (porosity and tortuosity). Limited data is available describing the hydrogen and methane diffusion through reservoir rocks. Moreover, minor attention has been given to the influence of temperatures and water saturation on effective diffusion through porous rocks.
To evaluate these effects, a series of experiments were carried out using an unsteady state method with fixed volumes. In this experimental setup, methane concentrations diffusing into a hydrogen-filled view cell are measured using infrared light spectroscopy. Diffusion through 100% water saturated rock was measured using pressure decay method. In this experiment, the partial pressure of the gas diffusing into saturated rock was measured and converted to concentration change. To interpret experimental data, 3D numerical models were constructed in a software covering the various physical processes. The concentration changes were matched with the results of numerical simulation to determine the diffusion coefficients.
The measured bulk diffusion coefficients within the temperature range 28-100 °C follow the kinetic gas theory. The effective hydrogen-methane diffusion coefficient through Berea rock also increases with temperature. 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, methane) in pure water and formation brine were measured and found to be negligible. Thus, the loss of gasses into formation brine can be assumed to be zero during effective diffusion measurements through partially and fully water saturated rock samples. Hydrogen diffusion in 100% water saturated Berea rock revealed that diffusion is a slow process for these conditions. This indicates minor diffusive transport of hydrogen through water saturated rocks (such as shales in the caprock). The results of effective hydrogen-methane diffusion measurements through partially saturated rock sample (at 40% and 60% water saturation) prove that water reduces effective pore space for gas mixing and, thus, diffusion coefficient is reducing with increasing water saturation. The data show that the tortuosity of the rock is increasing with water saturation increase. The reason is that gas has a longer travelling distance due to pore throats blocked by water.
Diffusion was shown in the literature to play an important role in underground hydrogen storage. However, limited data concerning effective diffusion coefficients was available in the literature. The study contributes a high-quality data set of effective diffusion coefficients with temperature, pressure, and water saturation. This data set is crucial to assess hydrogen losses through cap rock and hydrogen–hydrocarbon gas mixing in the reservoir related to high and low permeability layers for various conditions.