Research Of Heat Transfer Mathematical Models Of Hot Dry Rock Enhanced Geothermal System

At present,China's carbon emissions rank first in the world.Not only in order to cope with global climate change,China's own green industry upgrading also needs to accelerate the pace of reducing carbon emissions.In “two sessions” in 2021,carbon emission peak and carbon neutral have become the hot topics,and the goal to achieve the carbon emissions peak by 2030 and carbon neutral by 2060 has been included in the 14 th Five-Year Plan and the Long-Range Objectives Through the Year2035.Among them,the promotion and utilization of low-carbon clean energy is one of the important prerequisites for achieving the carbon emission peak.Hot Dry Rock(HDR)which is one of the low-carbon clean energy sources is not only rich in storage,but also clean and pollution-free,so it has a broad space for development.Secondly,although geothermal heating and power generation have been advocated early in China,most of the projects using geothermal energy are mainly use shallow geothermal at present,and the use of deep geothermal(especially HDR geothermal)is less.This may be because,first of all,the current research on the heat transfer progress and mechanism in the fractures in HDR Enhanced Geothermal System(EGS)is not thorough enough,and the calculation of the outlet water temperature and water volume of the geothermal production well under different conditions is not accurate enough,which leads to the greater uncertainty of engineering design.Secondly,for HDR EGS,the construction is relatively difficult and the economy has yet to be discussed,so it may bring higher investment costs and longer payback periods.Regarding the issue above,therefore,this paper developed a simple HDR EGS with double vertical wells suitable for commercial and residential heating in cold regions of China.The three-dimensional model of the system was systematically studied by means of numerical simulation calculations and the results obtained can provide an important reference for the engineering design and construction of HDR EGS.Aiming at the self-designed three-dimensional model of a simple HDR EGS with double vertical wells,a three-dimensional model for numerical simulation was established,and the following research was carried out:Firstly,through numerical simulation methods,the effects of fracture opening,well spacing,injection flow flux and the reservoir range on the heat extraction performance of HDR EGS are studied,the heat extraction performance of HDR EGS was evaluated from the five aspects of the outlet water temperature of the production well,the heat extraction rate,the pressure difference between two wells,the average rock temperature and the rock heat recovery,and a comprehensive ranking of the degree of influence was carried out.The results showed:With the increase of the fracture opening,the outlet water temperature,the heat extraction flux,the pressure difference decrease.With the increase of the well spacing,the outlet water temperature,the heat extraction flux,the pressure difference increase.With the increase of the injection flow flux,the outlet water temperature and the heat extraction flux decrease,and the pressure difference increases.With the increase of the reservoir range,the outlet water temperature,the heat extraction flux,the pressure difference increase linearly.Moreover,within the same thermal recovery time,the smaller the fracture opening,the higher the rock thermal recovery degree.The larger the well spacing,the higher the rock thermal recovery degree.The greater the injection flow flux,the lower the rock thermal recovery degree.The larger the thermal reservoir range,the lower the degree of rock thermal recovery degree.According to the comparative analysis of the influence of fracture opening,well spacing,injection flow flux and reservoir range on the outlet water temperature,heat extraction flux,pressure difference between the inlet and outlet and rock thermal recovery degree of HDR EGS,the reference order of different influencing factors on the heat extraction performance of HDR EGS from high to low is the injection flow flux,the well spacing,the fracture opening and the reservoir range.Based on engineering experience,it is recommended that the fracture opening of the underground fracturing of HDR EGS should be about0.002 m,and the well spacing should be 300 m to 600 m.It is recommended that the operating injection flow flux of HDR EGS is 72t/h to 108t/h.If the quality of circulating water required for heating can be guaranteed,it is recommended to consider appropriately reducing the reservoir range to reduce construction cost to a greater extent.Secondly,through the commercial software SPSS,the multiple linear regression analysis on a large amount of numerical simulation result data was carried out,and the empirical formula for estimating the outlet water temperature and heat extraction rate flux of HDR EGS was derived,which offers the process of designing,constructing and running the HDR EGS theoretical support and at the same time greatly reduces the calculation workload and calculation difficulty of the design,construction and operation personnel,in order to be able to more simply and conveniently use HDR geothermal resources.