The Effect Of Ultra-low Temperature On Mechanical Properties Of Shale

Shale reservoirs with low permeability are difficult to exploit,and fracturing technology is an effective means for shale oil and gas reservoir reconstruction.However,the hydraulic fracturing method consumes a large amount of fresh water resources and also causes environmental pollution problems.Liquid N₂ fracturing is an waterless fracturing method that uses liquid N₂ as the fracturing medium,which is clean and environmentally friendly and helps to form complex fracture networks.The development potential of shale oil and gas in China is huge,and the study of the mechanical properties of shale under the action of ultra-low temperature liquid N₂ can provide a reference for the design of liquid N₂ fracturing.In this thesis,a combination of experiment and theory is used to study the influence of liquid N₂ ultra-low temperature on the microstructure and macroscopic physical and mechanical properties of shale outcrops,and the damage mechanism of shale under the ultra-low temperature of liquid nitrogen is discussed.Through the scanning electron microscope（SEM）technology,it was found that the characteristics of shale microstructure changes after ultra-low temperature mainly include:obvious cracks at the cementation of mineral particles;clay mineral particles fall off,resulting in damage,and micro-bedding deformation;the original cracks are further developed and formed new cracks;micro-pore damage becomes larger to form new micro-pores,etc.The changes of macroscopic physical and mechanical properties of shale before and after the ultra-low temperature action of liquid N₂ under different bedding directions and saturated conditions were studied by carrying out acoustic wave tests and triaxial compression experiments.The results show that the degree of change in the physical and mechanical properties of shale is affected by the bedding direction,water content conditions and confining pressure.Under the condition of low confining pressure,after the ultra-low temperature action of the dry vertical bedding shale,the P and S wave velocities are obviously reduced,the dynamic mechanical properties are obviously weakened,the compressive strength and elastic modulus are decreased,while the physical and mechanical properties of the parallel bedding shale are weakened to a relatively small degree,mainly due to the weak bedding surface of shale is more likely to be damaged by liquid N_2.The weakening degree of physical and mechanical properties of shale is more obvious under the condition of water content,the water phase transformation expansion in shale will aggravate the damage of shale internal microstructure.Under the condition of high confining pressure,the micro-cracks and micro pores inside the shale due to the action of ultra-low temperature are closed,the changes of compressive strength and static mechanical parameters of shale before and after the ultra-low temperature action cannot reflect the degree of the ultra-low temperature damage.Based on damage mechanics,the microscopic ultra-low temperature damage coefficient of shale was established,and the damage degree of shale caused by liquid N₂ultra-low temperature was quantitatively characterized.At the same time,the law of energy evolution in the process of uniaxial compression is analyzed,and it is found that the energy required for shale destruction is significantly reduced after the action of ultra-low temperature.The uncoordinated shrinkage and deformation of mineral particles and the expansion of pore-water phase transformation are the main causes of shale damage.In general,damage will occur inside shale after the action of ultra-low temperature,which can reduce the energy required for shale fracturing,thereby helping liquid nitrogen fracturing to generate complex fracture networks.