Study of the effect of micro-pore characteristics and saturation degree on the longitudinal wave velocity of sandstone

In an attempt to investigate the effect of different saturation degrees and micro-pore characteristics on longitudinal wave velocity, the present study selects two groups of sandstones with different micro-pore structure characteristics as research objects. In view of the different degrees of saturation of the samples, improved acoustic emission is employed to measure longitudinal wave velocity and a comprehensive analysis is conducted on the variation of longitudinal wave velocity. The test results show that the longitudinal wave velocity of the sample with diverse porous structure, at different saturation levels, manifests itself in three stages. At the saturation ratio of 0–30%, the wave velocity of the sandstone samples presents an increasing tendency, while there is a slight change given for the saturation ratio of 30–70%, first declining and then slowly increasing. In the third stage, the velocity significantly increases with the saturation ratio of 70–100%. Regarding the samples with a single porous structure, the measured longitudinal wave velocity presents two stages. When the saturation ratio is below 75%, the longitudinal wave velocity decreases slowly with the increase of saturation degree, while there is a sharp increase of the velocity as the saturation ratio increases when the ratio exceeds 75%. Based on the capillary model and the test results of the samples with a single pore structure, a relationship is established between the micro-pore structure characteristics and the macroscopic longitudinal wave velocity of the samples at different water saturation degrees, and the longitudinal wave velocity displays a satisfied consistency with that of the samples with diverse pore structure.