| As a kind of green renewable energy,geothermal energy from dry hot rock has the characteristics of abundant reserves,high stability and high economic benefit,and has a broad application prospect.But buried depth,high temperature hot dry rock reservoir needs fracturing made thermal storage heat removal,and manufacturing artificial heat storage by means of artificial fracturing stimulation to develop hot dry rock geothermal energy project called enhanced geothermal systems(EGS).In view of the variability of geological conditions of fractured rock mass and the complexity of multi-physical field coupling of EGS,it is necessary to study the operating principle of EGS by numerical simulation,select the best operating parameters,and optimize the design scheme for application in practical engineering.Based on this,based on the EGS conceptual model,this paper designed multi-fracture reservoirs with different distribution characteristics to conduct coupling simulation with the wellbore.Numerical simulation software COMSOL was used to conduct simulation,and the influences of different fracture distribution characteristics on the seepage heat transfer and heat loss of the EGS system were studied,and various factors affecting the thermal mining efficiency of the system were discussed.The specific research content and results are as follows:(1)The mathematical model of seepage and heat transfer of fissure heat reservoir and wellbore was established,and numerical simulation soft COMSOL was used for simulation.The comparison and verification of numerical solution and theoretical solution and similar case verification were carried out,which proved the accuracy and rationality of the numerical model method adopted in this paper.(2)The three-dimensional numerical model of EGS for fractured thermal reservoir-wellbore was established,and the fractured reservoir models with different distribution characteristics were designed.The commercial software COMSOL was applied to conduct finite element numerical simulation for the model,and the change law of the overall seepage field and temperature field of the reservoir-wellbore EGS was analyzed.The variation law of temperature field between the wellbore and the formation: the heat absorbed by the water in the injection wellbore increases,while the formation temperature around the injection wellbore continues to decline.The formation temperature around the production shaft rises first and then slowly decreases.Variation law of reservoir temperature field: the low temperature zone first appears near the opening section of injection well,and gradually expands to the vicinity of production well with the advance of running time.It can be seen from the fracture heat storage temperature maps with three different distribution characteristics that the low-temperature region of the fracture surface is larger than the upper and lower matrix,and the size of the low-temperature cold front is closely related to the position and dip Angle of the three cracks.(3)Linear analysis of formation temperature and fractured reservoir temperature changes with time,select monitoring points and monitoring lines to simulate temperature changes.Comparative analysis shows that after 40 a operation,the exit temperatures of parallel type,cross type I and cross type II are 117.5℃,107.8℃ and104.4℃,respectively,and the decrease of thermal short circuit value is 12.1℃,13.7℃ and 14.2℃,respectively.The results show that the long-term parallel system has the least heat loss and better heat recovery performance.(4)The effects of injection temperature,injection flow rate,geothermal gradient and parameters of thermal reservoir properties(matrix permeability,fracture surface permeability,thermal conductivity)on EGS output wellbore outlet temperature were respectively discussed.The outlet temperature changes of wellshare-reservoir combination and wellshare-free combination were compared and analyzed.The influence of six factors on heat loss of production wellbore is also discussed.The reference order of influence degree on EGS thermal recovery performance from low to high is injection temperature,thermal reservoir property parameters,injection flow rate and geothermal gradient.Figure [45] table [12] reference [87]... |
PDF Full Text Request