The objective of this research is to investigate the changes of mineralogical and geochemical properties for Pohang basin sandstone and mudstone, resulted from the geochemical reaction with supercritical CO2 at the CO2 sequestration conditions. To investigate mineralogical and geochemical changes of rocks after the supercritical CO2 injection, the supercritical CO2-groundwater-rock reaction was reproduced in the laboratory at CO2 sequestration condition (50 ℃ and 100 bar). Rocks used in experiments were sandstone and mudstone in Pohang basin. For the experiments, rock samples were collected by using boring (390 m in depth) located near the coastal area at Yeonam-dong, Buk-gu, Pohang-si, Gyeongbuk, Korea and groudwater was sampled from the well (450 m in depth) located Heunghae-eup, Buk-gu, Pohang-si, Gyeongbuk, Korea. Rock samples used in the experiment were crushed into power. The high pressurized cell was filled in with 100 ml of groundwater and 30 g of powdered rock samples. The void space of the high pressurized cell was saturated with supercritical CO2 and maintained at 100 bar and 50 ℃ for 60 days. The changes of mineralogical and geochemical properties of rocks were measured by using XRD, XRF and BET and concentrations of dissolved ions in groundwater were analyzed on ICP/OES after 10, 30 and 60 days of the reaction, Rock samples used in the experiment cut into the slab (15 mm × 40 mm × 5 mm). The weight change of rock samples during the experiment was measured to calculate the dissolution rate constant (Kd) of rocks for 10, 30 and 60 days.
When the supercritical CO2 is stored in the underground aquifers, the weathering of rocks will occur actively according to the low pH condition of groundwater. The pH of groundwater was 8.72 before the experiment and decreased to 5.36 after 60 days and it maintained around pH 5.4 at CO2 sequestration condition. Results of XRD analyses indicated that the proportion of plagioclase and k-feldspar in the sandstone decreased and the proportion of calcite, pyrite, chlorite and smectite increased during the reaction. In the case of mudstone, the proportion of plagioclase, k-feldspar and mica decreased and the proportion of illite and smectite increased during the reaction. From SEM-EDS analysis, confirmed precipitant created after the supercritical CO2-groundwater-rock reaction and the precipitants were calcite, pyrite and kaolinite. The average specific surface area of sandstone and mudstone using BET analysis increased from 27.32 m2/g and 19.61 m2/g to 28.61 m2/g and 26.61 m2/g, respectively for 60 days of the reaction. Results of ICP/OES analysis, For the sandstone, concentration of Mg2+, Ca2+, Na+, Si4+, and K+ dissolved in groundwater increased from 1.99, 1.78, 137.70, 9.61 and 17.78 mg/L to 15.17, 34.37, 167.16, 14.9 and 16.16 mg/L, respectively for 60 days of the reaction. For mudstone, concentration of Mg2+, Ca2+, Na+ and K+ dissolved in groundwater increased from 1.99, 1.78, 137.70 and 17.78 mg/L to 39.55, 98.11, 132.90 and 31.00 mg/L, respectively for 60 days of the reaction. Results suggested that feldspars and calcite of rocks would be significantly dissolved when it contacts with supercritical CO2 and groundwater at CO2 sequestration site in Pohang basin.
The dissolution coefficient(Kd) value calculated from the weight loss of sandstone and mudstone were 0.00119 and 0.00134, respectively. The dissolution time for rocks were predicted using the first-order dissolution coefficients which were calculated from the experiment data. In the case of sandstone and mudstone, the time that 90 % of 1 g for the rock (0.9 g) dissolved by the reaction was 5.30 and 4.70 years, respectively, These results suggested that the dissolution of sandstone and mudstone in Pohang basin would quickly occur when the supercritical CO2 was injected.