Rate of H2S and CO2 attack on pozzolan-amended Class H well cement under geologic sequestration conditions
Zhang, L., D.A. Dzombak, D.V. Nakles, S.B. Hawthorne, D.J. Miller, B.G. Kutchko, C.L. Lopano, and B.R. Strazisar, “Rate of H2S and CO2 attack on pozzolan-amended Class H well cement under geologic sequestration conditions,” International Journal of Greenhouse Gas Control 27, 299-308, 2014.
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 CO2under 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.
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