The combined effects of pore structure and pore fluid on the acoustic properties of cracked and vuggy synthetic rocks

Many experimental observations show that the shear modulus of carbonate rocks changes in some cases upon fluid saturation. This observation is inconsistent with the presumption inherent to Gassmann's model that the shear modulus is independent of the fluid saturant. If the shear modulus change after saturation is ignored, the reliability of analyses of amplitude-versus-offset (AVO) and time-lapse seismology would be affected. So the elastic properties and velocities of saturated carbonates with complex pore structure are investigated. We made synthetic carbonate cores containing either void cracks or vuggy pores by first embedding predesigned aluminum foils or NaCl grains into the two-component matrix (carbonate cuttings and epoxy) and then leaching them out. We measured the ultrasonic P-and S-wave velocities of eight synthetic cores at different saturation conditions, and compared our measurements with theoretical predictions. The results indicate that changes in the shear modulus after saturation depend on both the pore structure and the pore saturant. Samples with cracks show shear stiffening because the viscous coupling is more active in narrow pores. Regardless of the pore structure, the shear moduli at kerosene saturation are larger than that at brine saturation, because the completely wetting kerosene provides a more shear-resistant solid-fluid boundary. The dynamic moduli changes with saturation increase as the secondary porosity increases. Gassmann's equation underestimates the velocities at shear stiffening and overestimates the velocities at shear weakening. The velocity deviation between Gassmann's prediction and experimental observations increases as the increase of shear modulus change.