Mechanical Behavior of Polyvinyl Alcohol Fiber-Reinforced Class G Cement Paste under H2S and CO2 Exposure at Elevated Temperature and Pressure

Summary Maintaining effective zonal isolation in elevated-pressure and elevated-temperature wells remains a critical challenge, particularly under exposure to carbon dioxide (CO2)- and hydrogen sulfide (H2S)-rich environments. Conventional Class G cement typically exhibits limited tensile capacity and low strain tolerance, which promote microcrack propagation, permeability increase, and progressive physicochemical degradation. Although previous studies have explored either chemical durability or mechanical reinforcement, these aspects are commonly addressed in isolation and under simplified laboratory conditions, failing to capture the coupled effects of mechanical confinement and long-term chemical exposure representative of in-situ well environments. To address these limitations, we investigated a fiber-reinforced cementitious system designed to mitigate brittleness and enhance strain capacity under realistic downhole conditions. Specifically, Class G cement incorporating 0.5 vol% polyvinyl alcohol (PVA) fibers was evaluated as a strategy to improve crack-bridging capacity and delay damage evolution. The experimental program was explicitly designed to reproduce field-relevant conditions, including curing and aging for 90 days in brine saturated with either CO2 or H2S at 88°C and 20 MPa. In addition, the mechanical response was assessed under both uniaxial and triaxial loading to account for the effect of confinement, which is often neglected in conventional experimental approaches, but plays a key role in in-situ performance. Mechanical behavior was characterized through compressive and flexural tests, while durability and degradation mechanisms were investigated using porosity and permeability measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), and pH monitoring, enabling direct correlations between microstructural evolution and macroscopic response. The results demonstrate that confining pressure governs deformation behavior, with specimens subjected to stresses above 20 MPa exhibiting pronounced plasticity before deviatoric loading. Chemical exposure led to significant reductions in compressive strength—up to 27% in brine + H2S and 45% in brine + CO2—as well as substantial elastic modulus degradation, associated with increased porosity due to dissolution and transformation of hydration products. While the incorporation of PVA fibers provided initial improvements in strain capacity and crack control, their long-term reinforcing efficiency under carbonated conditions was reduced, highlighting limitations of polymeric fibers under combined thermal and chemical aging. This behavior is attributed to the synergistic effects of carbonation reactions and degradation of the fiber/matrix interface. These findings reveal that, although fiber reinforcement partially mitigates the inherent brittleness of conventional cement systems, its effectiveness is strongly dependent on long-term exposure conditions and confinement. By integrating realistic chemical environments, elevated-pressure and elevated-temperature conditions, and triaxial confinement into a unified experimental framework, this study advances current methodologies and provides a more representative assessment of cement performance in the field. The results offer critical insights into both the potential and limitations of fiber-reinforced cementitious systems and contribute to the development of more durable and mechanically resilient materials for long-term zonal isolation in elevated-pressure and elevated-temperature wells.