Research on the Variation of Groundwater Temperature
Date Issued
2016
Date
2016
Author(s)
Tai, Tung-Lin
Abstract
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.
Subjects
groundwater temperature
borehole
aquifer
permeable zones
geothermal gradient
Type
thesis
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