Numerical Investigation Of EGS With Coupling Method Based On Fractal Characteristics Of Fracture System

The development and operation of enhanced geothermal system(EGS)is complex,which involves the multi-physical field and multi-scale effects covered temperature,hydrodynamic and stress field in fractured rock.As the core element of geothermal energy exploitation in EGS,mass and heat transfer of working fluid mainly occurs in fractures and fracture networks of hot dry rock,including natural joints,artificial fractures and geological faults.Therefore,based on the coupling thermo-hydro-mechanical(THM)in thermal reservoir,considering the fractal characteristics of fracture system,it is of great significance to carry out theoretical derivation and numerical simulation for the study of EGS around the core issue of revealing the mass and heat transfer mechanism between heat-carrying fluid(water)and high temperature thermal storage.This paper summarizes the fractal characteristics of core-scale single fracture and fieldscale fracture network,and constructs rough fracture surfaces by spectral synthesis method.THM coupling mathematical model of mass and heat transfer between water and EGS is established,which takes into consideration the coupling effects,including the change of solid properties caused by thermal stress in high temperature rock mass,the change of fluid flow characteristics caused by temperature and the evolution of fracture permeability caused by mechanical behavior in mining process.In order to verify the accuracy and reliability of the mathematical model and solution method,the numerical simulation results of core-scale and field-scale are compared with the experimental results and analytical solutions respectively.Fractal analysis is used to process the results of flow and temperature field in core scale,and single factor analysis and orthogonal test analysis are carried out to analyze the factors affecting average outlet temperature to discuss the influence degree of different indicators.Besides,a thermal recovery evaluation system is constructed for the field-scale EGS project with dual horizontal well system,and thermal recovery performance of fracture network models with different fractal characteristics is evaluated with the optimized parameters.The results show that the higher the fractal dimension is,the more irregular the fracture surface distribution is.Compared with the experimental results and analytical solutions,it is proved that the mathematical model and solution method in this paper are reliable.For the threedimensional model of core scale and field scale,the distribution characteristics of temperature,seepage and stress field are obtained by numerical simulation.It shows that there is strong THM coupling effect in the process of fluid flow and heat transfer,which is in good agreement with the existing general conclusions.The degree of heterogeneity and localization of fluid flow is very high on the rough fracture surface.It can be seen that the larger the fractal dimension is,the greater the heterogeneity of seepage field distribution is.The fitting formula of fractal dimension and equivalent hydraulic conductivity coefficient is obtained.Among the factors affecting the average temperature at the core outlet,the most obvious one is the average fracture aperture,followed by the thermal expansion coefficient of rock mass and injection temperature.According to the evaluation of the recovery performance of EGS at the field scale,it is found that the interconnected fracture network with higher bifurcation number has the higher the heat production and power generation;the energy efficiency of the unconnected fracture network is relatively high;the larger the fracture and bifurcation number,the higher the overall heat recovery and local heat recovery near the production well.