Study On Mechanical Behavior And Damage Mechanism Of Shale In Different Saline Environments

CO₂ emissions continue to increase globally,the problem of global warming is intensifying,and carbon neutrality has become a global hot term.Deep saline aquifers are one of the most promising sites for geological storage of CO_2.Shale formations,which are often used as the cover of deep saline aquifers,are usually in a high ground stress,high salinity environment,and the shale formations have a variety of bedding facies production.The stability and integrity of the caprock shale are crucial to the storage of CO₂ in deep saline aquifers.Based on mentioned above,this thesis investigates the effects of brine environment and bedding orientation on the mechanical behavior of shale using a combination of indoor tests and numerical simulations with ideal caprock shale as the research object,in order to provide a reference for ensuring the safety of CO₂ storage projects in deep saline aquifers,and the main research contents and conclusions are as follows:（1）Conventional triaxial compression tests were carried out on shale specimens saturated with different brine solutions to,and the effects of brine type and brine concentration on the triaxial strength and deformation properties of shale were investigated,and the damage and failure mechanism of shale under saline environments were determined by combining microscopic means.5%Na Cl solution saturation has the greatest effect on the deterioration of mechanical parameters of shale specimens,followed by 5%K₂SO₄ solution.The high concentration of Na Cl solution has a rebound effect on the mechanical properties of shale specimens,and the rebound increases with the increase of Na Cl concentration.Brine intruded into the shale specimens through the bedding planes,microporosity distributed along the shale bedding and the microcracks generated by hydration to achieve damage to the mechanical properties of the shale.（2）Based on the brittleness index of energy balance,the brittleness degree of shale under different saline environments was evaluated;the compression-permeability coupling tests of brine-saturated shale specimens were carried out,and the permeability evolution law of brine-saturated shale specimens during the whole loading process was analyzed.The brittleness degree of saturated shale decreased significantly in low concentration brine and increased significantly in high concentration brine.The permeability of the specimens was basically unchanged before the peak strength and showed a substantial increase after the peak strength.（3）Conventional triaxial compression tests were carried out on dry and saturated shales with different bedding orientations to investigate the mechanical properties of the shales with different bedding orientations under three environments,and the influence of the brine environment on the anisotropic characteristics of the shales was analyzed.The peak strength of the dry shale satisfies the"U"shape of high on both sides and low in the middle.The saturated shale exhibited more obvious anisotropic mechanical characteristics,and the 5%Na Cl solution produced a highly connected bedding plane in the shale specimens with a 0°bedding orientation,which led to the severe deterioration of the mechanical properties of the specimens.（4）Based on the indoor test results of shale with various bedding orientations in three environments,i.e.,dry,distilled water and 5%Na Cl solution.The numerical models of shale specimens in different environments were established by using PFC3D discrete element software,drawing on the idea of mechanical equivalence,and a simulation study of the mechanical properties of the shales in three environments was carried out.The microcrack evolution process of shale specimens and the displacement field characteristics after damage were analyzed.The bedding orientation determines the relationship between the main fracture surface and the bedding plane of the specimen.The conclusion that the brine-saturated specimens are more likely to failure along the bedding plane after the saline environment weakens the mechanical properties of bedding plane is verified.The thesis has 75 figures,21 tables,and 132 references.