在控制壓力積聚條件下台灣西部深層鹽水含水層二氧化碳地質封存數值模擬

Numerical Simulation of the Geologic Sequestration of Carbon Dioxide under Controlled Pressure Buildup in a Deep Saline Aquifer in Western Taiwan

吳承諺(Cheng-Yan Wu)

指導教授 ： 劉台生

國立中正大學/理學院/地球與環境科學系地震研究所/碩士(2013年)

摘要

為減緩溫室氣體對全球暖化的惡化，將溫室氣體的主要成份（二氧化碳，CO2）從大氣中捕獲並注入至深地層的碳捕獲及封存(Carbon Capture and Sequestration，CCS) 為目前世界上認為經濟可行的減碳技術。本研究假想台灣西部沿海的T火力發電廠(以下簡稱T電廠)為CO2排放源，以鄰近T電廠的沈積盆地（以下簡稱T盆地）下方深層鹽水含水層作為試驗封存場址(以下簡稱試驗場址)，並以TOUGH2 ECO2N (Pruess, 2005)模擬CO2在封存場址中的移棲。T盆地地層平緩且無特殊地質構造，而試驗場址考慮的地層由淺至深分別為卓蘭層、錦水頁岩、桂竹林層等三個岩層，其中低滲透性的錦水頁岩與高孔隙率的桂竹林層分別視為蓋層與儲集層。試驗場址中的錦水頁岩有往西尖滅之趨勢，考量蓋層的側向延伸性，故本研究以錦水頁岩等厚度為10 m的界線作為模擬區域西側的邊界。模擬步驟先決定CO2總注入量，再限定注入CO2後造成壓力積聚(pressure buildup)的增幅，最後再調整注入率或注入井數目以進行模擬。本研究基本案例假設以單口井將CO2注入桂竹林層，並且以每年4Mt(百萬噸)的固定注入率持續注入15年，其中4Mt約為T電廠10%年排放量，故總注入量為60 Mt。T盆地地層整體趨勢往東北方向微傾，而模擬區域內的地層大多往東傾，故CO2注入後因浮力作用累積在蓋層下方並往西方移棲。模擬結果顯示，壓力積聚增幅隨深度而遞減，故注入井頂端的壓力積聚增幅最大。壓力積聚太大時容易對岩層造成力學破壞，因此建議可降低注入率或以多井注入的方式分散注入量，此方法能有效降低壓力積聚。然而，多井注入雖然可降低壓力積聚，但注入井頂端的固態鹽沉降會增多，可能導致注入井堵塞。多井注入因各井的貫穿深度不同，導致注入時壓力積聚大小亦會不同，當注入井貫穿深度越深，壓力積聚增幅越小。在敏感度分析中，若改變桂竹林層水文地質參數則會影響CO2團(plume)在桂竹林層中的徑向擴展範圍大小，而改變錦水頁岩層參數則會影響CO2封存量及蓋層封閉性。

關鍵字

壓力積聚 ； 地質封存 ； 二氧化碳 ； ECO2N

並列摘要

Carbon capture and sequestration (CCS) is believed to be an economically feasible technology to mitigate global warming by capturing carbon dioxide (CO2), the major component of greenhouse gases, from the atmosphere and injecting it into deep geological formations. In this study, a coal-fired power plant (T-plant for short) located in the western coastline of Taiwan was targeted as the emission source. A massive sedimentary basin (T-basin) close to the T-plant was considered as a hypothetical sequestration site. Major rock formations (from top down) considered in the simulation include Cholan formation, Chinshui shale, and Kueichulin formation. The low permeability of Chinshui shale and high porosity of Kueichulin sandstone render them suitable as the cap rock and storage formation, respectively. Because Chinshui shale pinches out towards the west, the simulation domain uses the 10 m isopach of Chinshui as the western boundary. Migration of CO2 was simulated using the TOUGH2 ECO2N code (Pruess, 1995). The first step in simulation is to determine the total amount of injected CO2. Subsequently, the maximum ratio of pressure buildup as a result of injection is specified. Finally, the number and locations of injection boreholes or injection rates are adjusted heuristically such that the maximum ratio of pressure buildup in boreholes is within the specified upper limit. Totally 60 Mt of CO2 is injected into Kueichulin formation, which is uniformly distributed in 15 years and corresponds to a constant injection rate of 4 Mt/yr. This injection rate is approximately 10% of the annual emission rate of the T-plant. Because all rock formations in the simulation domain dip gently to the east, the injected CO2 generally migrates to the west as a result of buoyancy and is accumulated beneath the cap rock. Pressure buildup in the injection borehole decreases with depth, and the maximum pressure buildup occurs at the top of the injection borehole. Although the scheme of multiple injection boreholes can effectively reduce pressure buildup, salt precipitation at the top of injection borehole will be inevitably increased and may result in clogging of the borehole. Sensitivity studies indicate that changing the hydrogeological parameters of the Kueichulin formation will increase the radial extent of the CO2 plume, whereas changing the parameters of the Chinshui shale will affect the amount of sequestration and the sealing ability of the cap rock.

並列關鍵字

Pressure buildup ； Geological carbon sequestration ； Carbon dioxide ； ECO2N

參考文獻

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