Three-Dimensional Numerical Simulation and Optimization of Geothermal Exploration From Reservoir Containing Natural Fractures Under Thermal-Hydraulic-Mechanical Conditions

Geothermal energy is regarded as a sustainable, clean and reliable source supplier, and thus its development and utilization are of great significance. Geothermal energy is usually stored in hot dry rock (HDR), which is of high temperature (150-650 ℃), low porosity and low permeability. These properties result in a great challenge in heat extraction. Therefore, permeability enhancement in HDR is very important to heat extraction. The geological formations with naturally fractured characteristics are advantageous to the geothermal exploitation. When the fluid is injected into the geothermal reservoir, the range of the seepage area relates to the exploitation efficiency, where the well layout, including injection well and extraction well, plays a crucial role in the efficiency optimization of the geothermal resource. Although the feasibility of the heat extraction from enhanced geothermal system (EGS) have been confirmed by several field projects (e.g., Fenton Hill, Desert Peak, and Cooper Basin projects), many difficulties remain unresolved in the commercial production. Restricted by the deficiency of the field measurement and laboratory experiment, the numerical approach is an effective in investigating the geothermal exploitation performance, enabling the engineering process to run in a clear manner. Herein, a novel numerical model is developed to simulate the geothermal extraction process in the reservoir with natural fractures, where the three-dimensional (3D) model consists of two systems, namely as the matrix system and the fracture system. As the geothermal exploitation involves different physical processes in the fractured reservoir, such as heat transfer, fluid seepage and solid deformation, a full thermal-hydraulic-mechanical (THM) coupling process is simulated in the 3D model. The coupling THM process, as well as the governing equations is implemented in the COMSOL Multiphysics Finite Element (FE) solver. Firstly, the developed THM model is validated by an analytical solution of the thermoelastic consolidation problem. Then, based on the reliable model, several evaluation parameters, such as the operating life, the production temperature, the thermal energy and the recovery rates, are defined to evaluate the geothermal exploration performance. Afterwards, during the exploitation of ~30 years, the effects of the well layout (e.g., the number and the space of the injection well and extraction well) and the spatial location of the natural fracture on these evaluation parameters are investigated. This numerical work indicates that a reasonable design of the wells is essential to enhance the exploration performance in the reservoir containing natural fractures, thus is hopefully instructive in designing an commercial engineering project to extract geothermal resource.