H2S–CO2 reaction with hydrated Class H well cement: Acid-gas injection and CO2 Co-sequestration

Laboratory experiments were performed in order to determine the alteration in cement exposed to acid gas (H2S–CO2) and pure CO2 under simulated reservoir conditions. Cement samples were exposed for a period of 28 days at a temperature of 50 °C and a pressure of 15 MPa using pure CO2 and H2S–CO2 (21 mol% H2S) to simulate acid gas. The cement samples were partially submerged in aqueous solutions to include both saturated aqueous and supercritical CO2 phases. The cement exposed to pure CO2 was identical in alteration to those previously tested and described in that they exhibited the typical carbonation rims which result from the CO2/cement interaction. The H2S–CO2 exposed cement exhibited a carbonated zone similar to the CO2-only samples and underwent an additional sequence of oxidation–reduction and sulfidation reactions. Ettringite was observed in the interior region of the cement, and pyrite in the carbonated rim of the cement exposed to H2S–CO2. The mineralogical changes and alteration front are believed to be controlled by local porewater pH buffering. Although the process of secondary ettringite formation subsequent to the hardening of cement can lead to strength loss and degradation, ettringite induced mechanical damage was not observed in the samples.