利用跨孔奈米鐵示蹤劑試驗偵測裂隙水流路徑之研究

Delineation of Fracture Flow Paths Using a Cross-hole Nano-Iron Tracer Test

莊伯禹(Po-Yu Chuang)

指導教授 ： 鄧茂華

共同指導教授 ： 賈儀平(Yeeping Chia)

國立臺灣大學/理學院/地質科學研究所/博士(2017年)

https://doi.org/10.6342/NTU201701358

摘要

近年來國際上發展諸多地球物理技術，可間接推判裂隙岩層中優勢地下水流路徑，然而直接偵測技術仍在發展中。本研究針對調查技術瓶頸與關鍵課題，首次將奈米級零價鐵作為示蹤劑調查裂隙水流路徑，奈米鐵溶液已被廣泛的運用在地下水污染整治方面，研究中利用奈米鐵具備磁性的特性，設計一個磁鐵陣列偵測裝置，當奈米鐵投注於試驗井後，流經透水連通裂隙到達觀測井，並被置於觀測井內的磁鐵陣列吸附，藉由奈米鐵顆粒吸附之量化分布，可直接得知示蹤劑抵達觀測井的深度位置，進而估算地下水及滲流大小或污染物(溶質)傳輸可能擴散及延散範圍。
本研究選定位於南投縣信義鄉的和社水文地質試驗井場進行現地試驗，當地岩層裂隙大多導水性不佳，僅有少數透水裂隙，為確認試驗井之間的水力連通性，先藉由跨孔水力試驗，選定兩口具良好水力連通性之裸井作為試驗井，再運用熱脈衝流速儀量測兩口井之透水裂隙垂向分布情形，繼而依此規劃奈米鐵示蹤劑試驗之投注位置及導電度監測位置。從觀測井的導電度變化，可以得知示蹤劑經透水裂隙抵達觀測井之所需時間，同時利用磁鐵陣列奈米鐵吸附量的分布剖面，可判釋透水連通裂隙的深度位置。本研究發現奈米鐵示蹤劑抵達的位置與觀測井高度透水裂隙所在位置相符，然而奈米鐵示蹤劑的回收率僅有0.01%，可能會大幅限制現地應用價值。
經檢討改進後，本研究選擇一口裸井與一口全開篩井，進行第二次跨孔奈米鐵示蹤劑試驗。先利用熱脈衝流速儀調查兩口井的透水裂隙分布情形，並依此選定注入井1.5 m開篩區段，封塞其他區段後，將奈米鐵示蹤劑注入。由觀測井磁鐵陣列吸附奈米鐵的分布情形，可判釋示蹤劑經由裂隙抵達觀測井的深度。試驗結果指出奈米鐵顆粒的回收率高達7.5%，使用區段開篩的注入方式可有效增加奈米鐵顆粒流進岩層裂隙的數目，而奈米鐵吸附量最多的位置，也是熱脈衝流速儀所偵測到的透水裂隙所在位置。本研究也執行跨孔鹽水示蹤劑試驗，試驗成果用以比對兩種示蹤劑的傳輸與延散特性。此外利用MODFLOW和MT3DMS建置一個水文地質概念模型，計算相連透水裂隙之水文地質參數。本研究結果顯示，奈米鐵示蹤劑試驗搭配熱脈衝流速儀試驗，有潛力運用於偵測裂隙岩層中優勢水流路徑。

關鍵字

裂隙岩層 ； 流速儀 ； 奈米級零價鐵 ； 示蹤劑試驗 ； 水流路徑

並列摘要

Recent advances in borehole geophysical techniques have improved characterization of cross-hole fracture flow. The direct detection of preferential flow paths in fractured rock, however, remains to be resolved. Nanoscale zero-valent iron (nZVI) has been widely applied to in-situ remediation of groundwater contamination. In this study, a novel approach using nZVI particles as tracers was developed for detecting fracture flow paths directly. As nZVI particles are magnetic, they are attracted to magnets. This feature spurred the development of a magnet array for locating the position of incoming tracers. When nano-iron particles are released in an injection well, they can migrate through the connecting permeable fracture and be attracted to a magnet array when arriving in an observation well. Such an attraction of incoming iron nanoparticles by the magnet can provide quantitative information for locating the position of the tracer inlet.
Two nZVI tracer tests were implemented in fractured rock at a hydrogeological research station in central Taiwan. Generally only a few rock fractures are permeable while most are much less permeable. In the first field test, a series of field experiments were conducted in two open boreholes. A heat-pulse flowmeter can be used to detect changes in flow velocity for delineating permeable fracture zones in the borehole and providing the design basis for the tracer test. The fluid conductivity recorded in the observation well confirmed the arrival of the injected nano-iron slurry. All of the iron nanoparticles attracted to the magnet array in the observation well were found at the depth of a permeable fracture zone delineated by the flowmeter. However, a low mass recovery rate of approximately 0.01% would restrict the potential application of nZVI particles as tracers beyond the experimental range.
In the second field test, the tracer test was intended to carry out between an open hole and a screened well at the hydrogeological research station. Heat-pulse flowmeter tests were conducted to characterize the vertical distribution of permeable zones for locating the screened depth of the injection well. After the sealing of the injection well, the screened section was only 1.5 m. The nZVI slurry was released in the screened injection well. The arrival of the slurry in the observation well was detected by an increase in electrical conductivity, while the depth of the connected fracture was identified by the distribution of nZVI particles attracted to the magnet array. The mass recovery rate in the nZVI tracer test was approximately 7.5%, suggesting the mass recovery was successfully enhanced by hydraulically isolating a permeable segment in the injection well. The position where the maximum weight of attracted nZVI particles was observed coincides with the depth of a permeable fracture zone delineated by the heat-pulse flowmeter. In addition, a saline tracer test produced comparable results with the nZVI tracer test. Numerical simulation was performed using MODFLOW with MT3DMS to estimate the hydraulic properties of the connected fracture zones between the two wells. The study results indicate that the nZVI particle could be a promising tracer for the characterization of flow paths in fractured rock.

並列關鍵字

Fractured rock ； Flowmeter ； Nanoscale zero-valent iron ； Tracer test ； Flow path

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