Laboratory Measurements of Attenuation in Rocks at Ultrasonic Frequencies

Abstract

The spectral ratio method is used to calculate the quality factor (Q) in porous rock samples at ultrasonic frequencies (0.3 - 1.5 MHz). The data were collected using the pulse transmission technique with aluminum used as a high Q standard. The data set consists of dry, water and benzene saturated rocks at differential pressures from zero to one kilobar. Two sandstones, Berea and Kayenta, Bedford limestone, and Webatuck dolomite are studied. Water and benzene were chosen as pore fluid saturants to contrast the effects of two different pore fluids (density, compressibility, viscosity, dielectric constant, and wetting properties) at ultrasonic frequencies. The main features observed are: 1) The quality factor Q increases with increasing confining pressure; at low pressures the rate of increase is larger. 2) Q for saturated samples is generally lower than for dry samples. 3) The introduction of a fluid saturant into a dry rock increases S-wave attenuation more than P-wave attenuation. 4) In general, given the measurement error and the fact that these results are preliminary, the differences in attenuation between the two fluid saturations, water and benzene, are not large. Nevertheless, we observe that benzene-saturated attenuations are slightly higher than water-saturated values, particularly at lower pressures (less than 500 bars) for the P-wave.