Pressure and Fluid Effect on Frequency‐Dependent Elastic Moduli in Fully Saturated Tight Sandstone

Abstract

We developed a system to explore the effects of pressure and fluid viscosity on the dispersion and attenuation of fully saturated tight sandstones, especially at seismic frequencies. Calibration of the new system revealed that the system can operate reliably at frequencies of [2–200, 106] Hz. Tight sandstone with a “crack–pore” microstructure was tested under nitrogen gas (dry), brine, and glycerin saturation. A frequency‐dependent effect was not found for the dry case. However, apparent dispersion and attenuation for the undrained/unrelaxed transition was clearly observed for sample under brine or glycerin saturation, the magnitude of which was largely suppressed by increasing effective pressure. The measurement results illustrated that increasing the fluid viscosity or the effective pressure will shift the dispersion curve to the lower frequency range. A simple squirt‐flow model with dual‐porosity scheme was used to compare with the measurement results. Although the estimated values deviated slightly from the data, the trend fitted the saturated data relatively well, especially at low effective pressures. Therefore, considering the crack–pore microstructure of the tight sandstone, dispersion and attenuation are induced predominantly by the squirt‐flow stiffening effect from cracks to pores.

Fluid Substitution and Shear Weakening in Clay‐Bearing Sandstone at Seismic Frequencies

“…Thus, when comparing Gassmann's predictions with laboratory ultrasonic measurements, the effect of frequency on elastic moduli needs to be considered. That is why several laboratory facilities with a low-frequency measurement range have been developed during the past 40 years (Best et al, 2007;Fortin et al, 2014;Jackson, 2000;Jackson et al, 2011;Jackson & Paterson, 1993;Madonna & Tisato, 2013;McCarthy et al, 2011;Mikhaltsevitch et al, 2014;Murphy, 1985;Nakagawa et al, 2013;Peselnick & Liu, 1987;Saltiel et al, 2017;Spencer, 1981;Subramaniyan et al, 2014;Szewczyk et al, 2016;Tisato & Madonna, 2012;Wang et al, 2012;Yin et al, 2017). Murphy (1985) conducted laboratory measurements on granites using the resonant bar method at sonic frequency supplemented with the ultrasonic method and found that Gassmann's predictions worked better for low sonic frequency measurements.…”

Role of Saturation on Elastic Dispersion and Attenuation of Tight Rocks: An Experimental Study

“…The elastic properties of high‐ and intermediate‐porosity sandstones at different frequency ranges are relatively well investigated by many researchers (Batzle et al, 2006; Borgomano et al, 2017; Johnston et al, 1979; Mikhaltsevitch et al, 2014, 2016; Pimienta et al, 2017; Spencer & Shine, 2016; Yin et al, 2017). It is in general acknowledged that sandstones exhibit no significant dispersion and attenuation in dry conditions (Spencer, 1981; Winkler et al, 1979).…”

“…For two fully saturated Fontainebueau sandstones with relatively low porosity, Pimienta et al (2015b) reported that there exist two strong attenuation peaks associated with drained/undrained and undrained/unrelaxed transitions. Yin et al (2017) recently performed the forced‐oscillation measurements over seismic frequencies from 2 to 200 Hz and concurrently at an ultrasonic frequency of 10 6 Hz for fully saturated tight sandstone. It is found that the squirt flow associated with crack–pore microstructure system in tight sandstone is mainly responsible for the dispersion of Young's modulus and extensional attenuation.…”

A unified poroviscoelastic model with mesoscopic and microscopic heterogeneities

“…In seismic exploration, the development of models to predict the characteristics of seismic waves in porous media is a considerable challenge. Through numerous experimental progress, the main characteristics of seismic waves are the velocity dispersions and amplitude attenuations, which can be observed in a broad range of frequencies in fluid-saturated porous media [5][6][7][8]. Several theories have been proposed to explain these phenomena at various scales [9].…”

“…The compliant pores are mainly cracks with small aspect ratios, and the intergranular pores are stiff with high aspect ratios. The porosities of these two types of pores can be measured under different pressures [8]. Similar to the mesoscopic heterogeneity mechanism, in the high-frequency regime, the fluid in the compliant pores is easily isolated by the stiff pores and the fluid pressure has no time to equilibrate.…”