Chemico-microstructural and mineralogically-induced time-dependent mechanical alterations of the reservoir rock found in the peripheral zone of a CO2-based EGS is experimentally investigated. A series of uniaxial compressive and triaxial (under 30 MPa constant confining pressure with different temperatures; 25-300 °C) strength tests on granite specimens reacted in ScCO2+water medium for different time periods was performed by coupling acoustic emission (AE) technique. The results were compared with those for intact and water-reacted rock specimens in order to better understand this weakening phenomenon. Scanning electron microscopy (SEM) was also incorporated to evaluate the microstructural and mineralogical alterations. The results indicated that the mineralogical structure of Harcourt granite rock specimens was mainly altered by the dissolution of silicate minerals and plagioclase phase feldspar minerals. In addition, enhanced precipitation of secondary clay minerals including kaolinite, smectite, and illite was observed in the ScCO2+water-reacted rock specimens. The average compressive strength of ScCO2/water-reacted granite specimens found to be reduced with the increase of reaction period. Furthermore, 3.81%, 5.08%, 5.66% and 0.52% reductions in failure strength resulted under 30 MPa confining pressure in 6 months ScCO2/water-reacted granite specimens at 25 °C, 100 °C, 200 °C and 300 °C temperatures, respectively, compared to intact rock values at these temperatures. The post-failure analysis also revealed that the ScCO2/water reaction significantly alters the post-failure behaviour due to micro-mineralogically-induced weakening inside the rock pore structure. It was concluded that enhanced mineral precipitation and dissolution influence in time-dependent degradation of the surrounding rock mass which influence in long-term operation of the geothermal system. •Microstructural and mineralogically induced mechanical alterations in CO2-based EGSs.•Time dependant mineralogical and microstructural alterations in rock specimens were observed.•Changes in mechanical properties were evaluated.•Tri-axial tests were performed under high temperature and pressure conditions.•Post failure analysis of rock specimens was conducted.