Experimental investigation on H2S-induced oilwell cement degradation under high-temperature conditions for geothermal applications

H2S, a highly toxic and corrosive substance present in geothermal reservoirs, can significantly reduce the durability of geothermal well cement. High temperature is a prominent environmental characteristic in the extraction of geothermal resources; however, current research primarily focuses on the corrosive effects of H2S on cement under moderate to low temperatures. This study employed Class G oilwell cement to investigate the corrosion of H2S on cement under high temperature. The experiments were conducted in a high-temperature reaction vessel. The effects of H2S on cement structure, chemical composition, and mechanical strength were analyzed under high-temperature conditions via SEM-EDS, XRD, FTIR, nanoindentation, and unconfined compressive strength tests. The results indicate that the corrosive effects of H2S lead to formation of different reaction layers within the cement structure, forming of microcracks, and changes in material composition. Within the cement, secondary minerals with weaker bonding properties, such as pyrite, pyrrhotite, and gypsum, are generated. Simultaneously, the compressive strength of the samples decreases. The surface nano-hardness and elastic modulus of the cement decrease, while there is a significant increase of nano-hardness and elastic modulus in the interior of cement, due to changes in structure and chemical composition. In summary, high temperature in geothermal applications reduces the level of H2S-induced cement corrosion due to inhibition of ettringite formation.