Gassmann's fluid substitution and shear modulus variability in carbonates at laboratory seismic and ultrasonic frequencies

Abstract

Carbonates have become important targets for rock property research in recent years because they represent many of the major oil and gas reservoirs in the world. Some are undergoing enhanced oil recovery. Most laboratory studies to understand fluid and pressure effects on reservoir rocks have been performed on sandstones, but applying relations developed for sandstones to carbonates is problematic, at best. We measured in the laboratory nine carbonate samples from the same reservoir at seismic ͑3 to 3000 Hz͒ and ultrasonic ͑0.8 MHz͒ frequencies. Samples were measured dry ͑humid-ified͒, and saturated with liquid butane and brine. Our carbonate samples showed typical changes in moduli as a function of porosity and fluid saturation. However, we explored the applicability of Gassmann's theory on limestone and dolomite rocks in the context of shear and bulk modulus dispersion, and Gassmann's theory assumptions. For our carbonate set, at high differential pressures and seismic frequencies, the bulk modulus of rocks with high aspect ratio pores and dolomite mineralogy is predicted by Gassmann's relation. We also explored in detail some of the assumptions of Gassmann's relation, especially rock-frame sensitivity to fluid saturation. Our carbonate samples showed rock shear-modulus change from dry to brine saturation conditions, and we investigated several rock-fluid mechanisms responsible for this change. To our knowledge, these are the first controlled laboratory experiments on carbonates in the seismic frequency range.

HVSR of ambient noise in Ston (Croatia): comparison with theoretical spectra and with the damage distribution after the 1996 Ston-Slano earthquake

“…Although S-wave velocity in soil is generally considered not to be much influenced by water saturation, experimental data and other studies (e.g. Iida 1938;Fratta et al 2005;Adam et al 2006;Lee and Collett 2006;Mondol et al 2007) indicate that shear modulus may be sensitive to saturation with fluids due to weakening of the soil matrix. Increased water content may also lead to increase of bulk density, which also contributes to velocity decrease.…”

Acoustic properties in travertines and their relation to porosity and pore types

“…The Gassmann theory assumes that shear moduli remain constant during fluid substitutions. Baechle et al (2005) as well as Adam et al (2006) showed that this is not the case in carbonate rocks, which are in many cases affected by shear weakening and shear strengthening, limiting the applicability of Gassmann's theory for the velocity prediction in carbonates. The travertines in this study, however, are crystalline with a matrix regularly consisting of micritic peloids, bound by sparites.…”

Frequency-dependent wave velocities in sediments and sedimentary rocks: Laboratory measurements and evidences

“…In the above example with synthetic unconsolidated sand (see Figure 1), ultrasonic velocities of dry and water-saturated samples could well be described by assuming the matrix stiffness is not affected by the water. For natural rocks, this assumption is often not valid; interaction with water often results in significant rock softening (Khazanehdari and Sothcott, 2003;Adam et al, 2006). The underlying mechanisms are not fully understood yet.…”