Study On Heat Transfer Characteristics Of Enhanced Geothermal System Based On Heat Flow Coupling

As a deep geothermal energy stored in the earth,dry hot rock geothermal resources have become a sustainable new energy with good development prospects because of its wide distribution,large energy,clean and renewable characteristics.They play an important role in achieving the goal of carbon peak and carbon neutralization in China.However,the dry hot rock resources are deeply buried and contain little water.The exploitation of the energy stored in the dry hot rock can only be realized by creating artificial heat storage through hydraulic fracturing to form enhanced geothermal system(EGS).At the same time,in view of the diversity of geological conditions of underground heat storage fractured rock mass and the complexity of EGS operation process,numerical simulation analysis of seepage and heat transfer process in fractured rock mass has become the main method to predict EGS heat recovery,which can provide guidance for engineering design and operation.Based on this,this paper establishes a single fracture heat storage model,and then combines it with the wellbore to study the coupling process of EGS heat storage and overall heat flow through numerical simulation.The specific work and main results are as follows:(1)By establishing a three-dimensional single fracture heat storage model,the flow and heat transfer of fluid working medium in high-temperature heat storage are studied,and the system performance is evaluated from the outlet temperature,heat transfer,power generation,operation life and heat extraction rate.The results show that: firstly,the fracture surface of heat storage is the main heat exchange channel of fluid,and the flow advantage area is located between the opening section of injection well and the opening section of production well.Secondly,high-temperature thermal storage can obtain high heat recovery performance for a long time,but its operation life is shorter than that of medium and low-temperature thermal storage.Finally,three standards from low to high are defined from the method of evaluating the operating life of EGS heat storage to understand the heat extraction of heat storage.The higher the standard,the shorter the operating life of heat storage,the more sufficient the heat extraction,the lower the standard,the longer the operating life of heat storage,and the insufficient heat extraction.(2)The effects of seven factors such as initial temperature and thermal conductivity on the long-term heat recovery performance of EGS heat storage are comprehensively analyzed.The results show that the initial temperature,matrix permeability and injection flow are the main factors affecting the thermal performance of EGS thermal storage,while the thermal conductivity,injection temperature and fracture width have little effect on the thermal performance of EGS thermal storage.Although increasing the number of production wells can improve the uneven distribution of flow field and improve the heat recovery performance of the system,considering the existing drilling technology and cost,the well system still meets the production requirements.(3)Based on the established single fracture heat storage model,combined with the wellbore,the heat flow coupling process of EGS combined with the heat storage and wellbore is numerically simulated,the variation law of the overall temperature field is analyzed,and the integrity of EGS is evaluated.The results show that the low-temperature water is injected through the surface and flows downward along the injection wellbore,absorbs heat from the surrounding formation,and the temperature rises.With the operation of time,the range of the formation around the injection wellbore will be cooled due to heat conduction,and the temperature field in the heat reservoir also shows a similar change law.The water absorbs heat from the heat storage with high temperature and flows upward along the production wellbore.On the whole,it is in the state of releasing heat to the surrounding formation.(4)Based on the model of the combination of heat storage and wellbore,the change law of heat exchange considering the combination of heat storage and wellbore and ignoring the combination of heat storage and wellbore,as well as the maximum annual operating cost of heating with EGS combined with heat pump are analyzed.The results show that neglecting the combination of heat storage and wellbore will overestimate the heat transfer and produce a large error with the actual situation.And EGS combined with heat pump is used for heating in Northern Shenyang.Compared with boiler heating,the maximum annual operation cost can be greatly reduced.(5)Combined with the parameter sensitivity analysis of thermal storage,the variation law of four main factors on the heat loss of production wellbore is studied.The results show that the effects of ground temperature gradient and matrix permeability on the heat loss of production wellbore are positively correlated,while the effects of injection flow and fracture permeability on the heat loss of production wellbore are negatively correlated.