“Performance of API Class G and Alkali-Activated Wellbore Cements Under Acetogenic Conditions: An Experimental Study for Underground Hydrogen Storage”

Underground hydrogen storage (UHS) requires wellbore cement systems that can resist mechanical shrinkage and chemically aggressive environments induced by microbial acetogenesis. This study presents a comprehensive experimental evaluation of shrinkage control and acetogenesis resistance in conventional API Class G cement and alkali-activated materials (AAMs), with magnesium oxide (MgO) incorporated as an expansive additive. Thirteen binder systems, including API Class G, silica-modified API Class G, and fly ash–slag-based AAMs with and without MgO, were investigated using cylindrical specimens and shale–binder composite systems. Specimens were exposed to acetic acid solutions to evaluate for mechanical performance using ultrasonic pulse velocity (UPV) and uniaxial compressive strength (UCS), while degradation mechanisms were examined through X-ray diffraction (XRD), scanning electron microscopy with energy dispersive spectroscopy (SEM–EDS), and high-resolution X-ray micro-computed tomography (XMT) with Avizo-based three-dimensional reconstruction. Results show that MgO reduced early-age strength in API Class G cement due to hydration competition but significantly mitigated strength loss after acetic acid exposure through late-stage brucite-driven expansion. Slag-rich AAMs exhibited superior acid resistance compared to API Class G systems, attributed to stable aluminosilicate-rich gel residues and refined pore structures. Fly ash–dominant AAMs showed limited early strength development due to curing constraints. XMT analysis revealed that shrinkage-induced microannular voids governed interfacial degradation, while MgO activation under reservoir pressure partially closed microannuli and improved local sealing performance. The findings demonstrate that combined MgO modification and slag-rich AAM formulations offer promising pathways for enhancing long-term sealing integrity of hydrogen storage wells under acetogenesis-relevant conditions. Accordingly, this study represents a preliminary investigation intended to elucidate governing degradation mechanisms and comparative material performance under accelerated conditions.