Effects of supercritical CO2 on viscoelastic properties of shales

Laboratory uniaxial compression creep tests, with differential stress of 30 MPa hold for 3 h, were performed on Chang-7, Longmaxi (LMX) and Barnett shales to study the influence of SC-CO2 on short-term viscoelastic properties. To this end, the wet shale samples were treated with SC-CO2 with a pressure of 30 MPa and a temperature of 110 °C for 14 days. We analyzed the creep data using the fractional Maxwell model. To investigate microscopic structural alterations, the surface morphology of the same location, before and after SC-CO2-water exposure, was examined by SEM images. Compared with dry shales, dynamic and static elastic moduli decreased by up to 25.02% and 55.83%, respectively, but the creep deformation increased by 200% for LMX and Chang-7 shales, and 500% for the Barnett shale treated by SC-CO2. Compared to dry sample, there is an increase in calculated fractional orders of 0.02, 0.07, 0.22 for SC-CO2 treated samples, indicating that SC-CO2 treatment is likely to enhance shale creep. SEM investigation confirmed physicochemical mechanisms responsible for the observed elastic damage and creep enhancement, including mineral dissolution and swelling caused by SC-CO2. This work would further improve our current understanding of the time-dependent deformation of shale under chemical-mechanical coupling effects during CO2 capture utilization and storage.