Underground gas storage plays an important role in achieving energy security. Hydrogen has been proven to be an important player in energy transition, and, thus, underground hydrogen storage (UHS) will become a focal point in the future of gas storage. This brings uncertainties regarding the behavior of mixtures of hydrogen and natural gas in storage wells. Therefore, a deeper investigation must be conducted to characterize the impact of hydrogen exposure compared with natural gas (methane) on well elements. This study focuses on the cement interaction with the mentioned mixture to prove the readiness of existing and new wells for UHS.
Cylindrical samples were prepared out of three types of cement (class A, class G, Special Cement: SC) and were confined in autoclaves in contact with different mixtures of H2/CH4 under 100 bars of pressure and ambient temperature for varying periods of time starting from three weeks. Tests were conducted to investigate the impact of hydrogen exposure on different aspects, namely the mineralogical (X-ray Diffraction; Scanning Electron Microscopy; and Element Distribution Map), mechanical (Uniaxial-Compressive-Strength), and petrophysical (Gas-Permeability). The investigation aimed to establish a comparison of the samples’ characteristics before and after the gas treatment. The pressure was monitored and the gas inside the autoclave was analyzed at the end of each phase. Moreover, samples were visually inspected and weighed prior to and after each phase to evaluate any material deterioration.

A rim was observed around the tested samples (mainly in cement type A), proving gas diffusion. Sample drying out played a significant role in the changes seen in permeability, weight, and Uniaxial Compressive Strength (UCS). The scale of gas-permeability change was found to be around 10-1 – 10-2 mD, within measurement uncertainty. This leads to the conclusion that cement samples evaluated under the experimental conditions and durations with the different mixtures show strong similarities in the results. The observed changes cannot be associated to hydrogen since the increase in its percentage did not introduce major impacts nor scale up the observed effects and the exposition at 100% H2 and 100% CH4 environment shows comparable results. This conclusion is based on the comparison of the test performed with the gas mixture at 100 bar and ambient temperature, for an average experimental duration and exposure of 23 days in the laboratory.
The paper highlights the novel and multidisciplinary approaches implemented for this study with different gas compositions and sets a baseline for future experimentation regarding the effects of hydrogen on storage wells. The results confirm that hydrogen exposure would not lead to loss of integrity in the tested conditions and environments. The project was conducted through cooperation with the gas storage industry, service company and different university departments.