Rate of H2S and CO2 attack on pozzolan-amended Class H well cement under geologic sequestration conditions

•The rates of H2S and CO2 alteration of pozzolan-amended cement are quantified.•28-day average CO2 alteration rate is 0.016mm/day for brine-immersed cement.•H2S is able to penetrate to the interior region of cement.•Significant H2S alteration is only observed in the rim region of cement.•28-day average H2S alteration rate is 0.013mm/day for brine-immersed cement. Experiments were conducted to investigate the rates of H2S and CO2 alteration of pozzolan-amended wellbore cement (35vol% pozzolan–65vol% cement), so as to evaluate the potential impact of H2S and CO2 induced degradation of existing cemented wells present at acid gas co-sequestration sites. In the exposure experiments, pozzolan-amended cement samples were mixed, cured and exposed to mixtures of H2S and CO2 under lab-simulated geologic sequestration conditions (50°C and 15.2MPa) for 2.5, 9, 28 and 90 days. Measurement of the carbon alteration front was used to calculate the rate of CO2 alteration of pozzolan-amended cement exposed to a mixture of 79mol% CO2 and 21mol% H2S under geologic sequestration conditions in exposure periods of 0–90 days. Average CO2 alteration rates (rate of movement of CaCO3 precipitation front) were 3.3×10−3mm/day and 3.2×10−3mm/day for cement samples exposed to a 1% NaCl solution saturated with CO2 and H2S, and those in contact with a supercritical mixture of CO2 and H2S, respectively. Two scenarios were considered for measuring and quantifying alteration caused by H2S over the exposure periods of 0–90 days: sulfur-rich zone thickness, and sulfur alteration index. The average rate of H2S alteration determined by sulfur-rich zone thickness divided by exposure duration was 4.3×10−3mm/day for cement exposed to 1% NaCl solution saturated with CO2 and H2S, and the average rate of H2S alteration determined by sulfur alteration index divided by exposure duration was 8.2×10−3day−1. Cement exposed to a supercritical mixture of CO2 and H2S result in H2S alteration rates determined by sulfur-rich zone thickness divided by exposure duration of 3.1×10−3mm/day, and average rates of H2S alteration determined by sulfur alteration index divided by exposure duration of 6.3×10−3day−1. Sulfur alteration index results also show that H2S was able to penetrate to the core of pozzolan-amended wellbore cement after 2.5 days of exposure, though this was not readily apparent in the sulfur-rich zone thickness results. Sulfur-rich zone thickness is best used to describe high-level sulfur alteration i