Degradation pathways and mechanisms of oilwell cement exposed to H2S under high temperatures

The extraction of geothermal energy faces the hazard of H₂S, a highly toxic and strongly corrosive gas. H₂S exposure can lead to the failure of oilwell cement, decreased extraction efficiency, and even pose serious risks to operational personnels near the wellsite. High temperature is a prominent environmental feature in geothermal resource extraction. However, current research works primarily focus on the corrosion effects of H₂S on cement at moderate to low temperatures. This study utilizes Class G oilwell cement to conduct corrosion experiments of cement by H₂S under high temperature in a H₂S-rich reaction vessel. The impact of H₂S on the structure, chemical composition, and mechanical strength of oilwell cement is analyzed via SEM-EDS, XRD, nanoindentation tests, and unconfined compressive strength tests. The results indicate a reduction in compressive strength for cement samples corroded by H₂S. The surface nano-hardness and elastic modulus of cement samples decrease while the internal values of nano-hardness and elastic modulus significantly increase. Under the corrosion of H₂S, the structure of cement is characterized by a yellow and black surface layer and stratified cracks. The external surface of the cement exhibits a yellow color due to the formation of pyrite (FeS₂), while internally, pyrrhotite (FeS) and gypsum (CaSO₄^.2H₂O) are generated.