Effects of pressure, fluids, and pore structure on wave attenuation in loading-unloading tests of tight sandstones

The exploration of petroleum resources is currently extended to unconventional oil/gas reservoirs, such as tight sandstones. These reservoirs require theoretical and experimental studies on the wave response. This study conducts ultrasonic measurements on three tight sandstone samples under varying confining pressures and fluid conditions. To estimate the inverse quality factors of P-waves and S-waves, the spectral ratio theory is employed. With increasing pressure, the P- and S-waves velocities rise, while the attenuation declines. The pressure has the greatest influence on the anelastic properties. At the same pressure, the unloading process exhibits lower attenuation and higher velocities compared with the loading process. This is due to the fact that the cracks do not completely return to their pre-loading state after unloading. A rock-physics model is developed through the Voigt-Reuss-Hill average, the differential effective medium theory, and the squirt-flow model. The model results agree with the experiments. The modeling also shows that with increasing pressure, the attenuation peak in the water-saturated and oil-saturated state gradually shifts to higher frequencies, while the peak in the gas-saturated state shifts to lower frequencies. Water-saturated rocks are less pressure-dependent than oil-saturated rocks, while fully gas-saturated rocks undergo the greatest deformation. This study supports seismic exploration in tight oil and gas reservoirs.