溶解封存(Solubility trapping)為二氧化碳地質封存重要機制之一，瞭解其封存機制將有效提升於後續礦化封存、溶解量推估以及風險與安全評估方面之研究。本研究探討以溶解相二氧化碳於小尺度異質性鹽水層中，由重力及密度差異所造成之自然對流(natural convection)效應，研究工作中專注於液相二氧化碳於鹽水層中垂直向遷移行為討論，其中賦予岩層滲透性具不同程度之異質性變化．數值建構中利用單向降尺度法(one-way downscaling approach)於以解析由大尺度模擬區域縮減至小尺度模擬範圍，提高小尺度範圍模擬結果之效率與精確性，並以sequential Gaussian法建立滲透係數(permeability)隨機場，利用均值滲透係數(lnk)作為模擬區域之背景值，以滲透係數小擾動量( )與相關長度比率( )表示隨機場之變異程度。不同於以往研究以線性方程式或擾流行為參數控制模擬邊界驅動液相二氧化碳溶解，本研究以符合自然界條件之岩體異質性作為自然驅動性，搭配數值模式TOUGHREACT/ECO2N模擬不同條件案例下液相二氧化碳之遷移行為。由研究結果顯示，於固定之超臨界態二氧化碳飽和度進行溶解時，不同案例下液相二氧化碳仍具有同樣之擴散過渡帶，此過渡帶與岩層異質性無相關，為二氧化碳分子垂直擴散之區間，而過渡帶之下局部滲透係數變異性可以觸發指狀流和增強已溶解之二氧化碳的垂直對流，而指狀流之數量與型態變化取決於過渡帶底部邊緣滲透率之變異性，其後續之對流增量亦受控於隨機場滲透係數之變化．當異質性程度越高時，則二氧化碳之溶解率與溶解量將隨之增加，然在相同之變異係數上，當側向(x-direction)滲透係數相關長度高於垂直向(z- direction)時，二氧化碳之溶解率與溶解量明顯降低，因垂直向之滲透係數較低時，可抵制了重力驅動效應，液相二氧化碳於側向遷移明顯增加；二氧化碳之溶解率與溶解量於溶解初期時明顯有較大變化量，隨時間增加則趨向平緩，推測原因為流場內部流體密度趨近一致，減少密度流之效應發生．研究案例同時改變均值滲透係數大小，由結果顯示，均值滲透係數控制整體溶解行為．本研究所提出之降尺度法可有效解決大尺度數值模式之網格需求與解析效率，提升相關數值問題之精確度；而本研究之模擬結果可提供於二氧化碳溶解量與長期移棲行為之探討，針對封存量推估與安全風險評估機制使用。;This study simulated the natural convection of dissolved carbon dioxide (CO2) in a small-scale heterogeneous saline formation by using the ECO2N equation of state module in the TOUGHREACT model. A one-way downscaling approach that involves using a series of submodels in simulation procedures was proposed to efficiently simulate problems with high-scale discrepancies. The study evaluated the effects of different degrees of small-scale permeability variations on the vertical migration of dissolved CO2. The sequential Gaussian simulation model was used to generate unconditional random permeability fields for different natural logarithm of permeability (lnk) variations (i.e., lnk variances and correlations in x and z directions). The results showed an identical transition zone of dissolved CO2 near the top boundary, where a constant CO2 gas saturation was specified. The local permeability variations can trigger fingerings and enhance the vertical convection of the dissolved CO2. The number of fingerings depends on the variations of permeability near the front interface of the dissolved CO2 (i.e., the bottom edge of the transition zone for the dissolved CO2). However, the patterns and developments of fingerings are constrained by the permeability variations along the fingering paths. At the same mean lnk permeability, the convection fluxes increase with an increase in lnk variances. However, an increase in lateral correlations (i.e., increase in the correlation lengths in the x direction) can slightly reduce the convection fluxes at the same lnk variance. The highly variable flux rates of the dissolved CO2 occur early and the variations in the flux rate decrease with time.
