This work aimed to investigate the durability of Class G cement pastes exposed to severe environments found in oil wells (brine saturated with H2S and CO2). Furthermore, to evaluate the potential of adding PVA fibers to enhance the mechanical and rheological properties of these pastes, especially in well abandonment operations. To achieve these objectives, the following characterizations and tests were performed: physical characterization of porosity, permeability, and computed microtomography; chemical characterization using X-ray diffraction and measurement of the pH of the exposure solutions; unconfined and confined mechanical tests; and rheological characterization to determine the flow curve and immiscibility between fluids in the vertical duct in the presence of the fibers.The results obtained demonstrated that the exposure of the cement matrix to the degradation environments for three months resulted in a significant loss of mechanical strength, with reductions of approximately 36 per cent and 45 per cent in unconfined tests for the CO2 and H2S environments, respectively. In confined tests, a reduction in strength was observed with confinement of 10 and 20 MPa; however, a gain in strength was observed in both exposure environments when the confining pressure reached 40 MPa. The evaluation of PVA fiber addition in fresh state pastes, after hardened and exposed to an acidic environment revealed an increase in the average strength of the pastes, regardless of the confinement condition. The environment with H2S-saturated brine proved to be more aggressive to both the fibers and the cement matrix. PVA fibers played an important role in the early ages of the cement paste exposed to the CO2-saturated brine environment, contributing to the gain in material ductility under confinement. However, in the case of samples subjected to six months of exposure, the PVA fibers underwent a degradation process, compromising their structural function within the matrix. In the specific context of well abandonment operations, the incorporation of PVA fibers into the fresh cement paste demonstrated effectiveness in modifying the rheological properties of the cementitious matrix. The rheological tests revealed that the addition of the fibers positively modified the viscosity and yield stress of both the cement paste and the simulator fluid used (Carbopol), resulting in the stabilization of these phases over the drilling simulator fluids in vertical duct experiments. This stabilization behavior suggests the assurance of the abandonment plug integrity, preventing the migration of formation fluids. The volumetric fraction of PVA fibers directly influenced the viscosity and, mainly, the yield stress of the cement paste.