Ultrasonic Attenuation in Shale

Interpretation of the elastic wave response in shales is a challenging task due to the difficulty of understanding the roles of various elements in the shale microstructure, including clay platelets, rigid grains, pore structure and saturating fluids. In sandstones, the velocity-effective stress response is pronounced and follows a log-linear relationship. The influence of pore fluids on velocity is well described by Gassmann (1951). In contrast, the velocity-effective pressure behaviour of shales does not exhibit the same sensitivity at ultrasonic frequencies. The role of pore fluids and microcracks on the seismo-acoustic response of sandstones is reasonably well understood whereas similar mechanisms are difficult to identify in shales. Probably, some form of local flow in response to propagating elastic waves occurs but the mechanism is unknown at this stage. One experimental approach is to use an extended spectral ratio method (Toksoz et al, 1975, Siggins and Dewhurst, 2011). This technique was used to process the P-wave waveform data obtained with ultrasonic measurements on a Norwegian sea shale, at a range of effective stresses with pore pressure held constant. A generalised standard linear solid model (GSLS), was then fitted to the observed spectral responses.