The Influence of H2S and CO2 on the Triaxial Behavior of PVA Fiber Reinforced Class G Cement Paste Under Elevated Temperature and Pressure

Ensuring the long-term sealing capacity of oilwell cement is essential in maintaining zonal isolation, especially in wells subjected to elevated pressure and temperature conditions and the presence of corrosive agents like CO₂ and H₂S. Traditional Class G cement exhibits limited ductility and low tensile strength, which makes it prone to microcracking, loss of structural continuity, and chemical attack over time. This research investigates the effect of incorporating polyvinyl alcohol (PVA) fibers into Class G cement formulations to improve their mechanical performance and resistance to degradation in chemically aggressive environments. Cement samples—with and without 0.5 vol.% PVA fibers—were cured and then aged in brine saturated with either CO₂ or H₂S at 88 °C and 20 MPa for a duration of 90 days. A combination of triaxial compression testing, porosity evaluation, and microstructural characterization was employed to assess damage evolution and material alteration. Results demonstrated a substantial decline in compressive strength—approximately 27% in H₂S conditions and 45% in CO₂ environments—alongside increased porosity associated with calcium depletion, formation of bicarbonates, and modification of hydration products such as C–S–H. Under triaxial loading (above 20 MPa), samples showed signs of plastic flow preceding failure, indicating reduced mechanical stability. The inclusion of PVA fibers contributed to enhanced crack control, delayed propagation of internal damage, and partial retention of strength following exposure. Despite these benefits, chemical deterioration was still evident, suggesting that further development of fiber-reinforced cementitious systems is required for application in extreme subsurface settings. The study supports the feasibility of using fiber-reinforced Class G cement to improve durability and mechanical reliability in acid gas-rich wellbore formations.