The Impact of Acetic Acid Reaction on Microstructural and Mineralogical Changes in Shale Caprock: A Preliminary Study for Underground Hydrogen Storage Integrity

Hydrogen storage in depleted gas reservoirs triggers geochemical and microbiological reactions at the caprockreservoir interface, yielding significant implications on storage integrity. Acetogenesis is a microbial reaction observed during underground hydrogen storage (UHS) that produces acetate and converts it into acetic acid under protonation, potentially impacting the UHS process integrity. For the first time, this research explores the impact of the acetic acid + brine + caprock reaction on shale caprock mineralogy, microstructure, and physicochemical properties, where this preliminary study has been conducted under ambient conditions to obtain an initial assessment of the impact. A comprehensive mineralogical and micro-structural characterization, including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray fluorescence (XRF), Xray diffraction (XRD), micro-computed tomography (micro-CT), and inductively coupled plasma mass spectrometry (ICP-MS) have been conducted to assess the mineralogical and microstructural changes in shale specimens saturated with brine solutions with a range of acetic acid percentages (5 %, 10 %, and 20 %) to find the maximum possible impact. According to the conducted mineralogical analysis (EDS, XRF, and XRD), there is a significant primary mineral dissolution during the acetic acid interaction, where calcite and dolomite are the predominant minerals dissolved, evidencing the significant impact of the acetic acid reaction on carbonate-rich caprock systems during UHS. However, secondary mineral precipitation happened at high acidic concentrations (20 %). Interestingly, other common minerals in reservoir rocks (e.g., mica, pyrite) did not demonstrate rapid interactions with acetic acid compared to carbonates. The impact of these mineralogical changes on the caprock microstructure was then investigated through SEM and micro-CT, and the results demonstrate substantial enhancements in porosity and microcracks in the rock matrix due to the calcite and dolomite dissolutions, despite some microcracks being closed by secondary precipitations. This preliminary study evidences the significant impact of acidification on caprock integrity, which may occur during the acetogenesis reaction in UHS environments. These effects should be carefully considered in field UHS projects to eliminate the risks.