Investigations of fluid flow through fractures in Enhanced Geothermal Systems

Abstract

Natural and induced fractures play significant roles in the production of natural energy resources in deep reservoirs, such as conventional and unconventional oil and gas reservoirs, and geothermal reservoirs. In many cases, fracture networks that consist of individual fractures distributed in the reservoir provide main flow paths for the industrial fluid production from the reservoir. For example, hydraulic fracturing is a highly efficient technique that is widely applied to generate induced fractures in shale gas reservoirs for shale gas production in recent years. In simulations of fluid flow through fractures under reservoir conditions, fracture surfaces are always assumed to be parallel plates for the purpose of easier descriptions and calculations. However, the roughness of fracture surfaces should be considered in order to provide representations of the flow and consequently optimize simulation results.Geothermal energy, a clean and renewable energy, has become an important part of total energy consumption all over the world. In order to extract more geothermal energy from geothermal reservoirs, Enhanced Geothermal Systems (EGS) technologies have been developed. In the area of natural energy resource production, water and CO2 have been widely applied to enhance oil and gas recovery for the last three decades. Similarly, with consideration of reasonable costs, stable properties and environmental protection, water and CO2 have been used as working fluids for geothermal energy extraction. This study focuses on investigating behaviours of fluid flow through a single fracture with rough surfaces and mass and heat transfers for different working fluids based on parallel-plate and rough-walled discrete fracture network models.In this study, the research work covers four areas: 1) Rough fractures generated by the 3D printing technology are used to investigate water flow paths through fracture rough surfaces through laboratory experiments. Numerical models that strictly corresponds to fracture samples in laboratory experiments are developed based on the Lattice Boltzmann method. The results from laboratory experiments and numerical simulations are compared to demonstrate water flow paths through a single rough fracture. 2) Based on the Lattice Boltzmann method, flow behaviours of liquid and supercritical CO2 through a single rough fracture are simulated by integrating highly pressure and temperature dependent CO2 properties. 3) The heat and mass transfers for water and CO2 as working fluids are investigated based on parallel-plate and rough-walled discrete fracture network models by the finite element method with the integration of coupled hydraulic-thermal-mechanical processes. The corresponding heat extraction efficiencies are evaluated. 4) The miscible flow with different CO2 and N2 proportions is proposed for heat extraction from geothermal reservoirs. The effects of different CO2 and N2 mixing ratios on heat extraction efficiencies are compared and evaluated. Similarly, the discrete fracture model that integrates coupled hydraulic-thermal-mechanical process is simulated by the finite element method.The key findings of this study are: 1) Water flow through rough fractures present preferential flow paths on fracture rough surfaces, which are observed and quantified. Various factors including fracture inclinations, fractal dimensions, and mismatch wavelengths have direct influences on water flow paths. 2) The tortuosities of CO2 flow through a single rough fracture under different pressure and temperature conditions show similar trends with the average velocities. The values of tortuosity have a close relationship with the kinematic viscosity. 3) CO2 leads to faster pressure changes compared with water in both parallel-plate and rough-walled discrete fracture network models. The heat extraction for water as the working fluid can be higher that of CO2 as the working fluid in certain durations. 4) Through comparisons of miscible flow with different CO2 and N2 proportions, the miscible flow (40% N2, 20% N2) can be a more efficient working fluid than the one with larger N2 proportions. The concept of the optimized heat extraction efficiency range is proposed and validated based on the heat extraction efficiency curves.The results from this study contribute to a better understanding of fluid flow through fractures and a more accurate evaluation of geothermal energy extraction, which provides a foundation for further research in areas of natural energy resource production and CO2 sequestration.