Rate of CO_2 Attack on Hydrated Class H Well Cement under Geologic Sequestration Conditions

摘要

Experiments were conducted to study the degradation of hardened cement paste due to exposure to CO_2 and brine under geologic sequestration conditions (T= 50℃ and 30.3 MPa). The goal was to determine the rate of reaction of hydrated cement exposed to supercritical CO_2 and to CO_2-saturated brine to assess the potential impact of degradation in existing wells on CO_2 storage integrity. Two different forms of chemical alteration were observed. The supercritical CO_2 alteration of cement was similar in process to cement in contact with atmospheric CO_2 (ordinary carbonation), while alteration of cement exposed to CO_2- saturated brine was typical of acid attack on cement. Extrapolation of the hydrated cement alteration rate measured for 1 year indicates a penetration depth range of 1.00 ± 0.07 mm for the CO_2-saturated brine and 1.68 ± 0.24 mmforthe supercritical CO_2 after 30 years. These penetration depths are consistent with observations of field samples from an enhanced oil recovery site after 30 years of exposure to CO_2Vsaturated brine under similar temperature and pressure conditions. These results suggest that significant degradation due to matrix diffusion of CO_2 in intact Class H neat hydrated cement is unlikely on time scales of decades.