Influence of Fluid Distribution on Seismic Dispersion and Attenuation in Partially Saturated Limestone

Abstract

Quantitatively assessing attenuation and dispersion of elastic‐wave velocities in partially saturated reservoir is difficult because of its sensitivity to fluid distribution. We conducted experiments on homogeneous Indiana limestone samples, partially saturated by two methods: drying and imbibition which lead to different fluid distribution for a given saturation. Forced oscillations (from 0.004 to 100 Hz) and ultrasonic (1 MHz) measurements were done under confining pressure to measure the change of elastic moduli with frequency and their attenuation. Our measurements show that compressional (P‐)velocities are strongly sensitive to the sample’s saturation method. For high saturations (above 80%), obtained by drainage, compressional velocities are frequency dependent, and clear peaks of attenuation can be observed. However, at the same saturations obtained by imbibition, no dispersion or attenuation is observed. In addition, shear velocities show little variation with frequency, saturations, and fluid distribution. The dispersion and attenuation of P‐velocities are shown to be influenced by the pore fluid distribution, which was investigated using micro‐computer‐assisted tomographic (CT) scans. Furthermore, a numerical model developed within the framework of poroelasticity’s theory predicts well the experimental results, using the fluid distribution obtained from CT as an input. Our results show that the velocity dispersion was related to wave‐induced fluid flow at mesoscopic scale controlled by the geometry and distribution of the gas patches.

The Frequency Dependence of Poisson’s Ratio in Fluid-Saturated Rocks

“…Partially Fluid-Saturated Condition. For partially fluidsaturated rock, when seismic wave passes through and mesoscopic fluid flow mechanism dominates, shear modulus keeps constant, while bulk modulus increases monotonously with frequency [18,19,25]. Thus, Poisson's ratio increases with frequency in the intermediate frequency range.…”

“…Partially Fluid-Saturated Condition. Sun et al [18] quantitatively assessed the dispersion and associated attenuation of elastic properties in partially water-saturated Indiana limestone sample (from 0.004 to 100 Hz). They obtained partial saturation by two methods: drying and imbibition.…”

“…Thus, it is essential to investigate the frequency dependence of Poisson's ratio in partially/fully fluid-saturated sedimentary rocks. However, scholars in seismic rock physics commonly focus their attentions on the frequency dependence of elastic moduli and associated attenuations, such as Young's modulus and extensional attenuation, bulk modulus, and bulk attenuation [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. When they experimentally investigate the frequency dependence of elastic moduli and associated attenuations, they measure Poisson's ratio together but take it as a bridge to obtain other elastic moduli under assumption that the rock is isotropic.…”

On Frequency-Dependent Rock Experiments: A Comparative Review

“…FO is applied to measure properties of all sedimentary rock types: sandstones [21,33,43,79,89,90,, carbonates [21,33,64,79,92,99,100,119,120,[136][137][138][139][140][141][142][143][144][145][146][147][148][149], and shales [75,100,119,[150][151][152][153][154][155][156][157][158][159][160]. FO is also applied to igneous rocks [21,70,71,73,[161][162][163]…”

“…Mikhaltsevitch et al [147] is the continuation of Mikhaltsevitch et al [143,144] whose results are also modelled by modified Gassmann theory. Sun et al [148] studied the impact of partial saturation by (i) drying and (ii) imbibition: P-wave dispersion (and attenuation) is significant for (i) but not for (ii) whereas S-wave dispersion (and attenuation) is insignificant for both (i) and (ii) at > 80% RH. Mesoscopic WIFF controlled by geometry and pore fluid distribution is the main mechanism causing P-wave dispersion.…”