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Laboratory Seismic and Ultrasonic Properties of Carbonate Rocks

Adam, Ludmila 1; Batzle, Michael 1
1 Geophysics, Colorado School of Mines, Golden, CO.

Carbonates are important targets for rock property research, because they currently present over half of the major oil and gas reservoirs in the world. However, rock physics measurements and theories related to changing reservoir conditions, mostly relate to sandstones. Applying the results developed for sandstones to carbonates is problematic, at best.

In this abstract we study the elastic and visco-elastic properties of carbonate samples at seismic and ultrasonic frequencies for varying fluid saturation and pressure. The most popular theoretical framework is described by Gassmann's fluid substitution theory. We explore in detail some of the assumptions of Gassmann's relation, especially rock frame sensitivity to fluid saturation and bulk modulus frequency dependence. Several rock-fluid mechanisms are proposed to explain the rock shear modulus change resulting from a brine substituting a gas or liquid-butane in the pore space. We find that for our carbonate rock samples, at high differential pressures and seismic frequencies, the saturated bulk modulus of rocks with high aspect ratio pores is predicted by Gassmann's relation. Ultrasonic velocities should be used with caution when predicting these rock properties in seismic time-lapse data feasibility studies, because laboratory data show that velocities can increase from seismic to ultrasonic frequencies. Nevertheless, the measured dispersion seem proportional such that there is good agreement between laboratory seismic and ultrasonic elastic properties, for example, in the Vp/Vs ratio.

Intrinsic attenuation at low and high frequencies is also measured in our carbonate suite. Contrary to most observations in sandstones, we observe that the bulk compressibility losses dominate over the shear-wave losses for dry samples and samples fully-saturated with liquid-butane or brine. Attenuation modeled from the measured modulus corroborates this conclusion. On average, P-wave attenuation can increase by 150% when brine substitutes for a light hydrocarbon in these carbonate rocks.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009