Effect of the fractal distribution of the radius of CO2patches on the ultrasonic velocity and attenuation of tight rocks

Carbon dioxide (CO2) injection is currently being carried out to improve hydrocarbon recovery and reduce carbon emissions. In this study, we measure the ultrasonic velocity and attenuation during supercritical CO2 injection into three oil-saturated tight rock samples (one sandstone and two carbonates). The experiments indicate that P-wave velocity is very sensitive to the pore-fluid substitutions, whereas the shear modulus and velocity are independent of it. Using the classical Gassmann model, the bulk moduli at different CO2 saturations obtained from the data exceed the Gassmann-Voigt upper bound, which is attributed to elastic stiffening. To model the P-wave velocity and attenuation, we use a poroelastic (mesoscopic-loss) model with a fractal distribution of the radius of the CO2 patches, and find that this model performs better than the White model, which is based on a single radius.