地下水溫度變化之研究

Abstract

地下水溫度變化會受到氣溫、地層溫度、地溫梯度、季節性補注、水文地質特性與井水流動等因素影響，本研究藉由地下水位/水溫計在金門井場與和社試驗井場進行地下水溫監測，監測記錄指出，造成地下水溫的季節性變化主要受控於側向地下水流，從補注區流到監測井的時間會延遲水溫變化，呈現夏季水溫較低，冬季水溫卻較高之現象。 本研究藉由溫度探測儀進行井測，調查井孔垂向地下水溫度之分布情形，地下水在孔隙介質中的主要熱傳輸機制為傳導與對流，井孔內地下水溫度的改變受控於不同地層的熱傳導係數，但其變化也受到地下水流動而產生對流效應所影響。本研究嘗試在台大農場60公尺深的觀測井進行礫石含水層的溫度量測先導試驗，井孔溫度剖面顯示在42公尺處，開篩段的頂端有明顯轉折，此現象為地下水流入井中產生的強制對流效應，將造成孔溫剖面出現異常，故孔溫剖面可能顯示出井內的透水帶位置。此外，井內量測值並不能代表井外的含水層溫度，必須藉由抽出井內的滯留水，使井水逐漸被含水層中的地下水置換，當水溫隨著抽水過程逐漸趨於穩定，此時的觀測值才代表鄰近的含水層溫度。 本研究嘗試利用井孔溫度搭配熱脈衝流速儀量測之透水性指標，在和社試驗井場調查裂隙岩體的透水裂隙，相互比對發現在特定條件下量測的孔溫，可指示透水性較高的裂隙區段，特別是流速較快的情況。另外，為改進過去以井孔溫度分布估算地溫梯度的方式，本研究選定宜蘭縣龍德站四口觀測井，藉由抽水與水溫監測方式，量測出較正確的含水層溫度，進而估算出地溫梯度為89 ℃/km，比過去經由井孔溫度量測估算之地溫梯度高出7 ℃/km，建議未來採用抽水方式量測實際地層溫度，做為地溫梯度估算之依據。

Variations of groundwater temperature are affected by a variety of factors, such as surface temperature change, formation temperature, geothermal gradient, seasonal groundwater recharge, hydrogeologic characterization, and water flows inside the borehole. In this study, we use groundwater level/temperature monitoring to study the factors of temperature variations over time. The Kinmen and Heshe monitoring data indicates that seasonal changes of the groundwater temperature are mainly resulted from lateral flow of the deep recharge water, causing about 3－6 months’ time delay from the recharge area, which means groundwater temperature may become lower in summer and higher in winter. We use the logging device, temperature probe, to investigate the vertical distribution of borehole temperature. The major groundwater heat transfer in porous medium are conduction and convection. The temperature distribution in geological formation is primarily controlled by conduction, while heat convection due to flow can also modify the distribution. Then the pilot test of field measurement is conducted at a NTU 60m deep well in a gravelly aquifer to characterize the temperature profile of screened zone. However, the slope of temperature profile changes at approximately 42m deep, the top of well screen, and it points out the effects of forced convection in the aquifer. In addition, the measure borehole temperature may not represent the aquifer temperature near the observation well. The measure temperature in the screened section changes continuously in response to pumping, but stabilizes when borehole water volume is extracted, which represents the true aquifer temperature. Another field test is conducted at Heshe test site in the fractured rock formation to characterize the preferential flow area. Detection of the borehole temperature anomaly often indicates the lateral water flow inside the open holes. Compared with results of permeable index by using heat-pulse flowmeter, we find that temperature logging in certain condition is possible to locate some permeable fracture zones, especially the high flow velocity. The other application is the improved estimate of geothermal gradient at Lungte well station, the traditional way is often based on the measurement of borehole temperature. Consequently, we adopt pumping and temperature monitoring approach in 4 wells at the Lungte station to obtain a more precise formation temperature, and the geothermal gradient estimates from formation temperatures is about 89 ℃/km, which is 7 ℃/km higher than that obtained from the borehole temperature. Our test results suggest that it is essential to measure the true formation temperature in order to improve the calculation of geothermal gradient.