The fluid–shale interaction during carbon dioxide (CO₂) enhanced shale gas recovery processes may have significant influences on the mechanical behaviour of shale, and it is also of great importance for the long-term safety of CO₂ geological sequestration in shale formations. In this work, shale samples from the Longmaxi formation in the Sichuan basin were used to investigate the mechanical properties and the acoustic emission (AE) behaviour of shale upon saturation with different fluids (water, brine, and CO₂–brine mixture with different CO₂ phase states) at 45 °C. X-ray fluorescence (XRF) analysis results show that major element alterations occur after shale is treated with supercritical CO₂–brine and subcritical CO₂–brine, the element content of Ca, K and Al in supercritical CO₂–brine saturated shale was decreased by 16.7%, 5.8% and 6.3%, respectively. Field emission scanning electron microscopy (FESEM) analysis results indicate that significant surface structural changes occur in shale after saturated with CO₂ and brine. Furthermore, the uniaxial compressive strength (UCS) and elastic modulus (E) of shale with different fluid saturation were decreased. Supercritical CO₂ cause a greater reduction of UCS and E compared to subcritical CO₂ due to its higher adsorption capability and larger CO₂ adsorption induced swelling. The maximum reduction in UCS and E of shale was observed in the case of CO₂–brine saturated samples, and the influence of the CO₂ phase on the mechanical behaviour of shale under brine conditions is not negligible. The fracture propagation pattern changes in shale after fluid saturation are also explained by the AE analysis.