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Modeling coupled thermal-hydrologic-mechanical processes in fractured geothermal reservoirs using embedded discrete fracture method
Yu, Xiangyu
Yu, Xiangyu
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2022
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The Enhanced Geothermal System (EGS) energy resources have been regarded as a clean substitute for fossil energy. Exploration and research have been widely conducted in geothermal fields where the temperature gradient is promising for energy recovery. The EGS development relies on artificially created hydraulic fractures through which the injected low-temperature fluid can be flowing while being heated by the surrounding rocks. The injection/production induces the pore pressure and temperature change in the field, which in turn causes rock deformation under the geo-stress. The mechanical deformation affects the fluid flow and heat transfer by changing the hydraulic parameters such as porosity and permeability. Therefore, a coupled Thermal-Hydrologic-Mechanical (THM) model is essential to simulate the fracture-dominated EGS development for better planning and management.
In this dissertation, the Embedded Discrete Fracture Model (EDFM) is introduced to explicitly and mathematically model the fluid flow and heat transfer processes dominated by hydraulic fractures. A fully coupled model is firstly developed in which mass and energy balance, as well as momentum balance equations, are all solved by the Integrated Finite Difference (IFD) method. In the fully coupled scheme, stress tensor components (except for the principal stress in the z-direction) and the mean stress are treated as primary variables so that the normal stresses acting on the fracture faces can be assessed, from which the fracture aperture can then be determined. The assumption made in this scheme is that discrete fractures are subject to the same stress state as their containing mesh grid, which is not completely rigorous from the perspective of fracture mechanics. Therefore, secondly, a sequentially coupled model that adopts the eXtended Finite Element Method (XFEM) as its mechanical solver is developed. XFEM is widely used in fracture propagation simulations and is capable of obtaining the fracture aperture directly under dynamic pressure, temperature and stress states. Lastly, to accommodate the natural fractures induced by artificial stimulation, Multiple INteracting Continua (MINC) model is integrated into the sequentially coupled approach. Several intermediate-scale synthetic fractured geothermal reservoir models are established to investigate how various important parameters or fracture models impact the production rate and temperature in both fully and sequentially coupled models, such as permeability, thermal conductivity, injection rate/temperature, and natural fracture spacing. The fully coupled model is also applied to a field experimental EGS project and results are compared with existing literature and measured data.
To the best of our knowledge, despite plenty of research on the fully and sequentially coupled THM model using EDFM and XFEM, there is a lack of model development, investigations, and applications in the field of three-dimensional (3D) fully or sequentially coupled model for EDFM, 3D EDFM with XFEM and 3D EDFM-MINC with XFEM. This research will fill in this gap between 2D and 3D, propose a methodology for modeling the coupled THM process in fractured geothermal reservoirs and provide insights into the impact of THM parameters on the geothermal energy recovery.
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