Seismic attenuation in partially saturated Berea sandstone submitted to a range of confining pressures

摘要

Using the forced oscillation method, we measure the extensional-mode attenuation and Young's modulus of a Berea sandstone sample at seismic frequencies (0.5-50Hz) for varying levels of water saturation (similar to 0-100%) and confining pressures (2-25MPa). Attenuation is negligible for dry conditions and saturation levels <80%. For saturation levels between similar to 91% and similar to 100%, attenuation is significant and frequency dependent in the form of distinct bell-shaped curves having their maxima between 1 and 20Hz. Increasing saturation causes an increase of the overall attenuation magnitude and a shift of its peak to lower frequencies. On the other hand, increasing the confining pressure causes a reduction in the attenuation magnitude and a shift of its peak to higher frequencies. For saturation levels above similar to 98%, the fluid pressure increases with increasing confining pressure. When the fluid pressure is high enough to ensure full water saturation of the sample, attenuation becomes negligible. A second series of comparable experiments reproduces these results satisfactorily. Based on a qualitative analysis of the data, the frequency-dependent attenuation meets the theoretical predictions of mesoscopic wave-induced fluid flow (WIFF) in response to a heterogeneous water distribution in the pore space, so-called patchy saturation. These results show that mesoscopic WIFF can be an important source of seismic attenuation at reservoir conditions.