Study On Heat Transfer Process And Optimization Of Enhanced Geothermal System Based On THM Coupling Model

Hot dry rock geothermal resources are widely distributed,large reserves,and have the characteristics of clean,stable and renewable.In recent years,they have attracted extensive attention from various countries in the world.Because of the characteristics of deep burial,dense rock mass and low permeability,the Enhanced Geothermal System(EGS)is the most effective means of hot dry rock exploitation at present.In the EGS system,the transmission process of circulating working medium in fractured reservoir involves the coupling among temperature field,hydro field and stress field,namely thermal-hydro-mechanical coupling(THM).Therefore,it is of great significance for the design and operation of EGS to accurately evaluate the influence of THM coupling effect on reservoir parameters and reasonably predict the heat recovery capacity and thermal stability of EGS.Based on this,this paper established a dual media THM three-field coupling model to carry out numerical simulation research on the heat and mass transfer process of low-temperature working medium in fractured rock mass.By analyzing the variation rule of three fields in the thermal reservoir under the coupling action of matrix and fracture permeability and stress,the influence of permeability characteristics of fractured rock mass on EGS thermal recovery performance was revealed.The sensitivity analysis of operating parameters and reservoir physical property parameters was carried out,and the exploitation strategy of increasing the heat recovery rate and prolonging the operating life of the geothermal system was put forward.Firstly,based on the governing equations describing the heat transfer and geomechanical problems in porous media,and considering the variation of water’s thermophysical property parameters with temperature and pressure,a mathematical model of three coupling fields in EGS fractured reservoir was established.Through deriving the expressions of the one-dimensional thermal elastic rock pillars of temperature field in rock mass,the corresponding analytical solution of temperature field was obtained.By setting the initial and boundary conditions,the numerical solution of the three coupling model was compared with the analytical solution to validate the correctness of the model.Secondly,based on the Monte Carlo method,a two-dimensional random crack network was constructed,and the parameters of the governing equation,as well as the initial and boundary conditions were set.The THM field of the reservoir was analyzed when the matrix and fracture permeability change with the stress.The results show that under the combined action of thermal stress and water pressure,the matrix permeability decreases gradually,and the fracture permeability increases exponentially.Hence,the rate of recovery heat is mainly determined by the dynamic changes of the fracture permeability.However,the predicted fracture permeability is 41.8% higher than the actual permeability if the dynamic changes of the matrix permeability is ignored during the simulation.As a result,the predicted heat breakthrough time is advanced,which would underestimate the system running life of nearly two years.Therefore,it is necessary to consider the coupling effect of fracture and matrix permeability and stress at the same time when establishing THM coupling model to improve the prediction accuracy.Finally,based on the above model,the effects of operating parameters and reservoir physical property parameters on EGS thermal recovery performance were studied by changing the water temperature of the incident well,the injection-production pressure difference,the matrix thermal conductivity,and the layout of the injection-production well pattern.It is found that the improvement of injection well water and injection and production pressure difference is beneficial to improve the thermal recovery capacity of the reservoir,but at the same time,it will promote the occurrence of thermal breakthrough phenomenon of the reservoir,and reduce the operating life and thermal stability of the system.The changes of matrix thermal conductivity and effective stress coefficient have little effect on the heat recovery performance of EGS,which can be ignored.Increasing the well spacing in a reasonable range can effectively improve heat ming performance and thermal stability of reservoir.Increasing the number of incident Wells has no significant effect on the thermal breakout time of the reservoir,but will result in the accumulation of fluid flow and reservoir pressure,which will affect the thermal stability and thermal recovery performance of the reservoir.Therefore,the method of adjusting injection water temperature by stages,setting reasonable injection-production pressure difference and "two injection-production and one production" injection-production well pattern can improve the heat recovery rate and prolong the operation life of the system at the same time.