Study On Evolution Of Fracture And Surrounding Rock During Seepage-Heat Transfer Process Of High-Temperature Fractured Granite

In the development of hot dry rock geothermal system,the effective construction of artificial geothermal reservoir is the key to the success of deep geothermal development.The number,size and connectivity of the fractures in the reservoir directly affect the permeability and the heat exchange area of the artificial geothermal reservoir,which in turn affects the thermal energy exploitation efficiency of the hot dry rock system.Field practice shows that the permeability of the reservoir is gradually increasing during the production of the hot dry rock system.This indicates that during the long-term seepage and heat transfer process with the cool fluid injected into the high-temperature fractured rock mass,secondary cracking of the fractured rock mass occurs,which resulted in increased reservoir permeability.In view of this phenomenon,this paper studies the deformation and failure characteristics of the fracture and its surrounding rock during the seepage-heat transfer process of high-temperature fractured granite through experimental research,theoretical analysis and numerical simulation,which aims to derive the main factors leading to the deformation and failure.And then theoretical method for effectively modifying the artificial geothermal reservoir is proposed to increase its permeability and heat exchange area,and to improve the extraction temperature and efficiency of fluids.This has important theoretical significance and engineering application value for the research of efficient construction of artificial geothermal reservoirs.(1)The physico-mechanical properties of granites subjected to different temperatures and different cooling methods(slow cooling and rapid cooling with water)were systematically studied.Studies have shown that with the increase of temperature,the density,longitudinal wave velocity,uniaxial compressive strength,elastic modulus and tensile strength of granite subjected to both cooling methods continuously reduced,and the permeability continuously increase.At the same temperature,the density,longitudinal wave velocity,uniaxial compressive strength,elastic modulus and tensile strength of the rapidly cooled samples were lower than those of the slowly cooled samples,and the permeability was higher than that of the slowly cooled samples.During slow cooling,thermal cracking of the granite is mainly caused by the thermal expansion mismatch between minerals and the thermal expansion anisotropy inside the mineral.During rapid cooling,in addition to the above effects,there is also thermal cracking induced by temperature gradient.(2)Further analysis of the difference rate of various properties of granite subjected to the two cooling methods at the same temperature shows that the thermal stress formed by the temperature gradient in the specimen is the largest under the condition of 500°C and rapid cooling.Therefore,the difference rate of the longitudinal wave velocity,uniaxial compressive strength,and elastic modulus between the slowly-cooled and rapidly-cooled samples reach the peak at 500°C,and then decreased.Microscopic observation of thin sections showed that the thermal cycling treatments resulted in the appearance of a large number of intergranular,intragranular and transgranular cracks inside the granite,which directly led to the deterioration of its physical and mechanical properties and the enhancement of permeability.(3)The multi-functional high-temperature servo-controlled triaixal rock permeability testing system was used to carry out the seepage test of rough single-fractured granite at 16?500°C.The research shows that the high temperature causes the thermal expansion deformation of the fractured granite and the decrease of its elastic modulus.Under the combined action,the fracture aperature continuously decreases with the increase of temperature.In addition,the dissolution of the minerals on the rough-fractured surface caused by the high-temperature fluid flowing and mineral precipitation of the free surface of the fracture further lead to a decrease of the fracture aperature.The relationship between hydraulic aperature and permeability of the fracture and temperature are in accordance with the exponential function.Due to the variation of the fracture apearture and physical properties of fluid,the hydraulic conductivity firstly increases and then decreases with the increase of temperature.The fracture aperature and hydraulic conductivity at higher temperatures are more sensitive to the changes of seepage pressure.(4)With the multi-functional high-temperature servo-controlled triaixal rock permeability testing system,the convective heat transfer test of single-fractured granite under different temperatures and flow rates was conducted.And the numerical model of convective heat transfer of fractured rock was established and verified.The results show that the convective heat transfer coefficient shows a linear positive correlation with flow velocity at all the temperature levels.At various flow rates,the change in the temperature field of the specimens is mainly concentrated around the inlet of fracture.As the flow rate increases,the temperature reduction area around the inlet of fracture gradually expands.The water temperature in the fracture along the flow direction exhibits nonlinear distribution,especially when the flow velocity is lower,the nonlinear distribution characteristics are more obvious.(5)Using the topography scanner and micro-CT system,the cracking characteristics of fracture surface and its surrounding rock under the effect of seepage heat transfer were studied.And the mechanism of secondary cracking of the surrounding rock was numerically studied.The results show that under the effect of seepage-heat transfer,the number of micro-asperities in the fracture surface decreases,the local undulation degree decreases,and the overall roughness coefficient decreases.Influenced by the thermal stress induced by the temperature gradient,the initiation and propagation direction of the secondary cracks in the surrounding rock is approximately perpendicular to the main fracture.Due to the heterogeneity of the mineral composition of granite,the direction of secondary cracks may locally changes.The thermal stress induced by the temperature gradient in the surrounding rock gradually decreases along the flow direction,resulting in concentrated secondary cracks in the surrounding rock near the inlet of the fracture.(6)Based on the above conclusions,three theoretical methods for effectively improving the artificial geothermal reservoir are proposed.Firstly,replace the injected fluid.Secondly,intermittently inject the cryogenic fluid.Thirdly,change the position of the injection well.The research results can provide theoretical basis and experimental data for the effective secondary modifying of artificial geothermal reservoirs,and have important theoretical significance and industrial application value for promoting the hot dry rock geothermal development process in China.