CHARACTERISTICS OF HIGH-FREQUENCY ULTRA-ACOUSTIC WAVE SPECTRUM AND PORE SIZE IN LOW-PERMEABILITY SANDSTONE

In the study of reservoir geophysics, elastic models of porous medium are commonly used in investigating reservoir geophysics of underground formations. In this work, we investigated the effect of pressure and pore size on acoustic frequency response in low-permeability sandstone. For a 0.5-3 mu s pulse width and 725-4351 psi pressure, the dominant frequency and the bandwidth of the transmitted waves are sensitive to the median throat radius, which suggests that the dominant frequency and bandwidth of acoustic log data may depend on microporosity. To characterize the differences between transmitted and incident waves, we defined a new parameter, the spectrum ratio Q, which is the ratio of dominant frequency and bandwidth. Experimental results suggest that the relation between Q and the median throat radius, i.e., the Q-R function, is linear. The spectrum ratio Q also varies linearly with pressure. By establishing relationship between high-frequency ultra-acoustic wave spectra and pore size in low-permeability sandstone, our work suggests acoustic logging data can possibly be used to infer sandstone pore throat size for laboratory studies. Expanded laboratory studies are justified.