Role of Saturation on Elastic Dispersion and Attenuation of Tight Rocks: An Experimental Study

Using the forced-oscillation method, we measure the dispersion of Young's modulus, extensional attenuation, and Poisson's ratio of tight sandstone and carbonate samples at seismic frequencies (1–1000 Hz) under a constant confining pressure of 20 MPa and for a water saturation varying between 0% and 100%. The experimental data suggest that the dispersion of Young's modulus and attenuation of tight rocks is significant in a broad frequency band spanning over 1–1000 Hz. A comparison with the high-porosity and high-permeability sample data shows a contrasting dispersion and attenuation characteristics. For the tight sandstone, Young's modulus reaches a maximum dispersion of 16% at 60% water saturation and a 13% dispersion at 100% saturation. Attenuation is insignificant in dry condition and for water saturation ≤30%. In contrast with the peak attenuation occurring at very high water saturation (e.g., 80–100%) in partially saturated high-porosity rocks, peak attenuation of tight sandstone takes place at a water saturation of 60%. For the tight carbonate, the magnitude of dispersion (~3%) and attenuation are markedly lower for all saturation levels. In the explored frequency range (1–1000 Hz), Young's modulus increases monotonously, and no obvious attenuation peak is observed when saturation levels are greater than 10%. Using well-established theoretical models based on physical properties and microstructure of the tested rocks, we suggest that the observed attenuation characteristics are possibly attributed to the combined physical mechanism of microscopic (squirt) flow, mesoscopic flow in partially saturated rock, and shear dispersion due to viscous flow in grain contacts.