Enhancing Geothermal Well Integrity: Optimizing Class G Cement with Pumice and Eggshell-Derived Pozzolanic Additives

Abstract Geothermal energy presents a sustainable pathway to meet global energy demands while addressing carbon emissions, yet the extreme conditions encountered in geothermal wells—characterized by high temperatures, pressures, and corrosive environments—pose significant challenges to well integrity. This study focuses on optimizing Class G cement for geothermal applications by synergically integrating pumice and eggshell-derived pozzolanic additives. Cement samples with varying pumice-to-eggshell powder (PM/ESP) ratios were subjected to temperatures of 150°C and 300°C using an autoclave reactor to simulate diverse geothermal reservoir depths. The samples were analyzed for compressive strength, permeability, thermal stability, and elastic properties over 30 days to evaluate their performance. The percentage ratio of 75% pumice to 25% eggshell blend emerged as the optimal formulation, exhibiting remarkable improvements across all tested parameters. Compressive strength increased by 25.5% at 150°C (from 8.79 MPa to 11.03 MPa) and 33.8% at 300°C (from 9.69 MPa to 12.96 MPa) compared to control samples. Permeability values demonstrated a significant reduction, achieving ultra-low ranges from 0.00165 mD at 150°C to 0.00192 mD at 300°C, reflecting a denser cement matrix with minimal fluid migration pathways. However, this minor increase highlights the formulation's robustness under elevated temperatures. Elastic properties showed substantial improvement, with Young's modulus values of 73.6 GPa at 150°C and 72.5 GPa at 300°C, and Poisson's ratio increasing from 0.284 to 0.365, indicative of enhanced deformation resistance under thermal stresses. Microstructural analysis using X-ray diffraction (XRD) confirmed the synergistic effect of the PM/ESP blend, with the formation of beneficial crystalline phases such as clinoenstatite (MgSiO3) and xonotlite (Ca6Si6O17(OH)2). The reduced content of calcium silicate hydrate (C-S-H) gel, decreasing from 10.43% at 150°C to 5.78% at 300°C, suggesting the transformation of C-S-H into thermally stable phases like clinoenstatite and xonotlite, which enhance matrix densification and thermal stability of the cement. Brownmillerite content increased by 13.3%, from 21.98% at 150°C to 24.90% at 300°C, improving the cement's thermal resilience. These compositional changes underline the critical role of additives in enhancing mechanical and thermal performance of cement under geothermal conditions. The findings highlight the implications of high-temperature exposure, with the PM/ESP blend maintaining mechanical integrity and low permeability even at 300°C, demonstrating its suitability for extreme geothermal environments. This research introduces a novel, environmentally friendly cement formulation tailored for the rigorous demands of geothermal energy production. By optimizing cement performance at high temperatures, it addresses critical challenges in geothermal well integrity, offering the potential for enhanced economic viability, reduced environmental risks, and the advancement of renewable energy systems. These findings pave the way for more robust and sustainable geothermal energy infrastructure, contributing to the global transition toward cleaner energy sources.