Study On The Effect Of N₂/SC-CO₂ Mixed Gas On The Mechanical Properties Of Shale

Carbon capture and storage（CCS）technology is an effective way to reduce CO₂emissions and mitigate climate change in the future.There is a large amount of N₂/CO₂mixed gas in the industrial waste gas.In order to reduce the resource consumption during the separation and purification of N₂/CO₂mixed gas,the concept that shale reservoirs can effectively store CO₂was used to study the adsorption of N₂/SC-CO₂mixed gas by shale and the influence of shale mechanical properties.In this paper,the black outcrop shale of Longmaxi Formation in Yanzi Village,Shuanghe Town,Changning County,Sichuan Province is taken as the experimental research object,and the shale soaking test with different concentration ratios of N₂/SC-CO₂mixed gas under the condition of constant temperature and constant pressure is carried out.The pore structure parameters and mechanical parameters of shale under different concentrations of N₂/SC-CO₂mixed gas are characterized and analyzed by means of P wave velocity measurement,mercury intrusion,X-ray diffraction analysis（XRD）,uniaxial compression test（acoustic emission assisted monitoring）and Brazilian splitting test.The influence of different concentrations of N₂/SC-CO₂mixed gas on the micro-pore structure of shale and the deterioration mechanism of mechanical properties are explored.The uniaxial compression damage constitutive model of shale under N₂/SC-CO₂mixed gas is established.The main conclusions are as follows:（1）The adsorption capacity of shale to N₂/SC-CO₂mixture shows that with the decrease of N₂concentration and the increase of SC-CO₂concentration,the adsorption capacity of shale to N₂/SC-CO₂in the mixed gas system is lower than that of single component N₂and CO₂.With the increase of SC-CO₂pressure in the mixed gas,the adsorption capacity of SC-CO₂decreases,and the adsorption capacity of N₂is approximately linearly related to the N₂pressure.When the concentration ratio of mixed gas is N₂4.087 mol?L^-1and SC-CO₂7.67 mol?L^-1,SC-CO₂has the lowest density,the lowest viscosity,the strongest diffusion ability,the most lasting supercritical phase,and the adsorption capacity of shale to N₂/SC-CO₂is the highest,which is 0.656×10^-4mol?g^-1and 1.518×10^-4mol?g^-1,respectively.（2）By analyzing the influence of mixed gas on pore structure,the results show that with the increase of SC-CO₂concentration in mixed gas,the wave velocity attenuation rate,porosity,average pore size,permeability,quartz content and clay mineral content of shale decrease.The specific surface area,fractal dimension,calcite,dolomite,and brittleness index showed an increasing trend.When the concentration ratio of mixed gas is N₂4.087 mol?L^-1and SC-CO₂7.67 mol?L^-1,SC-CO₂in the mixed gas shows stronger ability to extract organic minerals and dissolve inorganic salts,resulting in the weakest heterogeneity of pore structure distribution in shale,more uniform pore structure,lower complexity and the most obvious change effect.（3）The physical and chemical effects of N₂/SC-CO₂mixture on shale change the mechanical properties of shale.After the action of mixed gas,the tensile strength,uniaxial compressive strength and elastic modulus of shale decrease,and the Poisson’s ratio increases.With the decrease of N₂concentration and the increase of SC-CO₂concentration,the tensile strength,uniaxial compressive strength and elastic modulus of shale increase,and the Poisson’s ratio decreases.When the concentration ratio of mixed gas is N₂4.087 mol?L^-1and SC-CO₂7.67mol?L^-1,the tensile strength of shale is 6.17 MPa,the uniaxial compressive strength is 95.65 MPa,the elastic modulus is 8.042 GPa,and the Poisson’s ratio is 0.33.The micro-crack fracture mode is a tensile-shear mixed crack,and the tensile crack is dominant,and the mechanical properties deteriorate most significantly.（4）Based on the Lemaitre strain equivalence hypothesis,the relationship between damage factor and strain and the damage evolution equation of shale in uniaxial compression process after N₂/SC-CO₂mixture are established.The pressure ratio of mixed gas was introduced to modify the elastic modulus of shale foundation,and the uniaxial compression damage constitutive model of N₂/SC-CO₂mixed gas shale with two parameters and correction terms was established.There are 53 figures,21 tables and 105 references in this paper.