Influence of pressure, temperature, and pore fluid on the frequency-dependent attenuation of elastic waves in Berea sandstone.

Abstract

The effects of pore fluid, effective stress, pore fluid pressure, and temperature on the frequency dependence of elastic wave attenuation in Berea sandstone are interrelated in a series of systematic experiments. The attenuation of both the extensional and torsional modes of cylindrical samples of the sandstone is measured on the frequency range 3--30 kHz. To simulate conditions within the earth, the sandstone is subjected to confining stress to 70.0 MPa and temperature from 24.0 \ifmmode^\circ\else\textdegree\fi{}C to 120.0 \ifmmode^\circ\else\textdegree\fi{}C. Confining pressure and pore fluid pressure are varied independently. Data for two different pore fluids, brine and n-heptane, suggest that a scaling law exists for the pressure and temperature dependence of the attenuation in terms of the pore fluid. The logarithmic decrement of the sandstone is almost frequency independent in a vacuum evacuated sample, but shows a linear frequency dependence, once the sample is saturated. Extrapolation of this linear trend to low frequencies suggests that the decrement in fluid-filled sandstone is effectively frequency independent at seismic frequencies (<100 Hz). The frequency dependence becomes more pronounced as either the effective stress or the temperature is decreased. When the difference between the external stress on the sandstone and the pore fluid pressure is large, the attenuation depends only on the effective stress and is relatively temperature independent. But at low effective stress, the attenuation increases linearly with increasing pore fluid pressure and decreases linearly with increasing temperature. While a specific model is lacking, the attenuation process is apparently influenced most strongly by chemical processes at the pore fluid-matrix interface accompanied by subtle changes in the sandstone matrix dimensions.