Three-dimensional Numerical Modeling Of Thermal Groundwater In The Bedrock Resenvior In The Dongli Lake Area Near Tianjin

In recent years, the development and utilization of geothermal resources in Tianjin increases gradually, causing many problems such as the decline in geothermal groundwater levels.In order to realize the sustainable utilization of the geothermal resources, this article analysesthe geological structure background, bedrock heat reservoir characteristics and the geothermal well datain the Tianjin area and the Dongli lake area. FEFLOW software is used to simulate and predict the changes in geothermal groundwater levelsand tempreture under different pumping schemes.The Tianjin area is located in north China fault depression basin, and geothermal groundwater resources are rich of low to moderate temperature. The Dongli lake area is located in the Cangxian uplift in the Tianjin geothermal field. Bedrock reservoirs mainly include geothermal groundwater reservoirs of the Cambrian system, the Ordovician system, andthe Wumishan Group of the Jixian system. The bedrock geothermal reservoirs are mainly composed of limestone and dolomite. The geothermal gradient in the geothermal fieldranges from5.0℃/100 m to 3.0℃/100 m, and the geothermal gradient is less than 1.0℃/100 m below the depth of 2000 m.The FEFLOW software is used to conducts numerical modeling.The top area of the model is about 468 km2, and the bottom area of the model is about 676 km2. The top elevation is-1666.7 m, and the bottom elevation is-4000 m. The model is divided into five layers vertically, and two areasare horizontally divided by the Cangdong fault. The model is discretized into 4970 units, and the eastern boundary is treated as aslope one. The northern and southern boundaries are setthe prescribed head boundaries, andthe eastern and western borderies, no-flow boundary. Production wells and injection wells are treated as source terms in the model. For modeling of heat transport, the reference temperatureis 85℃. As the bottom of the model contacts with the underground heat source, the heat flux of 0.0565 W/m2 inflows in the bottom boundary and 0.045 W/m2 outflows in the top boundary. Values of parameters are set vertically according to stratigraphic lithology and fracture characteristics, which inclues the coefficient of permeability and porosity.The dynamic monitoring data ofgeothermal wells T1, T2, T3, T1 B, T2 B and T3 B between August 2008 and July 2011 are used formodel identification. The monitoring data of August 2011 and July 2014 are used for model verification. The sensitivity of permeability coefficient, specific storage and the heat conductivity are analysed for validating the stability of the model.Three pumping schemes are designed, incluing the first scheme of present situation of pumping, the second schemeof increasing recharge rate to 92%, and the third schemeof reducing production of T2 and T3 and adding a new pumping well and a new injection well.Computation of ground water levels and temperature within the next ten years are conducted. The results indicate that the geothermal groundwater levels of pumping wells T1, T2 and T3 are in a decresing trend on the whole. In the first scheme, the geothermal groundwater levels drop nearly 20 m, with the lowest water level of-110 m. In the second scheme, the geothermal groundwater levels drop nearly 10 m, with the lowest level of-105 m. The drop of the geothermal groundwater levels reduce compared with the first scheme. Compared with the second scheme, the drop of geothermal groundwater levels in pumping wells T1, T2 and T3 decrease by 1 m, 1 m and 2 m in the third scheme. The temperature of the geothermal groundwater in the three schemes changes by only around 1 ℃.