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

Experiments were conducted to investigate the rates of H2S and CO2 alteration of pozzolan-amended wellbore cement (35 vol% pozzolan-65 vol% 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 15.2 MPa) 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 79 mol% CO2 and 21 mol% 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 × 10−3 mm/day and 3.2 × 10−3 mm/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 × 10−3 mm/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 × 10−3 day−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 × 10−3 mm/day, and average rates of H2S alteration determined by sulfur alteration index divided by exposure duration of 6.3 × 10−3 day−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 in the rim of samples. The results indicate that (1) an aqueous environment is more favorable for H2S attack on pozzolan-amended cement than a supercritical CO2 and H2S environment; (2) for 90 days of exposure significant alteration induced by H2S and CO2 occurs at regions very close to the fluid/cement interface; (3) H2S penetrates pozzolan-amended cement more rapidly than CO2 in aqueous contact environments. In contrast, under supercritical liquid environment, H2S and CO2 have similar penetration rates in pozzolan-amended cement.