Investigation on sandstone wave velocity variation and the stress response of pore structure

This study considered three different sandstones from the surrounding rock in the underground reservoir of the Ningtiaota coal mine in northern Shaanxi Province, China. Data regarding wave velocity, porosity, stress–strain values, and the crack pores aspect ratio distribution of three different sandstones undergoing loading–unloading conditions were obtained, and the mechanism of the relationship between rock wave velocity change and pore change is analyzed. The results found that the P/S wave velocity increased with the deviator stress growth. However, when the deviator stress reached a certain value, the S wave shows a more sensitive characteristic than the P wave, and the S wave velocity appears a significant slow down position. Through theoretical inversion, the high-pressure modulus of the rock and the aspect ratio of the closed pores under high pressure were obtained, and it was found that the reason for the slowing down of the S wave growth rate was the closure of the crack pore. Comparing the experimental data, it is known that the stress value at the S wave first slows down point means most of the crack pores are closed. Based on this study, it is proposed that the rock loading process is divided into the compression stage, crack pore closure stage, and damage development stage. In addition, based on the relationship between wave velocity and microstructure, S wave can be used as an effective parameter to predict rock stability, permeability, and safety.