Characterization of pozzolan-amended wellbore cement exposed to CO2 and H2S gas mixtures under geologic carbon storage conditions

Capture and subsurface co-sequestration of acid gas (a mixture of CO2 and H2S) is a geologic sequestration approach that can reduce the emission of both CO2 and sulfur-containing sour gas. To investigate the effects of co-sequestration on pozzolan-amended wellbore cement, samples were mixed, cured and exposed to mixtures of H2S and CO2 under geologic sequestration conditions (50 degrees C, 15.1 MPa) for various periods of time. The resulting samples were characterized using microscopy, X-ray diffraction, and microhardness testing. These analyses showed significant alteration at the surfaces of pozzolan-amended cement samples after 28 days of exposure to 21 mole% H2S in a mixture with CO2. Results of samples exposed for 28 days also revealed that the samples with higher pozzolan amount (65% pozzolan by volume) allowed faster CO2 and H2S penetrations than those with lower pozzolan amount (35% pozzolan by volume). Analyses showed that there was formation of carbonate species in the interior core region of pozzolan-amended cement samples with 65 vol% pozzolan after 28 days of exposure, which was not the case for the samples with 35 vol% pozzolan. In addition, the microhardness test results showed that the rim region of the 35 vol% pozzolan samples exhibited a higher microhardness than that of the core region for the same sample, whereas the different regions of the 65 vol% pozzolan samples exhibited similar microhardness values. In summary, the pozzolan-amended cement samples with 35 vol% pozzolan exhibited better H2S-resisting performance than the samples with 65 vol% pozzolan. Published by Elsevier Ltd.