Development and validation of thermo-hydro-mechanical simulator model for enhanced geothermal systems

Abstract

The thesis presents the development and validation of hydraulic fracturing and heat extraction models for Enhanced Geothermal Systems (EGS). The fracture models are developed using the Displacement Discontinuity (DD) Method which is an indirect Boundary Element approach. Two types fracture models are developed: based on the constant strength DD method and based on the Kelvin's fundamental-solution DD method. Analytical verification for each method is included. The DD model, based on the Kelvin's fundamental solution, presents a more robust and efficient technique for the fracture modeling. Various issues for the implementation of the Boundary Element Method for the fracture mechanics problems such as hyper-singularity and fracture tip singularity of stresses have been accounted in the numerical model. The Newtonian fracture fluid flow is modeled for steady and transient flow cases. Steady-state fracture fluid flow is modeled using implicit Finite Difference Method, and transient fluid flow is modeled based on the Galerkin's Finite Element approach. The fracture heat flow is modeled considering two cases: heat flow analysis during the fracture initiation and propagation processes, and long term heat circulation and extraction model for thermal energy production form an EGS reservoir. Two-dimensional and three-dimensional heat extraction models based on the Boundary Integral Equation formulations are formulated. In both the model, Laplace transform technique is used to deal with the time variable. The weak singularity cases have been considered for both two- and three-dimensional models. The thermal induced stresses due to differential cooling of the reservoir rocks are accounted for. Explicit hydro-mechanical and thermo-mechanical models for two-dimensional are established. Finally, the field validation of the heat circulation test is done using three-dimensional heat extraction. The validation of experimental result based on the analytical fracture model is also performed.