Evolution Of Physical And Mechanical Properties And Damage Mechanism Of High-Temperature Granite Under Liquid Nitrogen Cyclic Cold Shock

Geothermal energy in hot dry rock is a type of clean and renewable energy.There is a huge amount of hot dry rock resources in China.The development of hot dry rock is of great significance to ensure China’s energy security and promote the green and low-carbon transformation of energy.Hot dry rocks are mainly granite with low porosity and permeability in the deep,which needs artificial fracturing to construct the enhanced geothermal system.The liquid nitrogen cyclic cold shock method uses the huge temperature difference between ultra-low temperature liquid nitrogen and high-temperature rock to induce strong thermal stress,which can generate a more complex fracture network than hydraulic fracturing.Aiming at the key problem of thermal damage of granite under the effect of temperature alternation,the macro-meso-microscopic damage and deterioration characteristics of granite during heating and liquid nitrogen cooling cycles were studied by means of theoretical analysis,physical experiment,and numerical simulation.The influences of different liquid nitrogen cold shock parameters on granite damage were explored,and the mechanism of thermal damage of granite by liquid nitrogen cyclic cold shock was clarified.The main conclusions are as follows:（1）Combining thermocouples and infrared imager,the spatial and temporal evolution of the temperature field of high-temperature granite during liquid nitrogen cold-shock was monitored,and the heat and mass transfer characteristics of granite were analyzed.The central temperature of granite decreases“rapidly-slowly-rapidly-slowly”,which corresponds to the film boiling,transition boiling,nucleate boiling,and natural convection boiling of liquid nitrogen,respectively.The temperature of the end face decreases exponentially,and the cooling rate is positively correlated with the cold-shock temperature difference.The radial temperature gradient is larger in the initial period of the cold shock and near the edge position.The radial thermal stress calculated by the thermal stress model of the cylinder exceeds the tensile strength.Based on PFC^2D,a numerical simulation of thermal cracking of granite grains was carried out.It is found that the difference in thermal expansion/contraction of different mineral grains when the temperature change leads to the normal contact force of particles exceeding the tensile strength,resulting in tensile cracks.On the whole,the rapid cooling rate,high temperature gradient,and large grain deformation difference in the process of liquid nitrogen cold shock are the essential reasons for the thermal damage of granite.（2）The macroscopic physical properties of granite during the high temperature-liquid nitrogen cyclic cold shock process were characterized continuously and non-destructively by combining ultrasonic,resistivity,and nuclear magnetic resonance.Furthermore,the influence of liquid nitrogen cold shock variables on the physical property damage of granite was analyzed.The results show that the granite damage granite by liquid nitrogen quenching is more severe than that by air or water cooling.Under single liquid nitrogen cold shock,with the increasing heating temperature,the skeleton continuity weakens,the connected conductive paths increase,and the pore space expands,resulting in the decrease of wave velocity and resistivity and the increase of porosity.Among them,the wave velocity is the most sensitive to granite damage.400℃is the threshold for significant physical deterioration of granite.Under cyclic liquid nitrogen cold shock,with the aggravation of granite damage,the P/S wave velocity attenuates,the waveform distorts,and the frequency drifts.That is,the fractured core acts as a“low-pass filter”.Combined with the two-factor ANOVA with interaction,the heating temperature（i.e.,cold shock temperature difference）was determined to be the main control factor of granite thermal damage.（3）Uniaxial compression and Brazilian splitting tests were carried out on granite subjected to high temperature-liquid nitrogen cyclic cold shock.Combined with digital image correlation technology,the mechanical behavior responses of thermal damaged granite in compression and tension were studied.The results show that granite develops from single failure mode to compound failure mode,from brittleness to ductility,and the range of high-strain zone increases.The uniaxial compressive strength,elastic modulus,and tensile strength decrease under high heating temperatures or multiple cold shocks.Compared with the cold shock times of liquid nitrogen,the heating temperature has a more significant effect on the mechanical properties of granite,and 400℃is also the abrupt change point of mechanical properties.Influenced by the initial porosity,grain size,and bond strength,the sensitivity of mechanical damage of granite A,C,and B to heating temperature and cold shock times decreases in turn.（4）The mesoscopic and microscopic pore structures of granite during high temperature-liquid nitrogen cyclic cold shock were continuously measured by optical microscope and low field NMR.The evolution model of pore structure was analyzed on a multi-scale,and the influence of liquid nitrogen cold shock variables on pore structure was compared.During the cyclic cold shock process of liquid nitrogen,at a relatively low heating temperature（200-400℃）,new small-sized pores increase,and a few intergranular and intragranular cracks associated with quartz form;At a relatively low heating temperature（500-600℃）,the pores expand and connect to form large-sized pores,and intergranular and intragranular cracks develop cooperatively,forming a fracture network locally.More cold shock times induce more mesopores,while higher heating temperature promotes the increase of macropores.Granites A and C have large grain sizes and tend to form macropores and fractures;Granitic B has smaller grain sizes and tends to form more mesopores.Through multifractal characterization,the complexity and heterogeneity of one-dimensional pore size distribution and two-dimensional crack pixel distribution are enhanced under high heating temperatures and multiple cold shocks.Combined with digital core technology,three-dimensional pore-fracture models of damaged granites were constructed.Then a mesoscale fluid flow simulation was carried out based on the LBM method to evaluate the seepage characteristics of damaged granite.（5）Three-dimensional X-ray microscopic imaging was used to scan the granite after high temperature-liquid nitrogen cyclic cold shock.The results show that with the increase of heating temperature,the sparse,isolated,and local distribution of short and tortuous streamline changes into the dense,staggered,and integral distribution of long and smooth streamline,the dominant seepage channel is strengthened.The relative permeability and average velocity of damaged granite increase exponentially,which proves that the seepage-heat transfer capacity is improved.The middle and long streamlines of granite A and C are densely distributed,showing the pattern of“main seepage surface+multi-branch seepage channels”,and the overall seepage capacity is high;Granite B is dominated by meandering,isolated short and medium streamlines,showing a“single seepage channel”pattern,and its overall seepage capacity is low,which is not conducive to fluid seepage and heat transfer.The research results in this dissertation further improve the law of damage and deterioration of high-temperature granite by liquid nitrogen cyclic cold shock,deepen the basic research of liquid nitrogen cyclic cold shock fracturing of hot dry rocks,and provide theoretical and technical guidance for the development of the enhanced geothermal system in China.There are 128 figures,13 tab1es,and 255 references in this dissertation.