Optimisation of Class G cement geomechanical and sealing performance for underground hydrogen storage using surface response methodology

Geological storage of hydrogen in depleted reservoirs, salt caverns, and saline aquifers offers a promising pathway toward sustainable energy systems and a low-carbon economy. Underground hydrogen storage requires properly cemented wells to maintain structural integrity and prevent gas leakage. This study investigates the influence of fly ash and eggshell powder (FAESP) additives on the mechanical and sealing properties of Dyckerhoff Class G cement under simulated hydrogen storage conditions. Response surface methodology (RSM) is used as a statistical optimisation framework to evaluate the combined effects of FAESP composition on cement performance. The interaction between fly ash and eggshell powder concentrations is systematically analysed to optimise the cement rheological properties. Cement slurries were prepared and cured under high-temperature and high-pressure conditions of 294°F and 3000 psi with hydrogen injection to replicate subsurface environments. Material characterisation was performed using particle size distribution, SEM, and XRF analyses. The RSM model showed strong predictive capability, with average errors below 2.28% across all rheological properties. Results indicate that a 75%/25% FAESP blend is optimal, significantly reducing porosity and permeability while improving compressive strength and flexibility. RSM enables efficient experimental design, reduced testing costs, and reliable prediction of cement performance for hydrogen storage applications.