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Abstract

Hydrogen (H2) is a clean source of energy and a promising solution in the energy transition due to its vast energy content and potential of zero greenhouse gas emissions. Storing hydrogen to meet present and future energy demands requires a large storage volume which is only available in subsurface reservoirs such as salt caverns, depleted oil and gas fields, and deep saline aquifers. Despite the high demand for Underground Hydrogen Storage (UHS) technology as part of a full H2-value chain, there is especially limited knowledge of the transport behavior of hydrogen in porous media [1]. With its charging and discharging operations, the storage of hydrogen in a porous reservoir formation undergoes a transient flow process, influenced by coupled thermo-hydro-mechanical processes between hydrogen, the formation fluid, the solid components of the rock, and the prevailing temperature and pressure regime, which repetitively changes under geotechnical utilization [2]. In consequence of cyclic storage operations, variations in effective mechanical stresses can affect the pore space geometry and may lead to irreversible deformation and weakening of reservoir and cap rocks.
Here, we present first results of an experimental laboratory study focussing on fluid substitution experiments (gas replacing brine) on various sandstone core samples sourced from Bad Bentheim and the Stuttgart formation. The study was specifically designed to replicate the unique reservoir conditions of the Stuttgart formation at the Ketzin site in Germany (confining pressure = 150 bar, pore pressure = 25 to 75 bar, temperature = 37 °C). In the frame of the national-funded GEOZeit project, long-term flow experiments are carried out to determine the evolution of relative permeability of H2-brine and CH4-brine systems in dependence of the number of load cycles. Alongside, measurements of electrical resistivity and ultrasonic wave velocities at each brine/gas saturation state are performed. This enable us to derive the saturation level and to understand the spatial distribution of liquid and gaseous phases in the pore space of our sample material. The experiments are complemented by a range of additional tests, including chemical analyses and microstructural investigations using XRD, SEM, and optical microscopy. Our results are expected to improve the understanding of coupled hydromechanical processes and their impact on reservoir properties during geotechnical operations, and to also provide the necessary parameters for large-scale modelling and up-scaling, required to assess the feasibility of storage, production, and monitoring of hydrogen gas in porous geological formations.