Study on effect of N2/CO2 mixed gas injection on shale mechanical properties of shale

After the industrial waste gas is treated by desulfurization and ammonia injection, the main component of the gas is N2/CO2. Based on the idea of reducing energy consumption and using shale reservoirs to effectively sequester CO2, the study on the effect of N2/CO2 gas injection on the mechanical properties of shale was carried out. Taking the black outcrop shale of the Longmaxi Formation in Sichuan Province as the experimental research object, under the condition of constant temperature and pressure, the N2/CO2 mixture gas injection test with different concentrations and ratios was carried out. The uniaxial compression test and the Brazilian splitting test were used to analyze the N2/CO2 effect of CO2 concentration in CO2 mixture on mechanical properties of shale. The research results show that: after the shale specimen is soaked in the N2/CO2 binary gas mixture, with the increase of CO2 concentration and phase change in the gas mixture, the pore growth rate of the specimen shows a trend of first increasing and then decreasing, and the pore growth rate is 34.91%-110.6%; the strength and Poisson's ratio of shale specimens decreased first and then increased, and the elastic modulus increased first and then decreased, the loss rate of uniaxial compressive strength was 37.5%-69.1%, the loss rate of tensile strength was 35.3%-85.4%, the elastic modulus increased by 37.5%-54.7%, and the Poisson's ratio loss rate was 11.8%～20.6%; the change of CO2 concentration and phase state in the mixed gas has a great influence on the damage intensity of shale and the damage factor increases first and then decreases. The shale shows obvious brittle failure characteristics, and the main failure mode is splitting failure. Preliminary analysis shows that the pore structure of shale changes under the action of mixed gas. When CO2 is in gaseous state, shale adsorbs CO2, and the pores generate van der Waals force, which expands the shale matrix, expands natural pores and fissures, increases the increase in pore size, and reduces the strength; when CO2 is in a supercritical state, the solubility of SC-CO2 in shale organic minerals in the mixed gas decreases with the increase of SC-CO2 concentration, the increase of pore size decreases, and the rate of strength decay decreases. When the SC-CO2 concentration in the mixed gas is 11.33 mol/L, the ability to dissolve organic matter is the strongest, the shale has the highest porosity, the highest brittleness, and the most obvious deterioration effect of mechanical properties.