Dispersion analysis of acoustic velocities in rocks

Abstract

Acoustic velocity dispersions in three rocks saturated with water, normal decane (n-decane), and a heavy oil (in petroleum engineering, heavy oil is referred to as crude oil with viscosity higher than 0.5 Pa s), respectively, are calculated in this paper. The results show that the apparent velocity dispersion in light fluid (low-viscosity)-saturated rocks is relatively small, usually less than 3% to 5%, whereas that in the same rocks saturated with heavy oil is much larger. Such apparent velocity dispersion can be well explained by the "local flow" mechanism, which relates dispersion to the viscosity of the pore fluid, the pore geometry of the rock, and the effective pressure and temperature. The Biot velocity dispersion calculated using the Gassmann equation and the Biot high-frequency limit of the velocities in the rocks is very small, typically less than 2%. Such Biot dispersion can be explained in terms of the Biot theory, which relates dispersion to the viscosity of the pore fluid and the permeability of the rock in a way that is just opposite to the local flow mechanism. According to either the local flow mechanism or the Biot theory, the temperature dependence of the compressional wave velocities in heavy-oil-saturated rocks in the seismic frequency band should be no less than that observed in the laboratory at 0.8-MHz frequency, which means that the laboratory results can be applied to the field as a basis for seismic monitoring of thermal floods.

Laboratory study of fluid viscosity induced ultrasonic velocity dispersion in reservoir sandstones

“…The measurements all were taken at constant in-situ pressures and the measured velocities were directly coupled to the changes of the saturating fl uids, providing a good chance to investigate the effect of fl uid viscosity on velocity dispersion. The primary characteristics of the measured dispersion in saturated sandstones with varying fluid viscosity were consistent with previous studies (e.g., Winkler, 1985;Wang and Nur, 1990;Dvorkin et al, 1995;Batzle et al, 2001;Deng et al, 2003; and can be interpreted by two mechanisms of solid/fl uid interaction: the Biot mechanism of large scale average motion of the fl uid phase relative to the solid phase (inertial coupling) and the squirt-flow mechanism of grain-scale relative motion. Both theoretical concepts have a viscositydependent characteristic frequency, which roughly is the boundary between high and low frequency range.…”

“…For the past several decades, considerable efforts have been made to quantitatively understand and separate the mechanisms producing velocity dispersion (e.g., Wang and Nur, 1990;Dvorkin and Nur, 1993;Parra, 2000;Yang and Zhang, 2002;Ba et al, 2008;Wei et al, 2008;Nie et al, 2010). Although the velocity dispersion is subject to the joint effects of pore structure, fluid property, pressure, temperature, clay content, heterogeneity, and etc., the amount and position of velocity dispersion of reservoir rocks are strongly associated with fluid mobility in the pores, of which the fluid viscosity is one important factor but it works theoretically differently in the two most important mechanisms that are generally accepted for interpreting fluid induced dispersion in saturated rocks: the Biot flow mechanism of large scale average motion of the fl uid phase relative to the solid phase (Biot, 1956) and the local flow or squirt flow mechanism of grain-scale relative motion (e.g., Mavko and Nur, 1975;O'Connell and Budiansky, 1977;Murphy, 1984;Winkler, 1985Winkler, , 1986Mavko and Jizba, 1991;Dvorkin et al, 1994;Dvorkin et al, 1995;Batzle et al, 2006).…”

Poroelastic wave equation including the Biot/squirt mechanism and the solid/fluid coupling anisotropy

“…Yet, it is well known that the Biot theory cannot adequately explain the large velocity dispersion and the strong attenuation in many rocks. Many investigators have shown that the squirt-flow mechanism can be responsible for the observed large attenuation and velocity dispersion [3,[5][6][7]. Dvorkin and Nur [1], Dvorkin et al [2,8] have also shown that the squirt-flow mechanism results in much higher and realistic attenuation in saturated rocks than that predicted by the Biot mechanism.…”

Lithology effects on the fractures parameters using image log and petrophysical data

“…The velocity deviation log, which is calculated by combining the sonic log with the neutron or density log, provides a tool to obtain well information on the predominant pore type in carbonates [Wang and Nur, 1990]. Velocity deviation characterizes with three zones.…”