Title:

An engineering analysis method for deep geothermal energy

At present there are already many deep geothermal projects allowing us to have a better understanding of deep geothermal energy. However, there are still many issues to be solved for more reliable use of deep geothermal energy. This thesis proposes an engineering analysis method to assess the performance of a typical deep geothermal system, which is a doublet system with one injection well and one extraction well. A convective heat transfer boundary between the aquifer and overburden layers is applied for the axisymmetric problem based on the Lauwerier model for the first time. A new analytical solution is deduced using a series of Laplace transforms. The interaction between the injection well and extraction well is first neglected. Compared with other relevant analytical solutions, this new analytical solution comprehensively includes both heat conduction and heat advection in the aquifer and the heat flux between the aquifer and overburden layer. As long as the relevant parameters are obtained, the new analytical solution can intuitively illustrate the temporal and spatial temperature distribution within the aquifer. It can be used to determine the location of the extraction well and to evaluate the extracted geothermal power of the hot water aquifer. The convective heat transfer boundary at the interface does not only reflect the actual heat transfer process at the interface, but also models the heat transfer process in the overburden layer. It is shown that the dimensionless equivalent heat transfer coefficient is expressed as a function of the dimensionless injection rate and the dimensionless thermal conductivity of the overburden layer so the new analytical solution effectively incorporates the properties of the overburden layer. A series of FE simulations are conducted, and the analytical model is curve fitted to the FE results to evaluate the values of the dimensionless equivalent heat transfer coefficient. Based on the results of the curve fitting exercise, two empirical equations are proposed for typical cases. Applying the analytical solution coupled with these empirical equations and along with proper error estimates, it is possible to conduct a simple and rapid evaluation of the geothermal potential of a particular site. The revised analytical solution in this thesis is novel as there is no other analytical or semi analytical solution for the doublet system considering the heat conduction and heat advection in the aquifer and the heat flux between the aquifer and overburden layer. The revised analytical solution extended the analytical solution for a single injection well to a doublet scheme by considering the interaction effect between the injection well and the extraction well. The expression of the critical distance between two wells is obtained so that the best location of the extraction well in a doublet system can be determined. The spatial and temporal temperature distribution in the aquifer for a doublet scheme can be given by the revised analytical solution when the well distance is greater than the critical distance. It is found that it is valid to use a single well model to simplify a doublet scheme when the extraction well is far away from the injection well. The temperature of the extracted water against different time, injection rates and well distances can be obtained via the revised analytical solution. The revised analytical solution is compared with the experimental data and the numerical solutions and it is found that they match with each other well. The effect of a natural fault/fracture that exists in the aquifer on the performance of a doublet system, namely the temperature distribution and extracted temperature, is evaluated. By comparing the line model with the domain model, it is found that the line model is valid and computationally efficient. It is found that the acceleration effect of the fracture on thermal movement is the greatest when the fracture is located at the midpoint of the two wells. When the fracture is shifted towards the injection (extraction) well, the acceleration effect decreases and then becomes the deceleration effect. The deceleration effect of the fracture is the greatest when the fracture is located at the injection (extraction) well. The expressions of the critical angle for any position of the fracture in the doublet system are obtained. Equipped with these expressions, it is possible to decide whether the doublet system is still efficient during its life span once the cold water injection rate and the geometry and properties of the fracture are given.
