Effects of water and supercritical CO2 on the mechanical and elastic properties of Berea sandstone

The effects of fluid saturation on the elastic and mechanical properties of the Berea sandstone have been assessed under triaxial stress and temperature in the laboratory. Besides air (dry rock), two saturating fluids have been used: water and supercritical CO2. Samples were subjected to triaxial loading up to mechanical failure at a constant effective pressure and temperature of 10 MPa and 50 °C, respectively. Ultrasonic P- and S-wave velocities were monitored while increasing differential stress and pore pressure, and were used to calculate the rock's dynamic elastic moduli. The results indicate that the mechanical behaviour of the rock and its static drained modulus is virtually unaffected by the nature of the saturating fluid. In contrast, supercritical CO2 induces a strong reduction of the dynamic bulk modulus at low effective stresses, which cannot be explained by poroelasticity theory. At high effective stresses the laboratory-derived dynamic bulk modulus and the Gassmann-derived predictions agree within the experimental uncertainty. On the other hand, for all effective pressures tested, the laboratory-derived water-saturated bulk modulus agrees reasonably well with the Gassmann-derived bulk modulus. The dynamic shear modulus exhibits a dependence on the saturating fluid unexpected for a poroelastic medium, i.e., at all effective pressures the water-saturated shear modulus is higher than the dry one, while the CO2-saturated is lower than its dry counterpart. The observed discrepancies between the laboratory-derived and the Gassmann predictions of dynamic moduli could be explained by non elastic fluid-rock interactions. Quantitative microstructural analysis coupled with the observed stress dependency of the dynamic elastic moduli suggest that CO2 was adsorbed on kaolinite resulting in a reduction of the stiffness of the CO2-saturated rock compared to the dry or water-saturated rock. These results are relevant for near-wellbore rock behaviour in field injection scenarios where supercritical CO2 displaces the in situ pore fluids in dry or near-dry conditions.