Effects of Calcite Dissolution on Caprock's Sealing Performance Under Geologic CO_2 Storage

摘要

To assess the long-term integrity of geologic CO2 storage, samples modeling caprock were reacted under a supercritical CO2-water system at 10 MPa and 40 degrees C. The resultant effects on their sealing performance were analyzed. The use of calcite-containing artificial samples comprising silica particles with controlled size, configuration, and packing density quantified the relation between the dissolved amount of calcite and permeability. The solution analysis revealed that the calcium concentration was once increased by the initial calcite dissolution and was then decreased by calcite reprecipitation at the time of depressurization. In response to this mechanism, the initial increase and subsequent decrease in permeability were confirmed. Correlation between the Ca-leaching amount and permeability fell into two categories depending on porosity and specific surface area: The sensitivity to permeability rose when both parameters were large. This correlation was maintained also in the case in which the Ca-leaching amount changed along with the porosity change. The power-law fitting to the porosity-permeability relation suggested that two correlations were derived from the degree of heterogeneity. In actuality, the pore diameter distribution of samples with suspected heterogeneity indicated a broad peak of the spectrum, which supported the existence of heterogeneity. Using the formulated relation, the permeability increase was estimated as less than just 2%, even when calcite dissolved to its saturated concentration. Therefore, if any calcite is contained in caprock, the risk of CO2 leakage is apparently extremely low.