Carbonate sediments are prone to rapid and pervasive diagenetic alterations that change the mineralogy and pore structure within carbonate rocks. In particular, cementation and dissolution processes continuously modify the pore structure to create or destroy porosity. In extreme cases these modifications can completely change the mineralogy from aragonite/calcite to dolomite, or reverse the pore distribution whereby original grains are dissolved to produce pores as the original pore space is filled with cement to form the rock (Figure 1). All these modifications alter the elastic properties of the rock and, therefore, the sonic velocity. The result is a dynamic relationship among diagenesis, porosity, poretype, and sonic velocity. The result is a wide range of sonic velocity in carbonates, in which compressional-wave velocity (V P ) ranges from 1700 to 6600 m/s and shear-wave velocity (V S ) from 600 to 3500 m/s.Porosity is the main controlling factor in determining the sonic velocity in rocks but in carbonates the pore type is nearly equally important in the elastic behavior and resultant sonic velocity (Anselmetti and Eberli, 1993, 1997). Most of the current theoretical equations do not, or insufficiently, account for this modification of the elastic behavior by the pore type. Consequently, seismic inversion, AVO analysis, and calculations of pore volumes that are based on these equations are prone to large uncertainties in carbonates.We measured acoustic velocities on modern carbonate sediments and rocks in various stages of diagenesis to reveal the relationships between original composition, porosity, pore type, and velocity. The apparatus for these laboratory experiments, constructed by VerdeGeoScience, consists of an oil-filled pressure vessel that contains the ultrasonic transmitter-receiver pair with piezoelectric transducers and the sample. Miniplug samples of one inch (2.5 cm) diameter and 1-2 inches in length are positioned between two piezoelectric transducers and sealed from the confining oil in the pressure vessel. Confining and pore-fluid pressures are chosen independently to simulate most accurately insitu stress conditions of buried rocks. The pore-fluid pressure is kept stable at 2 MPa and the confining pressure is varied between 3 and 100 MPa, resulting in an effective pressure of up to 98 MPa. The pair of transducers generates one compressional wave signal (V P ) and two perpendicularly polarized shear wave signals (V S 1, V S 2) at central frequencies close to 1 MHz.Sonic velocity of carbonate sediment. Grain size and shape, sorting, and the ratio between grain and matrix influence acoustic velocity in unconsolidated carbonate sediment. Pure carbonate mud has an average porosity of 60% and V P of ~1700 m/s. At a compression of 170 MPa, the porosity is reduced to 29% and V P increases to 2250 m/s, while V S is between 900-1200 m/s. These mud samples have a low shear modulus and, thus, a behavior similar to materials that have no rigidity (liquids). Carbonate sand (ooids and skeletal grains) s...

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Understanding the effects of saturation on the acoustic properties of porous media is paramount for using amplitude versus offset (AVO) technique and 4-D seismic. Most laboratory research on saturation effects has been carried out in sandstones, despite the fact that about half of the world's oil and gas reserves are in carbonates. We conducted saturation experiments in carbonates with the intention to fill this gap. These experimental data are used to test theoretical assumptions in AVO and seismic analysis in general. Earlier studies have shown that the complex pore structures of carbonates produce poorly defined porosity-velocity trends. Although porosity is the most important factor to control sonic velocity, our data document that pore type, pore fluid compressibility and variations in shear modulus due to saturation are also important factors for velocities in carbonate rocks. Complete saturation of the pore space separated our samples into two groups: one group showed decreases in shear bulk modulus of the rock by up to 2 GPa, the other group showed increase by up to 3 GPa. This change in shear modulus questions Gassmann's assumption of constant shear modulus in dry and saturated rocks. It also explains our observation that velocities predicted with by the Gassmann equation under-and overestimates the measured velocities of saturated carbonate samples. In addition, the Vp/Vs ratio shows an overall increase with saturation. In particular, rocks displaying shear weakening have distinct higher Vp/Vs ratios.

To assess saturation effects on acoustic properties in carbonates, we measure ultrasonic velocity on 38 limestone samples whose porosity ranges from 5% to 30% under dry and water-saturated conditions. Complete saturation of the pore space with water causes an increase and decrease in compressional- and shear-wave velocity as well as significant changes in the shear moduli. Compressional velocities of most water-saturated samples are up to [Formula: see text] higher than the velocities of the dry samples. Some show no change, and a few even show a decrease in velocity. Shear-wave velocity [Formula: see text] generally decreases, but nine samples show an increase of up to [Formula: see text]. Water saturation decreases the shear modulus by up to [Formula: see text] in some samples and increases it by up to [Formula: see text] in others. The average increase in the shear modulus with water saturation is [Formula: see text]; the average decrease is [Formula: see text]. The [Formula: see text] ratio shows an overall increase with water saturation. In particular, rocks displaying shear weakening have distinctly higher [Formula: see text] ratios. Grainstone samples with high amounts of microporosity and interparticle macro-pores preferentially show shear weakening, whereas recrystallized limestones are prone to increase shear strengths with water saturation. The observed shear weakening indicates that a rock-fluid interaction occurs with water saturation, which violates one of the assumptions in Gassmann’s theory. We find a positive correlation between changes in shear modulus and the inability of Gassmann’s theory to predict velocities of water-saturated samples at high frequencies. Velocities of water-saturated samples predicted by Gassmann’s equation often exceed measured values by as much as [Formula: see text] for samples exhibiting shear weakening. In samples showing shear strengthening, Gassmann-predicted velocity values are as much as [Formula: see text] lower than measured values. In 66% of samples, Gassmann-predicted velocities show a misfit to measured water-saturated P-wave velocities. This discrepancy between measured and Gassmann-predicted velocity is not caused solely by velocity dispersion but also by rock-fluid interaction related to the pore structure of carbonates. Thus, a pore analysis should be conducted to assess shear-moduli changes and the resultant uncertainty for amplitude variation with offset analyses and velocity prediction using Gassmann’s theory.