Abstract
Abstract The long-term stability of well cement in corrosive environments is a critical challenge for plug and abandonment (P&A) operations in the oil and gas industry. Hydrogen sulfide (H2S) poses a significant risk to the integrity of cement barriers, as it promotes chemical and mechanical degradation over time. This study investigates the effect of long-term H2S exposure on three commercial cement formulations and compares their performance with a Portland G cement containing micro-silica as reference, by exposing samples to H2S at 100°C and 10bar in artificial seawater for 1, 3, 6 and 12 months. Unexposed samples of each cement formulation were used as a baseline to evaluate effect of exposure. Micro-Computed Tomography (μCT) was used to assess structural changes, while uniaxial compressive strength (UCS) tests were performed to evaluate mechanical strength. Additionally, sample weight and volume were measured after each exposure interval. All samples revealed an increase in volume, attributed to the precipitation of gypsum, and a progressive reaction front, with notably different propagation rates depending on cement composition, for longer exposure times. Most samples developed a thin surface layer of precipitated products, together with changes in color and texture. The lightweight cement showed distinct behavior characterized by large pores filled by large, high-density crystals. The stability of the precipitates varied significantly with composition and duration of exposure. Sample weight increased initially but decreased after prolonged exposure, indicating ongoing reactions and loss of material density. UCS testing showed decreased mechanical strength for all samples after 12 months exposure, confirming significant detrimental effects of H2S on the integrity of all cement compositions. These results highlight the importance of considering both chemical and mechanical stability when evaluating long-term barrier performance, especially under demanding conditions, such as presence of H2S.