Mapping impedance microstructures in rocks with acoustic microscopy

Abstract

Various methods can characterize microstructural properties of reservoir rocks with the ultimate goal of relating microstructure to seismic properties. Scanning electron microscopy, transmission electron microscopy, and optical microscopy have traditionally been used for such studies. They have identified lithology, pore space, interconnectivity of pores, grain size, and cementation as the most important factors controlling seismic wave velocity and attenuation. However, these techniques provide qualitative descriptions only. The acoustic techniques presented here, scanning acoustic microscopy (SAM) and acoustic sounding (AS), can map and, more importantly, quantify microstructure as variations in acoustic impedance. Ultrasonic stress waves are sensitive to local variations in elastic properties and are therefore particularly suited for characterizing microstructural properties of reservoir rocks. Reflections from impedance boundaries in grains and between interfaces in the sample are used to construct the microstructural image. This paper will show that acoustic microscopy can be a powerful tool for studying internal structure and pore geometry of reservoir rocks. Acoustic microscopy's basic principle is almost identical to that of reflection seismology. Images of surface and subsurface microstructures are prepared on the basis of reflected acoustic waves—that is, on the impedance changes in the sample. Acoustic waves on a sample are mode converted, partly transmitted into the sample, and partly reflected. The reflection coefficient and with it the signal intensity received by the transducer are determined by the elastic constants of the material. Changes in acoustic impedance in the sample that influence wave reflection characteristics can be studied by mapping the reflected waves. Because the working frequency of the acoustic waves can be varied, their penetration depth into the sample and the resolution of microstructural features can be controlled. I will present results from high-frequency SAM (0.1–2 GHz) and low-frequency AS (25–100 MHz). The resolution of SAM is about 1 μm at 1 GHz; AS …