| Hot dry rock(HDR)geothermal energy is a type of renewable energy with great development prospects in deep strata.The geothermal energy mining of hot dry rock uses an enhanced geothermal system(EGS).However,it is quite difficult to construct an artificial reservoir in this system.And the hot dry rock geothermal power station that has been built is far from reaching commercial scale.In this paper,the fault-mode HDR(F-HDR)geothermal development plan proposed by the team in 2008 is adopted,and the deep large-angle fault zone of the Yangbajing high-temperature geothermal field in Tibet,China is used as a natural artificial reservoir.The three-dimensional thermo-hydro-mechanical coupling numerical simulation method is used to study the variation law of temperature field,seepage field and solid stress field and output and lifespan in HDR geothermal development process.The main work and achievements are as follows:1.An equivalent continuous medium solid-flow-thermal coupling calculation model for dry hot rock geothermal development is established,which is the governing equations of temperature field,solid deformation field and seepage field.Discrete equations are given using the finite element method and the finite volume method,respectively.2.In the environment of Visual Studio 2015,a software THM-FF is developed using C++,QT and OpenGL.The software can be used for thermal-hydromechanical coupling numerical stimulation,as well as meshing,material and boundary conditions assignment,and data extraction of specified nodes or units.3.According to the fault structure of Yangbajing in Tibet,an injection well and two production wells are arranged along the horizontal direction at intervals of 6000 m to form an independent HDR geothermal mining system.The software THMF is used to simulate the thermal-hydro-mechanical coupling of HDR geothermal mining.This paper has the following conclusions:(1)At the initial stage of water injection,a temperature reduction zone is formed around the injection well rapidly.As the mining time increases,the low temperature zone mainly expands toward the production wells.The shallower the depth of the fault,the more prominent the expansion.After 22 years,the fault extends 2.5 km along the fault.It does not affect the operation of other geothermal mining systems.1# production well continuously produce 450°C high temperature water within 22 years of system operation;2# production well water temperature starts at the 8th year of system operation,and decreases exponentially from 450°C with running time.In 14 th year,the effluent temperature is reduced to below 200 °C.As the mining time increases,the rate at which the water temperature rises along the fault zone gradually decreases.(2)As the formation temperature decreases,the compressive stress of the rock mass decreases gradually,which leads to the improvement of the permeability of the rock,which creates favorable conditions for the seepage of the fluid in the fault.(3)In the early stage of mining,because of the high pressure gradient around the injection well and the difference in permeability of the inclined fault zone at the beginning of the mining,the injected water accumulates around the injection well.With the increase of mining time,the rock mass around the injection well exchanges heat with the water,reduces the compressive stress and improves the permeability.The water flow breaks through the penetration barrier,and the seepage channel gradually becomes smooth.On the 166 th day,the 2# production well began to produce water.On the 397 th day,the 1# production well began to produce water.As the mining time increases,the large pressure gradient around the two production wells causes most of the water in the surrounding fault zone to flow into the production wells,and the effluent flow gradually increases.The outlet water temperature and the mass flow of the two production wells have met the economic demand.The highest average flow rate of the 2# production well can reach 335kg/s,and the temperature above 200°C can be maintained.The average mass flow of 1# production well can reach up to 245kg/s,water temperature up to 445? can be maintained for more than 10 years;2# production well in the 15 th year,the average water temperature has been reduced to below 200?,in actual production,can be closed in the 15 th year 2# production well,waiting after the rock mass temperature rises,it continues to be mined;within 22 years of operation,the effective heat output is as high as 13,130 MW·a,which can work for 25 years for geothermal power plants with a surface installed capacity of 500 MW. |
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