| In the past century,with the impact of industrialization and population expansion,the demand for conventional fossil fuels has increased significantly,and the reserves of oil and gas resources have been obviously insufficient.At the same time,the large-scale utilization of fossil energy has damaged the ecological environment,disturbed and damaged the stability of the ecosystem,and human beings have to face very serious environmental problems.In order to protect the ecological environment and cope with climate change,countries all over the world have made great efforts to develop clean energy.The heat exchange between reservoir fracture and fluid is an important link in the process of geothermal exploitation of dry hot rock,so it is of great significance to study the mechanism of fracture seepage heat exchange.In this paper,a self-developed experimental system of seepage heat transfer in high temperature fracture is used.First,the fracture seepage test is carried out to study the seepage properties of sandstone flat fracture and hydraulic fracture under different conditions.Then,the fracture seepage heat transfer test is carried out.The test heating temperature is 50?190?,the confining pressure is 4?6 MPa,and the flow rate range is 5?30 m L/min.The distilled water is used as the thermal quality,and the straight fracture is used under different temperature and velocity conditions,the seepage heat transfer characteristics of single fracture in sandstone samples with fracture are studied.The test results of straight fracture and rough fracture are compared,and the influence of fracture surface roughness on heat transfer efficiency is analyzed.Through the systematic test and theoretical analysis,the main conclusions are as follows:(1)The sandstone samples of straight fracture and hydraulic fracture have been prepared,on the basis of which the seepage experiment has been carried out.The experimental results show that,on the whole,when the seepage pressure is increased and the flow velocity is increased from 5m L/min to 30m L/min,the fracture opening increases;with the increase of confining pressure,the fracture opening decreases;in the range of 50?190?,the fracture surface expands by heating,and the seepage channel decreases.Under the condition of 50?190?,confining pressure of 4?6 Mpa and flow rate of5?30 m L/min,the fracture closure degree is larger,the fracture opening is about 5um smaller than the straight fracture,that is to say,the permeability of fracture is lower.(2)At the same temperature and confining pressure,when the flow rate is 5?30m L/min,the permeability coefficient of each fracture surface increases with the increase of confining pressure;with the increase of confining pressure,the permeability coefficient decreases;within 200?,the fracture has closed due to the effect of confining pressure,and the change range of opening is relatively small.At this time,the increase of temperature promotes the movement of fluid molecules,and the hydrodynamic viscosity decreases the permeability coefficient of fracture surface increased slightly.It is found that the seepage capacity of each fracture surface in high velocity zone and high temperature zone is better than that in low velocity zone and low temperature zone.(3)In the aspect of flow velocity,with the increase of flow velocity,the heat boundary layer becomes thinner,the convective heat transfer coefficient of fracture increases.And the larger the flow velocity is,the larger the increase of heat transfer coefficient is,the more heat is transferred away;in the aspect of temperature,in the low-temperature area,the convective heat transfer coefficient and temperature basically show a linear positive correlation,in the high-temperature area The convective heat transfer coefficient no longer increases with the increase of temperature,which means that the temperature of rock itself will not directly affect the heat transfer coefficient,mainly considering the temperature difference between the temperature at the entrance and the fracture surface and the influence of the thermal physical parameters of water;in the aspect of confining pressure,the heat transfer coefficient increases with the increase of confining pressure.By comparing the influence of flow velocity and temperature of straight fracture on the heat transfer of seepage,it can be seen that the high flow velocity has greater influence on the heat transfer coefficient than the high temperature,that is,the state of fluid has more significant influence on the heat transfer efficiency.(4)In the aspect of flow velocity,the convection heat transfer coefficient has a linear positive correlation with flow velocity.And the larger the flow velocity is,the faster the increase of heat transfer coefficient is.In the aspect of temperature,when the temperature is 50?100?,the convection heat transfer coefficient has a positive correlation with temperature,and the linear relationship is approximately,when the temperature is over 100?,the heat transfer system.In the aspect of confining pressure,with the increase of confining pressure,the contact points of fracture increase,and the effective heat exchange area is small,which leads to the decrease of the overall intensity of convective heat exchange,so the heat exchange coefficient decreases with the increase of confining pressure.(5)Through the comparison of the experimental results of seepage and heat transfer between the straight fracture and the fracture,the heat transfer process of the fracture is more complicated due to the existence of the fracture roughness.At the same temperature level,increasing the roughness of rock fracture surface will increase the heat transfer coefficient to a certain extent;at the same flow rate level,in the low temperature area,compared with the straight fracture convection heat transfer,the intensity of the fracture convection heat transfer has increased,and with the increase of flow rate,the difference between the fracture convection heat transfer coefficient and the straight fracture convection heat transfer coefficient is larger.After entering the high temperature zone,the convective heat transfer intensity of fracture is slightly lower than that of straight fracture.According to the experimental results,the power-law relationship between the convective heat transfer coefficient and the flow rate is fitted,and its basic form is h=Cq~n.The fitting results show that the C value of the fracture is basically the same as that of the straight fracture,while the n value is higher,which shows that the roughness of the rock fracture enhances the intensity of the convective heat transfer to a certain extent. |
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