3-Dimensional Numerical Modeling Of Unsteady Thermal Groundwater Flow And Heat Transport

In some regions of China the problems that geothermal resources are wasted and under the overexploitation often occur, which lead to a continual falling in water levels at geothermal wells. A 3D mathematical model describing thermal groundwater flow and heat transport is established and the numerical methods of the mathematical model have been discussed. The simulation and prediction of water levels under natural and exploited conditions in different geothermal systems can provide a suitable basis for assessing potential geothermal resources and making relevant exploitation plan, which is of an important significance for the sustainable utilization of geothermal resources.According to universal conservation principle, the 3D mathematical model describing low-medium thermal groundwater is deprived again and the influence that the density, dynamic viscosity is changed with pressure and temperature is considered carefully. The pressure mathematical model is solved with the standard Galerkin finite element method. The Darcy velocity is solved by the finite element method which is raised by Yeh. For the temperature mathematical model, the standard Garlerkin finite element method, the streamline upwind Petrov-Galerkin method and the Petrov-Galerkin least square method can be used to solve it. The thermal groundwater numerical program FEMFH is complied with Fortran 90/95 and the numerical results of program FEMFH are verified with the 1D analytical solution, the 2D Elder proplem and the software FEFLOW.For a horizontal, homogeneous and isotropic confined aquifer when heat flow from below is present, the numerical results show that the equivalent hydraulic head in x direction when the temperature difference between the top and bottom boundaries increases, the hydraulic gradient slightly increases with the temperature difference by using the program FEMFH. The equivalent hydraulic head in z direction is affected by temperature, obeys a nonlinear changes and increases with the increasing elevation, which can be described by using a 2th order polynomial function. In the same condition fluid flux increases with the temperature of aquifer, which are mainly affected by the dynamic viscosity. The equipotential lines are not always orthogonal to the streamlines in a geothermal system and when the temperature increases, the skew intersection angles between the streamlines and the equipotential lines decrease. When the hydraulic gradient increases, the skew intersection angle of the node also increases. The streamlines is almost orthogonal to the equipotential lines. For the reference temperature, the value the hydraulic heads are changed with it and the flow nets also changes. In addition, when the temperature of the aquifer keep constant and the reference temperature is just equal to the temperature of the model, the equipotential lines are always orthogonal to the streamlines. When the values of the reference hydraulic conductivities changes, the flow nets almost keep unchanged.