以微流體技術探討侷限氣泡對非飽和帶中二氧化碳的傳輸的影響

Abstract

二氧化碳在非飽和帶中可以以氣相流動存在或以溶解態形式在孔隙與液體中運移，其傳輸行為不僅受氣相連通性控制，也會受到孔隙水流動、氣液界面質傳與侷限氣泡動態影響。尤其當溶解態二氧化碳脫溶析出至受困氣泡時，氣泡體積與內壓會隨之改變，進而造成流體通道孔喉阻塞、造成界面突破或跳躍式位移，更可能導致與流路重組；然而，這些極為複雜的孔隙尺度氣液傳輸過程難以從由傳統達西尺度模式直接觀測與描述。為釐清可溶性氣體、液相與侷限氣泡之間的交互作用，本研究利用微流體可視化技術，結合窄縮道毛細管、雙通道微模型與複雜孔隙微模型進行觀測實驗，並根據質傳理論建立對應數值模式，分析氣泡成長、孔喉受限、氣泡位移與濃度傳輸之間的關係。 結果顯示，溶解態二氧化碳進入侷限氣泡後會使氣泡體積增加，且孔喉幾何與接觸角遲滯會共同控制界面停滯、突破與體積變化。單窄道實驗指出，較窄孔喉需累積較高內壓才會突破，突破後界面位移與體積變化更明顯，但是雙窄道窄口突破後，卻沒有顯著差異；雙通道微模型則顯示，兩顆侷限氣泡與競爭流路同時存在時，氣泡成長會造成局部堵塞與壓力重新分配，進而導致氣泡斷裂(snap-off)，並誘發另一顆氣泡突破毛細壓力進入窄道。複雜孔隙微模型中，提高液體流量雖會加快濃度前緣推進，但氣泡成長、移動與跳躍會造成局部濃度起伏與流路重新分配。研究結果顯示，在高濃度二氧化碳液體傳輸中，二氧化碳不僅會解溶析出至侷限氣泡，並且會改變局部飽和度與流路結構的動態過程，不應僅被視為在氣體傳輸過程中，單純殘餘氣體與靜態障礙。

Carbon dioxide in the vadose zone can migrate either as a flowing gas phase or in dissolved form within the pore liquid. Its transport behavior is controlled not only by gas-phase connectivity, but also by pore-water flow, gas-liquid interfacial mass transfer, and the dynamics of trapped bubbles. In particular, when dissolved CO₂ exsolves into trapped bubbles, the resulting changes in bubble volume and internal pressure can block pore throats, trigger interfacial breakthrough or jump-like displacement, and reorganize flow paths. However, these highly complex pore-scale gas-liquid transport processes are difficult to directly observe and describe using conventional Darcy-scale models. To clarify the interactions among soluble gas, the liquid phase, and trapped bubbles, this study employed microfluidic visualization techniques, including constricted capillary tubes, double-channel micromodels, and complex pore micromodels, to conduct observational experiments. Corresponding numerical models were also established based on mass-transfer theory to analyze the relationships among bubble growth, pore-throat constriction, bubble displacement, and concentration transport. The results show that when dissolved CO₂ exsolves into trapped bubbles, bubble volume increases, while pore-throat geometry and contact angle hysteresis jointly control interfacial pinning, breakthrough, and volume changes. Experiments with single-constriction capillaries indicate that narrower pore throats require higher accumulated internal pressure for breakthrough, and that interfacial displacement and volume change become more pronounced after breakthrough. However, no significant difference is observed after breakthrough through the narrow opening in the double-constriction case. The pore-doublet micromodel further demonstrates that when two trapped bubbles coexist with competing flow paths, bubble growth can cause local blockage and pressure redistribution, leading to bubble breakup, interfacial displacement, and the movement of another bubble through the constriction. In the complex pore micromodel, although increasing the liquid flow rate accelerates the advancement of the concentration front, bubble growth, movement, and jump-like displacement induce local concentration fluctuations and flow-path redistribution. Overall, the results demonstrate that during the transport of CO₂-rich liquid, CO₂ not only exsolves into trapped bubbles but also dynamically alters local saturation and flow-path structure. Trapped gas should therefore not be regarded merely as residual gas or a static obstacle in the gas-transport process.