Microscopic NMR response and macroscopic mechanical behaviour of Chang 7 reservoir sandstone during CO 2 –brine exposure under nominal supercritical-CO 2 conditions for up to 60 d

To examine the pore-scale NMR and mechanical response of Chang 7 reservoir sandstone from the Ordos Basin during CO2–brine exposure under nominal supercritical-CO2 conditions, specimens were brine saturated and immersed in 5.00 wt% NaCl brine coexisting with a dense CO2-rich phase at 32°C and 7.5 MPa for 0, 10, 20, 30, 40, 50, and 60 d, then tested by NMR and triaxial compression tests. GR-HSE axial signal amplitude generally increased. CPMG-T2 spectra remained short-T2 dominated; total spectral area and medium- and long-T2 fractions differed. Peak strength and elastic modulus decreased overall with exposure, while peak strain increased. Failure evolved from single oblique shear to tensile–shear composite, then splitting-dominated composite. AE records differed among loading stages. These results provide laboratory-scale evidence.