Damage Characteristics And Fracturing Mechanism Of High-Temperature Granite Induced By Liquid Nitrogen

China possesses abundant hot dry rock geothermal resources.It is of great significance to realize the efficient exploitation of hot dry rock resources to alleviate energy crises and environmental problems.The high temperature and high in-situ stress of hot dry rock reservoirs pose challenges for conventional fracturing means.These challenges include high breakdown pressure,difficulties in creating complex fractures and the risk of induced seismicity,which result in low efficiency of reservoir stimulation.Based on the degradation effect of liquid nitrogen on hot dry rock,liquid nitrogen injection or fracturing is utilized as a pre-treatment method for hot dry rock reservoirs before conventional fracturing is performed.The purpose of applying liquid nitrogen is to reduce fracturing difficulty and create a better pre-condition for the formation of complex fractures in subsequent conventional fracturing.This fracturing method with the assistance of liquid nitrogen is expected to provide a more efficient solution for reservoir stimulation.Therefore,in this study,the effects of liquid nitrogen cooling on the mechanical properties,spatial statistical characteristics of acoustic emission(AE),damage evolution,and energy characteristics of high-temperature granite are studied.The fracturing forms utilizing liquid nitrogen are explored.The effectiveness and influence mechanisms of applying liquid nitrogen to assisted fracturing are also explored.Laboratory tests,theoretical analysis and numerical simulation methods are involved in this study.The main achievements are summarized as follows:(1)The degradation effect of liquid nitrogen cooling on the mechanical properties of high-temperature granite is revealed.Three-point bending test,Brazilian test,uniaxial compression test,and triaxial compression test with a confining pressure of 0?60 MPa are performed on heated granite(25?400 °C)after air cooling and liquid nitrogen cooling.It is analyzed the effects of granite temperature and confining pressure on the mechanical property degradation of granite after liquid nitrogen cooling.The process of liquid nitrogen cooling is numerically simulated,and the heterogeneous granite matrix is reconstructed by quartet structure generation set.It is revealed that the heterogeneity of thermal expansion coefficient promotes the thermal stress induced by liquid nitrogen cooling.(2)The neighbor distances of the AE events are evaluated,and the main AE events concentrated in the main failure zones are identified for heated granite after air cooling and liquid nitrogen cooling in triaxial compression failure.Integrating AE events into the percolation model,the high frequent,minimum,and maximum neighbor distances between AE events are determined.It is found that liquid nitrogen cooling results in a decrease in the high frequent and maximum neighbor distances between AE events in heated granite during triaxial compression failure.Utilizing the high frequent neighbor distances,the main AE events are successfully identified among the whole set of AE events.The spatial distribution characteristics of both the whole AE events and the main AE events are quantitatively evaluated in terms of the count,characteristic density,and fractal dimension.This provides a new perspective to quantitatively evaluate the effect of liquid nitrogen on the AE event distribution in heated granite during triaxial compression failure.(3)Crack distribution variable related to rock strength is proposed.A statistical damage constitutive model is developed for granite after thermal treatments,where the thermal damage variable is defined by the crack distribution variable.Based on fractal statistical strength theory,the crack distribution variable related to rock strength is defined.It is proven that liquid nitrogen can result in an increase in the number and complexity of cracks within heated granite.The crack distribution variable is used to define the thermal damage variable,which is more consistent with the physical meaning of damage.Through the newly defined thermal damage variable,the statistical damage constitutive model is developed for granite after thermal treatments.Then,the developed model is employed to theoretically analyze the effect of liquid nitrogen cooling on the damage evolution and energy characteristics during the triaxial compression failure of heated granite.(4)The secondary development of the coupled thermo-hydro-mechanical(THM)damage model is implemented.Numerical simulation is conducted to investigate the fracturing process induced by liquid nitrogen in granite under high temperature and high in-situ stress.The fracturing mechanism and performance of two fracturing forms utilizing liquid nitrogen(liquid nitrogen injection and liquid nitrogen fracturing)in high-temperature granite are analyzed.This THM damage model considers the fully coupled process of stress field,temperature field,seepage field,and mesoscopic element damage,as well as the heterogeneity of rock parameters and the physical properties of liquid nitrogen varying with temperature and pressure,which is implemented through the secondary development of COMSOL Multiphysics software using MATLAB.The fracturing process induced by liquid nitrogen is classified into two forms: liquid nitrogen injection and liquid nitrogen fracturing.By applying the established THM damage model,the differences between liquid nitrogen injection and liquid nitrogen fracturing are presented visually.Meanwhile,the fracturing mechanism of the two forms is systematically revealed.(5)A new idea of reservoir stimulation in hot dry rock,known as liquid nitrogen pre-fracturing assisted fracturing,is proposed.By applying the established THM damage model,the performance differences are investigated between the liquid nitrogen pre-fracturing assisted fracturing and the previously proposed liquid nitrogen pre-injection assisted fracturing.Under various reservoir conditions(temperature,insitu stress state,and heterogeneity),it is proven that liquid nitrogen pre-fracturing can effectively promote the formation of complex fractures,and significantly decrease initiation pressure and instability pressure.Moreover,liquid nitrogen pre-fracturing exhibits advantages in promoting complex fractures and decreasing instability pressure compared to the previous liquid nitrogen pre-injection method.This dissertation includes 119 figures,14 tables and 204 references.