利用毛細管與雙通道微模型探討侷限空氣對非飽和層二氧化碳傳輸過程的影響

Abstract

地表下二氧化碳傳輸因為土壤碳排、碳匯、與地質碳封存等溫室氣體與氣候變遷相關議題而受到關注。在非飽和層此空氣-水-土壤系統中是必須考慮空氣對流體傳輸的影響，以及二氧化碳在液-氣介面的動態傳輸過程。然而此機制不易在達西尺度模式描述，即，缺乏將此孔隙尺度現象深尺度為達西尺度模式可用之模型；為此，本研究以孔隙尺度實驗初步探討孔隙中水分與空氣分佈對於二氧化碳傳輸的影響與傳輸行為特性。本研究孔隙尺度實驗包含毛細管實驗與雙通道微模型實驗。毛細管實驗旨在相同飽和度與孔隙幾何下，液相的不同分段形式對於二氧化碳傳輸過程的影響；而雙通道微模型旨在檢驗新水中的溶解二氧化碳能否能擴散至孔隙介質中的受空氣侷限的殘餘水。對於兩實驗皆有建立以及相對應數學模式說明所發現之氣體-水-溶解二氧化碳互動行為在。毛細管實驗發現：在空氣存在的情況下，隨著液相分段數增加，氣相體積發展以及液相傳遞速度呈現遞減的趨勢。當二氧化碳從液相進入氣相的過程中，會導致氣泡體積增加。雙通道微模型實驗結果顯示，新水中的二氧化碳可能透過氣相傳輸或者角落薄膜流(corner film flow)進入殘餘水。透過比較數學模式模擬結果與實驗結果發現，考慮介面質傳與侷限空氣段影響等建立的模式具備足夠的預測準確度。然而模式在模擬實驗後期呈現低估濃度與氣泡體積的現象，推測可能原因有濃度較高的情況下亨利定律無法成立、蒸發影響、流道幾何形狀等其他尚未考量的因素。論文最後探討模式應用於雙通道微模型上無法預測以及仍需解決的議題。

The subsurface transportation of carbon dioxide draws a great attention due to its relevance to greenhouse gas emissions, carbon sinks, and geological carbon sequestration in relation to climate change. In the unsaturated zone, the air-water-soil system, the impact of air entrapment on fluid transport and the dynamic transfer process of carbon dioxide at the liquid-gas interface must be considered. However, those mechanisms are not easily described in Darcy-scale models, i.e., lacking models that can bridge the pore-scale phenomena to the Darcy-scale models. Therefore, our study preliminarily investigates the influence and transport behavior of water and air distribution in pores on carbon dioxide transport through pore-scale experiments. The pore-scale experiments in this study include capillary tube experiments and dual-channel micro-model experiments. The capillary tube experiments aim to examine the influence of different liquid phase configurations (numbers of air segments) under the same saturation and pore geometry on the process of carbon dioxide transport. The dual-channel micro-model is designed to explore whether dissolved carbon dioxide in new water can diffuse into the air-restricted residual water in the pore medium. Mathematical models have been established for both experiments to explain the discovered gas-water-dissolved carbon dioxide interaction behaviors. The results of capillary tube experiments show that in the presence of air segments, as the number of liquid phase segments increases, the development of the gas phase volume and the rate of liquid phase transmission show a decreasing trend. The process of carbon dioxide transitioning from the liquid phase to the gas phase leads to an increase in bubble volume. The results of the dual-channel micro-model experiments showed that carbon dioxide in new water may enter the residual water through gas-phase transport or corner film flow. By comparing the simulated results from mathematical models with experimental findings, it was determined that the models, which take into account interfacial mass transfer and restricted air segments, exhibit sufficient predictive accuracy. However, the models tend to underestimate the concentration and bubble volume in the later stages of the experiments. This phenomenon could be attributed to factors not yet considered, such as the breakdown of Henry's law at higher concentrations, evaporation effects, and geometric shapes of flow channels. In conclusion, the paper explores the unresolved issues and limitations of applying the model to the dual-channel micro-model, emphasizing the need for further investigation and resolution.