A Study Of The Storage Capacity Of Geothermal Reservoirs And The Exploitation Dynamics Of Thermal Water In Tianjin

Geothermal resources are proved to be clean and cheap. It is significant to research in storagecapacity. But over- exploitation makes the water level decline continually in Tianjin and can bringdisaster and resourses drying up. Therefore, it is essential to study the changes of the exploitation in thisarea.Geothermal groundwater resources are found to have occurred in deep-seated aquifers in thenortheastern North China Plain near Tianjin, China. Meso- to Neo-Proterozoic and Peleozoic carbonaterocks on the Cangxian uplift are capable of yielding 60-300 m3/h of 50-100℃ water from the wellsranging in depth between 1000 and 4000 m. Conductive heat flow of 64.48-124.35 mW/m2 from the deepcrust is responsible for these anomalous geothermal resources.In this paper, we analyze the lithology, pore character, permeable coefficient and tectonic characterof and the Wumishan group reservior, and mainly analyze the permeability and the distributing ofgeothermal wells. It is proved that the permeability is relation to the structure of thermal reservoir. InTianjin fault and the Baitangkou East fault, the permeability of the Ordovician system reservoir is strong,the yield respectively is 160 m3/h,186 m3/h, the temperature respectively is 63℃,70℃. Thepermeability of the Wumishan group reservoir is very good in the east of the the Baitangkou East fault(the yield is 303 m3/h, the temperature is 91℃), and strong in the Liqizhuang fault and Haihe fault(the yield is 205m3/h, the temperature is 90℃,88℃).Long-term exploitation of thermal groundwater in deep-seated aquifers has led to a continual fallingin water levels at geothermal wells. Periodic fluctuations in the falling water levels due to exploitation ofthermal groundwater in a pumping period followed by a non-pumping period can also be observed. Onthe other hand, temperature may affect the water level at geothermal wells. The observed water levelscan not reflect the realistic water levels of thermal groundwater in deep-seated aquifers. Calibration ofwater levels is necessary. The calibrated water levels show complex periodic changes and trendingchanges. In this paper, mathematical models containing the sum of a periodic term and a trending termare established to describe the changes in water levels of thermal groundwater. The trending term isfitted by using a linear function, and the periodic term, by Fourier series. The periodic term is analyzedwith the spectrum method and determined with the least square method. The Fourier series have 10 and12 items. The fitting is verified with the mean square error of the calibrated and computed water levelsand reasonable fitting is obtained. The mathematical models developed can be used to predict thechanges in water levels of thermal groundwater in the deep-seated aquifers and good results are alsoattained.By the end of 2004, the number of the geothermal well is 251 and the yield is 2467.9×104m3/a.Thethermal groundwater is mainly used for the heating, aquiculture, crop, bath, industry and producingmineral water, et al. But there is a series of the problems in the reconnaissance and exploitation of thegeothermal resources. This paper puts forward to the geothermal resources continuable utilizationproject- reasonable exploitation, reinforcing resources protection, improving utilization, enhancingmanagement study and reinjection technic.