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

Abstract Laboratory experiments were performed in order to determine the alteration in cement exposed to acid gas (H 2 S–CO 2 ) and pure CO 2 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 CO 2 and H 2 S–CO 2 (21 mol% H 2 S) to simulate acid gas. The cement samples were partially submerged in aqueous solutions to include both saturated aqueous and supercritical CO 2 phases. The cement exposed to pure CO 2 was identical in alteration to those previously tested and described in that they exhibited the typical carbonation rims which result from the CO 2 /cement interaction. The H 2 S–CO 2 exposed cement exhibited a carbonated zone similar to the CO 2 -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 H 2 S–CO 2 . 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.

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