| Hot dry rock geothermal energy is a stable and renewable clean energy,and the efficient development and utilization of hot dry rock geothermal energy is conducive to optimizing energy structure and reducing environmental pollution.Due to the characteristics of large buried depth and poor permeability,it is necessary to use hydraulic fracturing technology and other artificial means to transform the reservoir connectivity before the development and utilization of hot dry rock geothermal energy to achieve the purpose of thermal energy extraction.In the process of reservoir reconstruction and injection-production cycle,the temperature of reservoir rock will decrease under the action of low temperature fluid,which will lead to the change of physical and mechanical properties of reservoir rock and further affect the fracture characteristics of reservoir rock.In order to investigate the influence of temperature and cooling method on the physical and mechanical properties of granite,the thermal expansion coefficient,permeability,longitudinal wave velocity,fracture toughness and other parameters of granite were tested by thermomechanical analyzer,permeability test device,ultrasonic detection analyzer,rock press and other test equipment.The influence mechanism of temperature and cooling methods on the physical and mechanical properties of granite was analyzed by observing the microstructure of granite and combining with numerical simulation results.In addition,the hydraulic fracturing test of high temperature granite was carried out under real-time conditions,and the fracture characteristics of granite under the action of low temperature fluid were studied.The main research contents and conclusions are as follows:(1)The influence of temperature and cooling methods on the physical properties of granite is studied by measuring the thermal expansion coefficient of granite at different temperatures,the mass loss rate,longitudinal wave velocity and permeability of granite after different cooling methods(natural cooling and water cooling).By observing the microstructure of granite,the microscopic mechanism of the change of physical properties of granite is revealed.The results show that the thermal expansion coefficient of granite increases with the increase of temperature between 20°C and 500°C.When the temperature reaches 600°C,the thermal expansion coefficient of granite decreases greatly.In the range of20°C～600°C,the mass and longitudinal wave velocity of granite gradually decrease with the increase of heat treatment temperature.Temperature has little effect on the quality of granite and has a significant effect on the longitudinal wave velocity.In the temperature range of20°C～400°C,the permeability of granite changes little,and the permeability of granite increases rapidly after exceeding 400°C.The increase of microcracks is the main reason for the decrease of longitudinal wave velocity and the increase of permeability of granite.The cooling method is an important factor affecting the physical properties of granite.At the same heat treatment temperature,compared with natural cooling,the longitudinal wave velocity of granite after water cooling is smaller and the permeability is larger.(2)The influence of temperature on the fracture toughness and crack propagation characteristics of granite is studied by conducting semi-circular bending tests on granite specimens at real-time temperature,and using DIC technology and CT technology to collect and analyze the images of specimens during and after failure,respectively.The results show that the mode-I fracture toughness of granite increases first and then decreases with the increase of temperature.At 200°C,the mode-I fracture toughness of granite reaches the maximum.The mode-II fracture toughness of the granite does not change significantly from room temperature to 200°C,and decreases with increasing temperature after 200°C.With the increase of temperature,the length of fracture process zone(FPZ)at the tip of prefabricated crack under peak load increases gradually,while the fracture energy density increases first and then decreases.At room temperature,there are few branch cracks near the main crack surface of the failed mode-I SCB specimen.With the increase of temperature,the branch cracks near the main crack surface gradually increase,the damage zone gradually increases,and the roughness coefficient of the main crack surface increases.Compared with room temperature,the phenomenon of mineral shedding on the fracture surface at high temperature is more obvious.(3)The SCB test and Brazilian splitting test are carried out on granite after natural cooling and water cooling,and the failure process of granite specimens is monitored by acoustic emission system.The influence of cooling method on the mechanical properties of granite is studied and the mechanism of the change of mechanical properties of granite is analyzed by numerical calculation.In addition,the relationship between the relevant physical and mechanical parameters is also discussed.The results show that under the two cooling methods,the mode-I fracture toughness of granite reaches the maximum at 100°C,and the mode-I fracture toughness of granite decreases gradually after 100°C.The mode-II fracture toughness of the granite does not change significantly from room temperature to 100°C,and decreases with increasing temperature after 100°C.The tensile strength of granite decreases with the increase of temperature.The cooling method has little effect on the tensile strength and fracture toughness of granite between 20°C and 200°C.When the temperature exceeds200°C,compared with natural cooling,the deterioration degree of tensile strength,mode-I and mode-II fracture toughness of granite after water cooling gradually increases with the increase of temperature.The large thermal stress generated by granite during water cooling is the main reason for the weakening of its mechanical strength.There is a positive correlation between the tensile strength and mode-I fracture toughness of granite.With the increase of temperature,the proportional coefficient between the tensile strength and mode-I fracture toughness of granite increases.There is a strong positive correlation between the mechanical parameters and the longitudinal wave velocity of granite after cooling.(4)The hydraulic fracturing test of granite specimens at real-time temperature is carried out to study the fracture characteristics of granite during hydraulic fracturing.The results show that the failure mode of granite specimens at room temperature is tensile and shear mixed failure.Compared with the unstressed state,the proportion of shear cracks in the specimens under triaxial stress increases.The temperature significantly affects the fracture characteristics of granite.With the increase of temperature,the phenomenon of fracturing fluid infiltrating into the rock matrix during the water injection process is more obvious.With the increase of temperature,the failure pressure of granite gradually decreases,and the suddenness of failure gradually weakens.The granite specimen at 250°C shows the phenomenon of crack propagation before reaching the failure pressure.Under high temperature conditions,granite specimens tend to produce complex fracture networks.The failure pressure of granite is also affected by the water injection rate.As the water injection rate increases,the failure pressure of granite specimens increases approximately linearly.(5)Based on the mechanical parameters and the failure pressure of granite,the accuracy of the relevant failure criteria at room temperature is evaluated,and the failure pressure of high temperature granite is predicted under the condition of considering thermal stress.The results show that the stress intensity factor criterion can predict the failure pressure of granite at room temperature more accurately than the tensile stress criterion.The thermal stress induced by low temperature during water injection is an important factor affecting the failure pressure of high temperature granite.The stress intensity factor criterion established under the condition of considering thermal stress can accurately predict the failure pressure of high temperature granite. |
