Numerical Study On Seepage-heat Transfer Characteristics Of Supercritical CO₂ In The Fractured Reservoir

At present,the fracture channel in fracture reservoir is treated as thin film,regardless of the fracture thickness.The heat transfer model of rock matrix and fracture channel fluid adopts the local heat balance model based on the instantaneous equal temperature of rock matrix and fracture fluid or the convective heat transfer coefficient is usually assumed to be constant.It is not accurate to calculate heat extraction.Based on this problem,this paper simulates the convective heat transfer characteristics of supercritical CO₂ in fractured channels and the effect of different parameters on the heat extraction capacity is studied.In this paper,fractal interpolation method is used to construct fracture surface channel,and the distribution law of convective heat transfer characteristics of supercritical CO₂ in fracture channel is studied.The effect of mass flow,inlet temperature and reservoir pressure on convective heat transfer characteristics are analyzed and the variation law of Nu is obtained.The results show that:（1）the fracture surface morphology has an important influence on the distribution of convective heat transfer coefficient.The convective heat transfer coefficients at the upper and lower walls show the opposite trend.This is caused by the different thickness of the boundary layer due to the difference of the velocity gradient between the upper and lower walls.The convective heat transfer coefficient along the flow direction changes with the fluctuation of fracture surface.There is a maximum at the peaks and a minimum at the valleys.（2）The mass flow and reservoir pressure have great influence on convective heat transfer coefficient.The value of fully developed segment Nu is 6.5.Based on Monte Carlo method,the physical model of discrete fracture network is established,and the mathematical model of seepage-heat transfer coupling is established.The influences of injection temperature,pressure difference,ground temperature gradient,volume heat capacity of rock matrix,thermal conductivity of rock matrix and fracture permeability on the seepage characteristics,heat extraction performance and heat compensation capacity of dry hot rock are analyzed.The results show that:（1）injection temperature,pressure difference between inlet and outlet,geothermal gradient and fracture permeability are sensitive to seepage characteristics.Among them,the pressure difference and the fracture permeability have the greatest influence on the seepage characteristics,followed by the injection temperature,and finally the geothermal gradient.（2）Reducing injection temperature,pressure difference and fracture permeability,or increasing geothermal gradient and volume heat capacity can increase the heat extraction per unit mass of working fluid;reducing injection temperature and fracture permeability coefficient,or increasing pressure difference,geothermal gradient and volume heat capacity can increase the total heat extraction.（3）reducing the injection temperature and volume heat capacity,or increasing the pressure difference,geothermal gradient,thermal conductivity and fracture permeability can improve the heat recovery rate of surrounding rock.