Characterization of the unidirectional corrosion of oilwell cement exposed to H2S under high-sulfur gas reservoir conditions

Abstract

The corrosion of H₂S on oilwell cement is considered to be a great challenge for wellbore integrity and environmental safety in the exploitation of high-sulfur gas reservoirs. In this study, the corrosion performance of oilwell cement exposed to humid H₂S gas and H₂S-rich brine was investigated using designed unidirectional samples. Compressive strength, microhardness, porosity, gas permeability, SEM, EDS, and XRD analyses were conducted to compare the dissimilarity of H₂S attack in the two exposure scenarios. The experimental results show that the corrosion degree of cement exposed to humid H₂S gas was lower due to the dense gypsum layer formed on the cement surface and this layer inhibited the inward penetration of H₂S by blocking its diffusion. On the contrary, a porous and loose amorphous silica gel section was formed on the headspace of the brine-exposed cement due to the dissolution and migration effects of brine, which facilitated the penetration of H₂S to the interior of the cement. The degradation mechanism of cement and the effects of exposure scenario on cement properties are proposed.