| The heat extraction performance of the enhanced geothermal system(EGS)is significantly affected by the complex thermo-hydro-mechanical(T-H-M)coupling process,giving rise to the need of in-depth understanding of the effect of the fully T-HM coupling on the utilization efficiency and performance of EGS.Geothermal reservoirs are composed of rock matrix and discrete fractures.Based on the local non-thermal equilibrium theory,a three-dimensional numerical model coupled with T-H-M is established to simulate the heat extraction process of EGS.The results show that the main flow path is in the fracture surface,and the flow dominant area is concentrated between the production and injection wells.The fracture displacement changes due to the variation of reservoir temperature and pressure,which alters the reservoir permeability and finally determines the transport characteristics in the reservoir.The fracture permeability increases remarkably near the injection and production wells,but it tends to be weakened in the marginal region.Therefore,with the progress of the production,the flow dominant region is strengthened while the region with weak flow is weakened,resulting in sufficient heat extraction in the central region and insufficient heat extraction near the marginal region.To improve the heat extraction of EGS,the fluidity of the marginal region needs to be improved.The EGS sub-regions with different Depths,injection flows,and widths are calculated,and the average outlet temperature at 50 years of stable thermal storage operation is obtained.Therefore,under different commercial injection conditions,the EGS subregions that meet Standard1 and Standard2 are obtained with different combinations of depth and width,two standard of EGS life assessment are defined from maximum thermal efficiency and maximum power generation,which provides a basic guidance for the future research.The average outlet temperature and production flow of 3D-EGS with different injection flow rate are analyzed,the larger the fracture width is,the smaller the injection pressure will be.Therefore,the fracture structure is particularly important during the construction of the thermal storage.The width of fracture should reach a certain value to effectively reduce the injection pressure and improve economic efficiency.In view of the existing problems of double 3D-EGS,triplet 3D-EGS is proposed.Compared with double 3D-EGS,triplet 3D-EGS can improve the distribution of flow field,so its local and overall thermal recovery are higher.Based on the EGS of triplet,the influence of different well spacing on the average outlet temperature and overall thermal recovery is studied.Increasing the spacing between production wells properly can improve the average outlet temperature and overall thermal recovery,but the spacing between production wells and injection wells needs to be reasonably selected.The difference of thermal conductivity of matrix rock mass is studied,the difference of thermal conductivity of matrix rock mass has little effect on the average outlet temperature and overall thermal recovery of EGS.The effect of injection temperature of 20?,40? and 60? on the production temperature of EGS and overall heat recovery are studied.It is proposed that injecting low-temperature water of different temperature in different period of life can not only maximize the service life of EGS,but also improve its overall heat recovery.The higher the permeability of the main fracture surface and the higher average production temperature of EGS,has the longer the operation life and the higher the overall thermal recovery.EGS can be optimized by changing the fracture width,well layout and injection temperature. |
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