Excess carbon dioxide emission was considered as the most important cause of increased trend of global warming. Significant amounts of research were devoted to the reduction of CO 2 emission and CO 2 sequestration. Sequestration of CO 2 in empty oil reservoirs was considered as one of the most promising options. However, the effects of metal release and mobility as a result of CO 2 injection were not studied in detail, particularly under super-critical CO 2 (scCO 2 ) conditions. In this study, the release of selected metals immersed in distilled water for varying amounts of time in the absence and presence of scCO 2 was assessed in simulated conditions at 90°C and 24 MPa. Significant increases in dissolution of Fe, Sr, and Ba by 3, 8, and 24 times were found when the storage rock sandstone or caprock shale was immersed in DI water for different time period. However, in the presence of scCO 2 , the dissolution of these metals was reduced by 80% for Fe, suggesting permanent sequestration of scCO 2 into carbonate minerals. The trend in changes of pore water chemistry in the sandstone and shale after being immersed in DI water showed dissolution of Sr-bearing mineral and precipitation of Ba-bearing mineral.
This research is aimed toward an understanding of the effects of the chemical characteristics and mineral compositions of sandstone and formation water based on saline water-rock-supercritical CO 2 interaction simulation experiments. These experiments were conducted to assess whether toxic trace elements could be dissolved and released in formation water from sandstone in a CO 2 storage layer after CO 2 geological sequestration, thus affecting groundwater quality. The experimental results reveal that the concentrations of Cd and Pb in the water under examination exceeded the national primary drinking standard as a result of saline/fresh water-rock-supercritical CO 2 interactions after 40 d of sandstone immersion in saline/fresh water and 20 d of interaction. In addition, the Mn concentration in the saline/fresh water exceeded the national secondary drinking standard after 40 d of sandstone immersion and 20-80 d of interaction. However, Cd, Pb, and Mn were released to a greater extent (in terms of concentration, 2-fold for Cd, 7-fold for Pb, and 1.7-fold for Mn) in the presence of salinity, revealing that salinity may enhance the dissolution of Cd, Pb, and Mn after 20 d of saline water-rock-scCO 2 interaction. After a long period of supercritical CO 2 -sandstone interaction, the trace metals previously mobilized can be immobilized again by an increase in alkalinity due to aragonite dissolution.
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