Study On Water-Cooling Cracking Modes And Hydraulic Fracture Propagation Law Of High-Temperature Granite

With the continuous development of economy and society,energy and environmental problems are increasingly prominent.As a new type of clean energy,the development and utilization of dry hot rock can effectively alleviate the energy crisis,improve the ecological environment and help the sustainable development of economy and society.At present,mainly through hydraulic fracturing technology for dry hot rock reservoir reconstruction,in the process of hydraulic fracturing,due to the effect of sudden temperature difference,high-temperature rock mass has different degrees of damage,which affects its hydraulic fracturing mode and increases the control difficulty of reservoir reconstruction.In addition,because granite is a common reservoir medium,it is of great significance to study the characteristics of water-cooled fracture and the law of hydraulic fracture expansion of high-temperature granite for the development and utilization of dry and hot rocks.In the past,researches on high-temperature rock and its cooling by water mostly focused on the mechanical index and ultimate failure mode,and made many important progress,but failed to fully consider the law of physical field change in the process of temperature rise and cooling and the influence of cooling times on its deformation and fracture behavior.In addition,the previous research on high-temperature rock hydraulic fracturing mainly focused on the final fracture mode,and the research on the law of hydraulic fracture propagation and evolution and the characteristics of physical field change was less.Therefore,based on laboratory test and numerical simulation,the fracture mechanics characteristics of granite under high-temperature and water-cooling conditions are studied,and the evolution mode of hydraulic fracture under different temperature,confining pressure and stress difference is analyzed.The main works are as follows:(1)The mechanical properties and fracture mode of high-temperature granite are studied.Firstly,based on the uniaxial compression test in laboratory,the strength,deformation characteristics and ultimate failure mode of granite at different temperatures are analyzed.Then,based on the numerical simulation,the particle discrete element numerical model is established.Through the comparative analysis with the test results,the applicability and accuracy of the numerical model are verified,the fracture evolution characteristics of granite during the heating process are investigated,and the degradation mechanism of mechanical properties of high-temperature granite is discussed.(2)The fracture mechanics characteristics of high-temperature granite cooled by water are studied.The peak stress,modulus of elasticity,peak strain and final failure mode of high-temperature granite after water-cooling are obtained by uniaxial compression test.Meanwhile,considering the influence of cooling times,the strength and deformation characteristics of high-temperature granite after repeated water cooling are further analyzed.Based on the numerical simulation,the characteristics of crack evolution,contact force change and temperature field change in the cooling process of high-temperature granite are investigated,and the mechanism of mechanical property degradation of high-temperature granite after water-cooling is discussed.(3)Study on the evolution model of hydraulic fracture propagation of high-temperature granite.First of all,based on the laboratory test,the water pressure curve,fracture pressure and final failure mode of high-temperature granite under different confining pressure conditions are obtained,and the initiation and propagation mode of high-temperature granite hydraulic fracture is analyzed.After that,the numerical model of granular discrete element hydraulic fracturing is established,and the validity of the model is verified based on the test results.The law of the initiation and evolution of hydraulic fractures,the characteristics of the change of contact force and the law of the change of temperature field in the process of high-temperature granite hydraulic fracturing are analyzed.(4)The hydraulic fracturing failure mode of high-temperature granite under the influence of many factors was studied.Based on the numerical simulation,the fracture pressure,contact force distribution and final failure mode of high-temperature granite under different stress difference conditions are obtained,the influence of temperature and stress difference on the initiation and expansion of granite hydraulic fracture is investigated,and the expansion mode of high-temperature granite hydraulic fracture under different stress difference conditions is analyzed.