Study On Seepage And Heat Transfer Characteristics Of Single Fracture Granite Under High Temperature And High Pressure

Hot dry rock is a renewable resource with great potential,and heat extraction from hot dry rock is mainly carried out by enhanced geothermal system.One of the key factors affecting geothermal energy extraction from enhanced geothermal system is the permeability and thermal conductivity of artificial fracture network.Therefore,the study on permeability and heat transfer efficiency of fracture network is the basis for further development of hot dry rock.In fact,fluid flow and heat exchange in artificial fracture network is a four-field coupling process,which is affected by temperature field,stress field,seepage field and chemical field.The single fissure granite is taken as the research object in this paper,using the constant voltage mode to seepage condition,designed the different temperature（50℃、100℃、150℃、200℃）and confining pressure（5 MPa、10 MPa、15 MPa、20 MPa）under the condition of tension fracture,shear fracture,heat transfer experiment of single fissure granite seepage flow after chemical stimulus,focuses on the different temperature and confining pressure on the single fissure permeability granite and the influence of heat transfer coefficient.The influence of tensile fracture,shear fracture and chemical stimulation on heat transfer of granite with single fracture in seepage flow is analyzed.The main work and conclusions of this paper are as follows:（1）The variation law of seepage heat transfer characteristics of tensioned granite with single fracture under different temperatures and pressures is proved.The results show that the hydraulic opening and permeability decrease with the increase of temperature and pressure.The influence of temperature and confining pressure on the heat transfer efficiency of fluid passing through the fracture shows two different trends.The increase of temperature is beneficial to the heat transfer of fluid,while the increase of confining pressure is inhibitory to the heat transfer efficiency of fluid.（2）The difference of permeability and heat transfer coefficient between shear fracture and tensile fracture is revealed.The experimental results show that the hydraulic opening ratio and permeability ratio of shear sample to tensile fracture sample are higher than those of tensile fracture sample at higher temperature.In other words,when the temperature is high and the confining pressure is high,shear action has a certain effect on permeability improvement.When the confining pressure is large,the heat transfer coefficient of shear fracture is sensitive to the confining pressure.With the increase of the confining pressure,the ratio of heat transfer coefficient decreases,and the greater the confining pressure,the slower the ratio of heat transfer coefficient decreases.（3）The seepage heat transfer characteristics of single fracture granite treated with different chemical irritants are obtained.Test showed that the chemical irritant effect after sample permeability decline,general mud acid stimulation of rock permeability reduction is relatively slow,the permeability ratio higher than that of hydrochloric acid,this is due to the mud acid in hydrofluoric acid and hydrochloric acid and fracture surface of quartz,potash feldspar,sodium feldspar reaction,karst mineral composition,and producing fluorine sodium aluminate（Na₃Al F₄）,and other secondary precipitation.With the increase of confining pressure,the ratio of heat transfer coefficient of chemical stimulation 2#is slightly higher than that of chemical stimulation 1#sample,and the mineral corrosion and dissolution in chemical stimulation 2#sample are more.With the increase of confining pressure,the soil acid solution is beneficial to the heat transfer of granite when the confining pressure increases.This paper has 42 pictures,11 tables and 81 references totally.