Geochemical and petrophysical responses of Bandera Grey sandstone to CO2-rich cushion gas and acetic-acidified brine during hydrogen geo-storage

Hydrogen geo-storage in porous media is a key option for large-scale energy storage, yet abiotic geochemical interactions in CO 2 -rich systems containing low concentrations of organic acids remain insufficiently understood. This study investigated the coupled geochemical and petrophysical responses of Bandera Grey sandstone exposed to 2 wt% NaCl brine containing 0.01 wt% acetic acid and a gas mixture of 50% H 2 , 40% CO 2 , 5% CH 4 , and 5% N 2 at 42 °C and 1350 psi for 60 days. Acetic acid was used as a controlled proxy for potential microbial organic acids rather than as a representation of active microbial processes. Pre- and post-aging characterization included XRD, XRF, FTIR, SEM-EDX, ex-situ pH and dissolved-ion measurements, gas chromatography, and porosity and permeability measurements. The mineralogical results indicate localized dissolution of carbonate cement, particularly dolomite, while the quartz-feldspar framework remained largely preserved. Ion chemistry and ex-situ pH measurements were consistent with carbonate buffering within the combined CO 2 -rich, acetic-acid-containing system. SEM-EDX revealed localized mineral-specific surface alteration, while the integrated evidence suggested possible pore-throat modification without pervasive framework degradation. Hydrogen remained the dominant component of the recovered gas phase, although its normalized fraction decreased from 50% to approximately 42%. This change was interpreted as apparent gas-phase redistribution influenced by gas-brine partitioning, differential solubility, depressurization, and sampling rather than quantitative hydrogen loss. Porosity and permeability showed directional changes of 0.9–3.1% and 3–11%, respectively. Because these changes were comparable to the reported ±10% analytical uncertainty and were obtained from only two core plugs, they were treated as indicative trends rather than confirmed enhancements. Overall, the results indicate localized carbonate-cement alteration under the tested abiotic batch conditions. The findings represent early-stage behavior and should not be directly extrapolated to long-term or field-scale hydrogen-storage performance.