Numerical Simulation Study Of The Wellbore Heat Transfer And The Thermal-hydraulic Coupling In Random Fracture Reservoirs

As the world's energy demand continues to grow,traditional fossil energy has been depleted day by day.Geothermal energy,as a potential clean and renewable energy,plays an important role in the adjustment of energy structure.Enhanced geothermal systems(Enhanced Geothermal System,EGS)have attracted wide attention because of their sustainable supply of electricity.EGS integrity evaluation of thermal mining effect will play a decisive role in the long–term efficient and stable mining of thermal energy.In the past,the study of thermal mining effect of the system has ignored the heat loss of wellbore part.Or regard geothermal reservoir as equivalent porous medium.For more accurate evaluation of the EGS thermal mining effect,the numerical simulation study between wellbore heat transfer and thermal–hydraulic coupling in random fracture reservoirs is carried out in this paper.The whole thermal mining capacity of the system is studied by combining theoretical research,laboratory test,numerical simulation and field application,and the influencing factors of thermal mining are analyzed.The main contents of the thesis are as follows:1.Based on the characteristics of geothermal well body structure,a mathematical model of heat transfer between wellbore and formation in open system is established,and a continuous expression of dimensionless time t _D is introduced to solve the problem.By comparing with the analytical solution,it is verified that the numerical solution has strong reliability for long–term geothermal energy mining,and the equivalent thermal conductivity in the annular space is solved by equivalent ring space as solid.By analyzing the factors affecting heat transfer between wellbore and formation,it is concluded that annular space is the key part to reduce wellbore heat loss.2.The concept of"heat loss power"is put forward,and the influence of heat flux temperature and heat loss power on heat recovery effect is evaluated synthetically.The higher the injection temperature,the higher the wellhead heat Production temperature and heat loss power.With the linear increase of injection temperature,the wellhead water temperature and wellbore heat loss power also increase linearly,which has a great influence on the wellbore heat loss,but the heat loss power decreases with the increase of injection flow rate.The linear change of injection flow will lead to the deviation of wellhead water temperature and heat loss power.Therefore,considering the heat flux temperature and heat loss power,it can reflect the mining effect of heat energy.3.The seepage test of fracture network and the heat transfer characteristic test of dry hot rock fissure rock mass are carried out respectively.By loading different confining pressure values and adjusting different model pressure values in the seepage test of fracture network,the extraction flow rate is recorded under the condition that the injection flow rate remains unchanged,and the good diversion ability of the crack can be obtained.Then the seepage and heat transfer characteristics of dry hot rock fissure rock mass are tested.By analyzing the temperature drop of 6 temperature measuring points with time,it is concluded that the water temperature at the near nozzle end decreases faster and the water temperature at the far nozzle end decreases slower.In addition,using the experimental conditions as known conditions,the numerical simulation analysis was carried out compared with the laboratory tests.The simulation results of the temperature drop at the six temperature measuring points were compared with the experimental results.The results showed the same trend,but the simulation results were slightly higher than the experimental results.The problem is that the boundary condition of the simulation process is adiabatic impermeable boundary,but it is impossible to completely adiabatic and heat loss in the test.With the increase of test time,errors will occur,but the temperature decreases with time.The numerical simulation method is feasible for studying seepage and heat transfer process of fractured rock mass.4.Based on the seepage theory and the three conservation laws of mass,momentum and energy,the mathematical model of seepage and heat transfer of rock mass in single fissure and fissure network is established.The boundary conditions are given.The finite element numerical simulation is carried out by using the COMSOL Multiphysics and compared with the analytical solution.The maximum relative error between the numerical solution and the analytical solution is 3.9%.The feasibility of the numerical model of the fractured rock mass is verified.It is applied to the well system of single-fracture reservoir to simulate the seepage and heat transfer process,and to analyze the average temperature of the production well,the production flow rate,the heat opening rate and the heat extraction rate of the reservoir under the change of the parameters such as bedrock permeability,fracture permeability,fracture opening degree,injection flow rate,production pressure and fracture number.The purpose of it is to provide reliable theoretical basis for actual reservoir development.5?The actual reservoir contains a variety of fracture structures.The statistical parameters of these fracture systems can be obtained by field measurement.By using the statistical parameters with certain general distribution characteristics,the discrete fracture network rock system with the same statistical characteristics as the real formation can be obtained.Compared with the equivalent continuous medium model,the discrete fracture network model can more truly predict the permeability of fractured rock mass flow and heat transfer process.At present,in the two commonly used fracture network models,the geometric shape of polygonal model in three-dimensional space is quite complex,so baecher model is relatively simple.According to Monte Carlo method,under the condition of given statistical parameters,matlab programming is used to generate three-dimensional random fracture network rock mass system,in which the fracture surface is replaced by a circular plane.Then it is imported into COMSOL for simulation calculation,so the graphics file formed by Matlab is generated in the form