Numerical Simulation Of Seepage Heat Transfer In Deep Artificial Fractures Based On Discrete Element

With the rapid development of social economy,the issue of sustainable development has increasingly become the focus of the world.Energy and environmental issues are major issues that all human beings have to face.However,the use of conventional fossil energy has caused great environmental pollution,and is very limited and non-renewable.High temperature rock mass geothermal energy,also known as HDR(hot dry rock)geothermal resource,is a clean and renewable resource with huge reserves,and it is widely distributed.Extracting the heat stored in it is an Enhanced Geothermal System(EGS)that is artificially transformed.The problems encountered in this process all urgently need the support of thermal-hydro-mechanical-chemical(THMC)coupling theory and analysis technology of fractured rock mass,so that the engineering properties of rock mass can be improved,the efficiency of resource extraction can be improved,and save the construction investment to a great extent.For the above-mentioned problems,this paper mainly studies the theoretical analysis and numerical simulation of the seepage heat transfer problem of fractured rock mass involved in the development and utilization of deep underground geothermal resources.The specific research contents and main conclusions are as follows:Firstly,the physical properties of rock samples from ZR1 drilling in Zhacanggou geothermal field in Guide Basin were selected to measure their physical properties.On this basis,the seepage and heat transfer model of single-fracture rock mass was established by discrete element 3DEC program,and the rock mass temperature and fluid temperature were analyzed.The characteristics of the change with time are analyzed,and the influence of injection flow rate,injection fluid temperature,fracture opening and heat transfer coefficient on the seepage heat transfer of fractured rock mass is analyzed.The results show that after the model runs for a period of time,a low temperature area is formed near the inlet of the fracture,and with the operation of time,the low temperature area gradually expands to the water outlet,and the temperature of the rock mass gradually decreases.The greater the fluid injection speed,the faster the fluid temperature drop rate;the lower the fluid injection temperature,the faster the fluid temperature drop;the greater the fracture opening,the faster the fluid temperature drop;the greater the heat transfer coefficient,the faster the fluid temperature drop.On the basis of the study of seepage heat transfer in single-fissure rock mass,the seepage heat transfer model of parallel double-fractures rock mass is established,and the variation characteristics of rock mass temperature and fluid temperature with time are also analyzed.The distance between the fractures is analyzed,and the effect of the distance between the two fractures on the seepage heat transfer is analyzed.The results show that,like the single-fracture rock mass,after a period of operation,a low-temperature zone will also be formed along the water inlets of the two fractures,but the shape of the low-temperature zone will show significant differences with the difference between the two fractures.,the temperature of the rock mass in the middle area of the two fractures decreases faster than that on both sides of the fracture,and when the distance is larger,the temperature of the rock mass in the middle area of the two fractures decreases more slowly than that on both sides of the fracture.In order to better explore the influence of different parameters on the seepage heat transfer of the fractured rock mass,based on the above-established numerical models of the single-fractured rock mass and the parallel double-fractured rock mass,combined with the orthogonal test method,32 sets of numerical values were carried out.Sensitivity analysis of the relevant parameters affecting the seepage and heat transfer of the single-fracture rock mass and the parallel double-fractures rock mass was carried out.The results show that the specific heat capacity of the rock mass is the biggest factor affecting the seepage heat transfer temperature of the fractured rock mass,the fluid velocity and the fracture opening are the more influential factors,and the thermal conductivity of the rock mass and the fluid-rock heat transfer coefficient are the less influential factors..In general,along the fracture direction,the influence of each parameter on the seepage heat transfer temperature of the fractured rock mass shows a decreasing trend.Finally,based on the magnitude and sensitivity of the relevant parameters obtained in the above research,and based on the Zhacanggou geothermal field in the Guide Basin,by analyzing the geothermal characteristics of the reservoir and the hydraulic fracturing results of the previous related reservoirs,the depth of the formation was selected.For the 3000-3050m reservoir,considering the computing power of the discrete element 3DEC program,a geothermal model with a size of100m×100m×50m and a horizontal fracture running through it was established,and its heat production capacity was evaluated.The results show that after the model runs for 5 years,the maximum temperature of the reservoir drops from the initial 150.02?to 126.79?,and the temperature of the production fluid drops to 63.08?.In 5years,it can generate 7.311×10¹³J of heat,which is equivalent to saving about 2.5×10~6kg of standard coal,reducing carbon dioxide emissions by about 4.75×10~6kg,and reducing sulfur dioxide emissions by about 6.1×10~4kg.