The effect of pore pressure depletion and injection cycles on ultrasonic velocity and quality factor in a quartz sandstone

Abstract

Passing seismic waves generate transient pore-pressure changes that influence the velocity and attenuation characteristics of porous rocks. Compressional ultrasonic wave velocities [Formula: see text] and quality factors [Formula: see text] in a quartz sandstone were measured under cycled pore pressure and uniaxial strain conditions during a laboratory simulated injection and depletion process. The objectives were to study the influence of cyclical loading on the acoustic characteristics of a reservoir sandstone and to evaluate the potential to estimate pore-fluid pressure from acoustic measurements. The values of [Formula: see text] and [Formula: see text] were confirmed to increase with effective stress increase, but it was also observed that [Formula: see text] and [Formula: see text] increased with increasing pore pressure at constant effective stress. The effective stress coefficient [Formula: see text] was found to be less thanone and dependent on the pore pressure, confining stress, and load. At low pore pressures, [Formula: see text] approached one and reduced nonlinearly at high pore pressures. The change in [Formula: see text] and [Formula: see text] with respect to pore pressure was more pronounced at low versus high pore pressures. However, the [Formula: see text] variation with pore pressure followed a three-parameter exponential rise to a maximum limit whereas [Formula: see text] had no clear limit and followed a two-parameter exponential growth. Axial strain measurements during the pore-pressure depletion and injection cycles indicated progressive viscoelastic deformation in the rock. This resulted in an increased influence on [Formula: see text] and [Formula: see text] with increasing pore-pressure cycling. The value [Formula: see text] was more sensitive in responding to the loading cycle and changes in pore pressures than [Formula: see text]; thus, [Formula: see text] may be a better indicator for time-lapse reservoir monitoring than [Formula: see text]. However, under the experimental conditions, [Formula: see text] was unstable and difficult to measure at low effective stress.

An Experimental Study of Ultrasonic Swave Polarization by Transmission Through Fractured Porous Media

“…Slabs were of equal thickness and lengths but different widths for being cut out of a circular 54 mm diameter standard core retrieved from a glass sandstone outcrop explained in detail by Frempong et al (2007). The length of the finished sample specimen was 108mm with a depth of 65mm including five interfaces, with each slab being approximately1cm in thickness.…”

Static and dynamic effective stress coefficient of chalk

“…Biot's (1941) theory was developed on the basis of linear elasticity and reversible strain. Therefore, the coefficient α is typically calculated from the density and velocity of ultrasonic sound wave propagation in dry rocks (Banthia et al, 1965, Todd andSimmons, 1972;Christensen and Wang, 1985;Mavko and Jizba, 1991;Prasad and Manghnani, 1997;Frempong et al, 2007;Mavko and Vanorio, 2010); which produces very small linear elastic strain. We denote it as dynamic effective stress coefficient and use the same symbol α as Biot (1941) because Biot's (1941) derivation is for a purely elastic system, which can only be calculated from dynamic measurements.…”

“…Based on laboratory measurements of stress dependent sonic velocity, several authors have noted that α is a function of stress (e.g., Banthia et al, 1965;Todd and Simmons, 1972;Christensen and Wang, 1985;Engstrøm, 1992;Frempong et al, 2007) although in the ideal case, α should be constant. Failure to satisfy the assumptions of Biot's (1941) theory, such as constant grain contact area and drainage condition could be reasons for nonconstant dynamic effective stress coefficient.…”

“…Pressure dependent dynamic effective stress coefficient α is measured by several authors (e.g., Banthia et al, 1965;Todd and Simmons, 1972;Christensen and Wang, 1985;Mavko and Jizba, 1991;Hornby, 1996;Prasad and Manghnani, 1997;Frempong et al, 2007;Mavko and Vanorio, 2010). In addition, Geertsma (1957), Nur and Byerlee (1971), Frempong et al (2007) as well as Omdal et al (2009) design experimental setups and conduct mechanical tests to measure the static effective stress coefficient. Although most studies are made on sandstones, Banthia et al (1965) study Austin chalk, and Omdal et al (2009) study chalk from the Stevns outcrop in Denmark.…”

Geotechnical Site Investigation Using S-waves with Implications for Ground Motion Analysis

“…The final sample specimen consisted of all six rectangular slabs, stacked together to offer different normal or planar fracture contacts or interface areas subjected to load as the slabs were of equal thickness (or depth) and lengths but different widths. Sections of slabs were cut out of a standard circular 54-mm-diameter-core typical bedrock retrieved from a glass sandstone outcrop, additional details of which can be obtained from [1]. The length of the finished sample specimen was about 108 mm, with a depth of 65 mm (including five interfaces) given some tolerance of about 1 mm, with each slab being approximately 1 cm in thickness.…”