Study on the evolution of mechanical properties of hot dry rocks after supercritical CO2 injection

Characterizing the evolution of mechanical properties of hot dry rock (HDR) after supercritical CO₂ (CO₂(sc)) injection is crucial for assessing the heat extraction rate and reservoir security of CO₂ based enhanced geothermal systems. This study designed the experiments of triaxial seepage and mechanical properties considering no CO₂(sc) injection, CO₂(sc) injection, and alternating injection of water-CO₂(sc) (AIWC) in granite at 150–300 ℃. The experiments can reveal the mechanical properties of HDR in single-phase CO₂ zone, CO₂-water two-phase zone and dissolved CO₂ liquid phase zone in HDR reservoir. The results indicate that the failure mode of the rock samples primarily exhibits sudden instability after no CO₂(sc) injection and AIWC, whereas it predominantly manifests progressive instability after CO₂(sc) injection. Compared with 25 ℃, the uniaxial compressive strength (UCS) after no CO₂(sc) injection at 150–300 ℃ decreased by 13.86%–32.92%. After CO₂(sc) injection, the UCS decreased by 40.79%–59.60%. After AIWC, the UCS decreased by 27.74–40.48%. This shows that the strength of rock mass in the single-phase CO₂ zone is lower than that in the other two zones, and this weakening phenomenon increases with the increase of temperature difference. At the same temperature, the elasticity modulus after AIWC was greater than that after no CO₂(sc) injection and CO₂(sc) injection. With no CO₂(sc) injection, when the temperature was increased to 200 ℃ and 300 ℃, intergranular cracks and transgranular appeared respectively. After AIWC, mineral crystals such as calcite were precipitated on the surfaces of the connected large cracks, accompanied by kaolinite clay minerals. This increases the frictional contact of the mineral particles and enhances the stability of the HDR reservoir.