Mechanical Properties And Damage Mechanism Of Hot Dry Rock Under Cyclic Thermal Shock

As a clean and renewable resource,geothermal has a broad prospect of development and utilization,especially Hot Dry Rock(HDR),which has huge reserves,wide distribution and stable source,and is of great significance to guarantee national energy security and relieve ecological environment pressure.During the exploitation and utilization of HDR,it is often necessary to adopt the method of artificially forming geothermal reservoir to extract deep geothermal energy from low permeability rock mass,namely enhanced geothermal system(EGS).The fracturing of HDR reservoir has always been one of the key technologies in EGS,and it is also a barrier and challenge to achieve economic and efficient EGS.Thermal stimulation caused by low-temperature fluid is expected to be an effective auxiliary means for geothermal reservoir fracturing.When ultra-low temperature fluid(such as liquid nitrogen)is injected into hot rock mass,it will suffer thermal shock due to the huge thermal stress caused by instantaneous cooling,resulting in cracking.The physical and mechanical properties and damage mechanism of HDR under cyclic thermal shock are of great significance to study the reservoir excitation technology,establish an artificial heat storage with good permeability,connectivity and large fracture specific surface area,and realize an EGS with high efficiency and good sustainability.In this dissertation,the physical and mechanical properties and mesoscopic damage mechanism of HDR under high temperature,after cooling from high temperature,and after cyclic thermal shock were investigated using the methods of laboratory test,numerical simulation and theoretical analysis.The effects of mesoscopic factors such as mineral composition,grain bond strength,bond homogeneity and grain size homogeneity on the characteristics of mesoscopic cracking and hydraulic fracturing of HDR were analyzed.The main conclusions are as follows:(1)A grain-breakable model considering the effect of temperature was established which introduced several mesoscopic factors affecting rock thermal cracking such as mineral composition,grain bond strength and grain size.The mechanical properties and mesoscopic cracking mechanism of granite were revealed under high temperature and after cooled by different cooling methods.The results show that the heterogeneity of rock meso-composition is the fundamental reason affecting its thermal cracking characteristics.The initiation and propagation of heat induced meso-crack in rock is closely related to its mineral composition,grain size and bond strength.Thermal cracks in granite change gradually with the increase of temperature under high temperature,and are mainly inter-granular cracks caused by tensile failure.Medium and coarse-grained granites with larger grain size and weaker bond strength have more thermal cracks and lower strength under high temperature.Granite after water cooling has more thermal cracks and exhibits more obvious inter-granular cracking compared with that after air cooling.The strength and elastic modulus are lower and the differences are gradually remarkable with increasing temperature.(2)A series of cyclic thermal shock and tensile experiments on hollow cylindrical granite were conducted under unconstrained and triaxial stress conditions.The physical and mechanical properties and mesoscopic damage mechanism of granite were revealed under different heating temperatures,thermal shock cycles,thermal shock methods and constraint conditions using optical microscopic test and discrete element numerical simulation.The results show that the P-wave velocity and tensile strength of the granite decrease significantly after cyclic thermal shock while the saturated water absorption and thermal cracks show the opposite trends.The change after the first thermal shock is the most significant,and becomes stable after 5 cycles.Compared with water cooling,the P-wave velocity and tensile strength of granite after liquid nitrogen cooling are lower while the saturation water absorption is higher,more thermal cracks are observed,and the effect of thermal shock is more significant.The initiation and propagation of thermal cracks are limited and thermal shock impact on granite is weakened by confining pressure when the temperature and confining pressure are low.On the contrary,the confining pressure can limit the initiation of thermal crack in the early stage of thermal shock cycles,but it will aggravate the damage of granite with the increase of cycle.(3)The statistical damage constitutive models of thermal damaged rock under compression and tension were established with different statistical distribution functions based on Griffith criterion.To solve the problem of complicated parameters in statistical damage constitutive model and difficulty in reflecting the damage evolution law directly,a thermal damage constitutive model based on meso-crack parameters was proposed,in which the thermal and mechanical damage variables were defined considering the inter-and intra-granular crack parameters from the definition of damage variables.The results show that the three constitutive equations based on Griffith criterion can well-describe the stress-strain curves of granite after cyclic thermal shock.The theoretical results of the three constitutive models using different statistical functions have little difference before the peak stress,while the stress drops the most under Weibull distribution after the peak stress,which is closer to the experimental result.The thermal damage constitutive model based on meso-crack parameters is simple in expression and easy to understand.Besides,it has fewer model parameters and can well-reflect the evolution law of damage variables and damage mechanism of rock during cyclic thermal shock and compression.(4)The theoretical solutions of temperature and stress fields of borehole surrounding rock under thermal shock were derived using the theory of heat transfer and elasticity,and the criteria of crack initiation and propagation of borehole surrounding rock under the impact of thermal shock were proposed with the help of fracture mechanics.The borehole surrounding rock model considering grain breakability was established using block discrete element software,by which the macro and meso influencing factors on cracking characteristics of borehole surrounding rock during cyclic thermal shock were obtained.The results show that the temperature and stress of surrounding rock are related to the distance from borehole and thermal shock time.The meso heat-induced cracks mainly distribute near the edge of borehole and increase with the increase of thermal shock cycle during cyclic thermal shock.The smaller the burial depth of HDR,the higher the initial temperature and the lower the temperature of cooling fluid,the more meso-cracks and the larger the damage range after thermal shock.The inter-and intra-granular cracking characteristics of reservoir rock during cyclic thermal shock are influenced by mesoscopic composition,in which the quartz content has the most significant effect.(5)A combined stimulation method of cyclic thermal shock and hydraulic fracturing for HDR reservoir was proposed,and a hydraulic fracturing model of HDR was established considering the grain breakability.The initiation and propagation mechanism of hydraulic fractures in HDR after cyclic thermal shock was revealed,as well as the macro and meso influencing factors.The results show that cyclic thermal shock can reduce the initiation pressure of HDR and increase the complexity of hydraulic fracture network obviously.The meso failure of HDR during hydraulic fracturing is main inter-granular tensile failure.The smaller the burial depth of HDR,the higher the initial temperature and the lower the temperature of cooling fluid,the smaller the initiation pressure after cyclic thermal shock,and the higher the complexity of hydraulic fracture network.The hydraulic fracturing characteristics of HDR after cyclic thermal shock are also affected by its mesoscopic composition,in which the grain bond strength is the most important.There are 163 figures,44 tables,and 181 references.