土壤氣體地球化學監測技術評估

在大量二氧化碳氣體灌注地下進行封存後，如何確保無洩漏之虞，持續監測是驗證整體計畫成效的一環，至關重要。相較於鑽鑿觀測井量測深井流體、震測成像技術、現地微震觀察、示蹤劑注入法，抑或大氣中二氧化碳濃度量測等各項技術，地表土壤氣體的監測是直接且成本低廉的方法之一。在採用密閉氣罩法針對永和山地區進行土壤二氧化碳與甲烷氣的通量調查，本研究累積完成十個固定樣點的土壤氣體成分分析，以及全年十二次的調查結果，並參考美國經濟地質局所屬的墨西哥灣區碳研究中心（Gulf Coast Carbon Center）的研究進展，引進偵測氣體洩漏的最新判讀技術。全年數據顯示，永和山地區二氧化碳通量平均為15.62 g/m2/day，略低於台灣斷層帶的平均二氧化碳通量（約17 g/m2/day），顯示該地區並無旺盛二氧化碳逸氣作用，地質活動相對平穩。土壤氣體分析結果顯示永和山地區二氧化碳的平均濃度為3.06%，高濃度異常值可能受作物或農耕活動影響。鑑於近年來國際地質封存計畫的實務經驗，以及本所計畫項目為期一年以上的觀察，土壤氣體濃度的監測方法仍有諸多缺點及限制。包括區域性的土壤逸氣特徵，常受壤土組成、緊實程度、水汽含量等因素影響，濃度高低原本就各自差異。植物根系和微生物的呼吸作用、土壤內有機物的分解作用，皆能增加土壤中整體二氧化碳的濃度含量。空氣或土壤中的溫度，也證實影響上述生物作用，左右著土壤二氧化碳的通量變化。單日、季節性或氣候性的變化，常造成氣體洩漏監測判讀錯誤，為土壤二氧化碳逸氣的基準值判識帶來困擾。為提昇封存場址洩漏監測的成本效益，參考美國經濟地質局所屬的墨西哥灣區碳研究中心（Gulf Coast Carbon Center）的最新發表的氣體洩漏的偵測技術，應用氣體濃度比值法判讀二氧化碳來源。此項技術取代了單純以氣體絕對濃度的高低作為判別依據，而是基於土壤或地層內反應作用的趨勢，作為判讀標準。此法直接以地層中氮、氧、二氧化碳和甲烷等氣體間的化學關係進行觀測對比，檢視系統內是否有外來洩漏氣體的加入。類似於傳統地球化學的濃度比值對比法，此法相較於絕對濃度量測法更為客觀，而且具有廣泛的應用性，省卻了長期的背景值量測，而且能就封存灌注各施作階段進行有效監測，可降低洩漏判定的不確定風險，增進辨識外來氣體的能力。

關鍵字

二氧化碳；地質封存；洩漏監測；土壤氣通量

並列摘要

It is crucial to the geological carbon sequestration (GCS) that continued monitoring is to verify the efficiency of the project, and to detect if there is a gas leak after injection of massive carbon dioxide into underground reservoirs for long-term storage. There are several detection techniques, such as monitoring the fluids in the observation wells, Vertical Seismic Profiling (VSP), microseimic monitoring, injected geochemical tracer technique, and measuring the ambient CO2 concentration. Compared with those methods, soil gas monitoring is one of the most cost-effective techniques. This study accomplished a year-long measurement of the soil gas fluxes and concentrations in the Yunghoshan area. Our data indicate that the average CO2 flux is 15.62 g/m^2/day, slightly lower than the average value in the fault zone of Taiwan. In addition, the fluxes of methane are relatively stable without distinct variation. The results indicated that there was no active gas leak and the regional geology is stable. The results of soil gas analysis show that the average concentration of CO2 is 3.06% in the vadose zone, and the high anomaly values may be affected by the local agricultural activities.According to the experience of international GCS projects and our one year-long observation, there are some disadvantage and limitation about the soil gas monitoring technique. Although long-term monitoring is necessary to the whole project, the high variability of the soil gas which usually makes the high anomalies nonrepresentative will hinder the application of this technique to detect deep exogenous leakage of CO2, if there is any. Therefore, more efficient methods of soil gas monitoring are still in demand. In order to enhance the efficiency of monitoring in the geologic storage site, the newly developed CO2 leakage detection technique has been employed through the collaboration with Gulf Coast Carbon Center (GCCC) in the University of Texas at Austin. The method could be applied the gas concentration ratios to distinguish the sources of vadose zone CO2. Instead of using the absolute gas concentrations as discriminating criteria, this approach based on the in-situ vadose zone background processes to examine chemical relationships between N2, O2, CO2, and CH4 to promptly distinguish a CO2 leakage signal from exogenous input into the vadose zone. Similar to traditional geochemical analysis by concentration ratio comparison, this approach is more objective than solely determination by the absolute vadose zone CO2 content, and could be widely applied. It can not only economize on years of background measurements to quantify variability of natural vadose zone CO2, but can also detect deep exogenous leakage with more efficiency during each stage of the GCS project. Thus this technique can reduce the uncertainty and risk of erroneous determination, and improve the ability of distinguishing deep exogenous leakage of CO2.

並列關鍵字

Carbon Dioxide ； Geological Carbon Sequestration ； Soil Gas Monitoring

國際替代計量

土壤氣體地球化學監測技術評估

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