of coordinate transformation,and the script file is imported into Auto CAD for graphics implementation.Finally,it is imported into COMSOL to complete the modeling of random fracture grid reservoir model.6?Taking the existence of two groups of orthogonal fracture networks in the reservoir as an example,a stochastic fracture grid reservoir model including the open hole section of injection and production wells is established.According to the coupling model of fracture network reservoir,the seepage and heat transfer process of the reservoir is simulated,and the average temperature,flow rate and thermal recovery of the production wells are analyzed under the conditions of fracture number,average trace length and spacing of injection and production wells.This paper analyzes the influence of fracture statistical parameters and wellbore layout on the thermal recovery effect of reservoir,realizes the generation and simulation process of 3D fracture network reservoir model,and outputs the important result data affecting the thermal recovery effect,which has important practical significance for guiding the reservoir construction and evaluating the thermal recovery ability of the system.7?The transient heat transfer models of injection well wellbore and formation,random fracture network reservoir seepage and heat transfer model and production well wellbore and formation transient heat transfer model are established respectively.The average water temperature and flow rate at the bottom of injection well are taken as the average water temperature and flow rate at the open hole section of injection well in random fracture network reservoir to simulate the reservoir,and the average water temperature and flow rate at the open hole section of production well are obtained by simulation.The water temperature and flow value are taken as the initial value of water temperature and flow at the bottom of the production well,and then the transient heat transfer simulation of the production well is carried out,and the average water temperature and enthalpy value at the wellhead of the production well are finally obtained.This method can not only evaluate the heat transfer between injection wells and production wells,but also simulate and analyze the changes of temperature,pressure and flow rate in the random fracture grid reservoir.It can also consider the coupling effect between wellbore and fracture grid reservoir.At the same time,it improves the calculation efficiency.The whole simulation of the circulation flow channel of injection well,random fracture grid reservoir and production well in EGS heat recovery process is also realized.8?This paper analyzes and evaluates the exploitation of EGS reservoir in Gonghe basin,Qinghai Province.In the simulation,the variation of water viscosity with temperature is considered,the flow and heat loss of top and bottom strata are considered,and the compressibility of water and bedrock is not considered.The conclusion based on this is more realistic.Based on the reservoir model,the effects of injection temperature,injection flow rate,production pressure,fracture permeability and fracture opening on Wellhole and wellhead temperature,wellhead temperature and enthalpy,production flow rate and thermal recovery rate,as well as Wellhole pressure and flow impedance of injection well are analyzed.It is concluded that injection temperature has a greater impact on flow impedance.Injection flow rate has a great influence on the flow impedance in the early and middle stage of production,while production pressure has a great influence on the production temperature,enthalpy,production flow rate,thermal recovery rate and flow impedance.The increase of production pressure will increase the fluid loss,reduce the production flow rate,hinder the fluid flow to the production well,and increase the flow impedance;fracture permeability has a great influence on the production temperature,flow rate and injection resistance.The greater the permeability,the more obvious the influence.When k_f increases 2 times from 5.0×10^-11m² to1.0×10^-10m²,the Wellhole temperature of 30 years production well can be reduced by 4.8%;when k_f increases 2 times from 1.0×10^-10m² to 2.0×10^-10m²,the Wellhole temperature of 30years production well can be reduced by 16.2%;similarly,with the same permeability increment,the production flow can be increased by 21.59%and 56.89%,the thermal recovery rate can be increased by 13.73%and 17.35%,and the Wellhole pressure of injection well can be increased by 21.59%and 56.89%,respectively.The results show that the seepage force decreases by 2.58%and 7.39%respectively,and the flow impedance decreases by 21.53%and 45.12%respectively,which indicates that the permeability has a great influence on the fracture seepage flow,and then affects the heat recovery effect.Compared with the fracture permeability,the fracture opening has less influence on the thermal recovery effect of the system,mainly because the subtle change of the fracture opening has little influence on the fracture seepage flow.9?In the early stage of exploitation,the bottom water temperature of the production well is higher due to the seepage and heat transfer with the bedrock,which absorbs heat from the bedrock.The wellhead temperature is lower than the bottom temperature because it gradually loses heat to the surrounding strata with the decrease of well depth.With the increase of time,more and more heat is taken away from the bedrock by the water flow,which leads to the decrease of the temperature of the bedrock at the bottom of the reservoir,which makes the temperature at the bottom of the production well decrease.The temperature difference between Wellhole and wellhead decreases with the increase of mining time.It can be seen that considering the heat loss of the wellbore is of great significance to the early,early and middle stage of the system thermal recovery,and the early,early and middle stage of the system thermal recovery rate and heat recovery rate increase fastest,that is,the period of maximum system efficiency.Ignoring the heat loss of the wellbore will lead to the overestimation of the system thermal recovery effect.In order to reduce the heat loss,we can take pumping in the production well.In this way,heat loss can be effectively reduced and heat utilization efficiency can be improved.