Effect of exposure environment on the interactions between acid gas (H2S and CO2) and pozzolan-amended wellbore cement under acid gas co-sequestration conditions

•Pozzolan-amended cement was exposed to supercritical CO2–H2S and brine with CO2–H2S.•Cement with 35vol% pozzolan was altered more when exposed to brine.•Cement with 65vol% pozzolan was altered more when exposed to supercritical CO2–H2S.•Difference in exposure environment did not result in difference in mineral formation. Laboratory experiments were conducted to determine the effect of exposure environment on the integrity of pozzolan-amended Class H cement under geologic acid gas co-sequestration conditions. Cement was exposed to two potential subsurface storage environments: (1) a supercritical mixture of CO2/H2S and (2) a CO2/H2S saturated brine. Results show that cement alteration is dependent on the amount of pozzolan addition. The alteration rate and mechanism also differ for the two exposure scenarios. Cement containing 35% pozzolan amounts by volume (hereafter referred to as 35vol% pozzolan cement) exposed to the aqueous environment was more vulnerable to alteration, compared with the same pozzolan content cement exposed to a supercritical mixture of CO2/H2S (21mol% H2S). Specifically, 35vol% pozzolan cement exposed to the aqueous environment exhibited a higher level of mineral dissolution (e.g., C-S-H, 3CaO·SiO2, 2CaO·SiO2, etc.) and a higher degree of sulfur alteration than the same pozzolan content samples exposed to the supercritical mixture of CO2–H2S (21mol% H2S). Different from the 35vol% pozzolan cement, the cement containing 65% pozzolan by volume (hereafter referred to as 65vol% pozzolan cement) was more susceptible to alteration in the supercritical CO2/H2S (21mol% H2S) environment, compared with the same pozzolan content cement exposed to the aqueous environment. Increasing the H2S mol% in the supercritical phase from 21mol% to 40mol% increased the alteration of cement exposed to both exposure environments. The 65vol% pozzolan cement was more resistant to H2S and CO2 alteration than the 35vol% pozzolan cement if the H2S content was high (i.e., 40mol%), while the 35vol% pozzolan cement was more resistant to H2S and CO2 alteration than the 65vol% pozzolan cement if the H2S content was relatively low (i.e., 21mol%). It was observed that while the two related exposure environments resulted in different degrees of cement alteration, the exposure environments did not result in the formation of different minerals.