Influence of water saturation on horizontal and vertical motion at a porous soil interface induced by incident P wave

To improve quantitative interpretation of seismic data, we analyze the effect of fluid substitution in a porous and fractured medium on elastic properties and reflection coefficients. This analysis uses closed-form expressions suitable for fluid substitution in transversely isotropic media with a horizontal symmetry axis (HTI). For the HTI medium, the effect of changing porosity and water saturation on (1) P-wave moduli, (2) horizontal and vertical velocities, (3) anisotropic parameters, and (4) reflection coefficients are examined. The effects of fracture density on these four parameters are also studied. For the model used in this study, a 35% increase in porosity lowers the value of P-wave moduli by maximum of 45%. Consistent with the reduction in P-wave moduli, P-wave velocities also decrease by maximum of 17% with a similar increment in porosity. The reduction is always larger for the horizontal P-wave modulus than for the vertical one and is nearly independent of fracture density. The magnitude of the anisotropic parameters of the fractured medium also changes with increased porosity depending on the changes in the value of P-wave moduli. The reflection coefficients at an interface of the fractured medium with an isotropic medium change in accordance with the above observations and lead to an increase in anisotropic amplitude variation with offset (AVO) gradient with porosity. Additionally, we observe a maximum increase in P-wave modulus and velocity by 30% and 8%, respectively, with a 100% increase in water saturation. Water saturation also changes the anisotropic parameters and reflection coefficients. Increase in water saturation considerably increases the magnitude of the anisotropic AVO gradient irrespective of fracture density. From this study, we conclude that porosity and water saturation have a significant impact on the four studied parameters and the impacts are seismically detectable.

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Reply by the author to H. N. Collins

Geophysics ◽

10.1190/1.1486948 ◽

1985 ◽

Vol 50 (3) ◽

pp. 505-506

Author(s):

R. L. Carlson

Keyword(s):

Linear Regression ◽

Bulk Density ◽

Deep Sea ◽

Water Saturation ◽

P Wave ◽

Fluid Density ◽

Weighted Linear Regression

In our paper on P‐wave anisotropy in deep‐sea carbonates we assessed the degree of water saturation in the samples by a weighted linear regression of [Formula: see text] on Φ in the form [Formula: see text] (1) where [Formula: see text] is bulk density, [Formula: see text] is grain density, Φ is fractional porosity, and [Formula: see text] where [Formula: see text] is fluid density.

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Acoustic signatures of partial saturation

Geophysics ◽

10.1190/1.1444305 ◽

1998 ◽

Vol 63 (1) ◽

pp. 132-138 ◽

Cited By ~ 77

Author(s):

Rosemary Knight ◽

Jack Dvorkin ◽

Amos Nur

Keyword(s):

Water Saturation ◽

Large Change ◽

P Wave ◽

Velocity Variation ◽

Pressure Curve ◽

Velocity Saturation ◽

Acoustic Signatures ◽

Capillary Equilibrium ◽

Lithologic Unit ◽

Water Gas

The relationship between elastic wave velocities and water saturation in a water/gas reservoir depends strongly on whether saturation is heterogeneous (patchy) or homogeneous. Heterogeneity in saturation may result from lithologic heterogeneity because under conditions of capillary equilibrium, different lithologies within a reservoir can have different saturations, depending on their porosities and permeabilities. We investigate this phenomenon by generating models of a reservoir in which we control the distribution of lithologic units and theoretically determine the corresponding velocity‐saturation relationship. We assume a state of capillary equilibrium in the reservoir and determine the saturation level of each region within the reservoir from the corresponding capillary pressure curve for the lithologic unit at that location. The velocities we calculate for these models show that saturation heterogeneity, caused by lithologic variation, can lead to a distinct dependence of velocity on saturation. In a water‐gas saturated reservoir, a patchy distribution of the different lithologic units is found to cause P-wave velocity to exhibit a noticeable and almost continuous velocity variation across the entire saturation range. This is in distinct contrast to the response of a homogeneous reservoir where there is only a large change in velocity at water saturations close to 100%.

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Incorporating mechanisms of fluid pressure relaxation into inclusion‐based models of elastic wave velocities

Geophysics ◽

10.1190/1.1598110 ◽

2003 ◽

Vol 68 (4) ◽

pp. 1173-1181 ◽

Cited By ~ 3

Author(s):

S. Richard Taylor ◽

Rosemary J. Knight

Keyword(s):

Elastic Wave ◽

Water Saturation ◽

Laboratory Data ◽

Fluid Pressure ◽

P Wave ◽

Porous Rocks ◽

Low Frequencies ◽

Wave Velocities ◽

Exact Agreement ◽

Flow Through

Our new method incorporates fluid pressure communication into inclusion‐based models of elastic wave velocities in porous rocks by defining effective elastic moduli for fluid‐filled inclusions. We illustrate this approach with two models: (1) flow between nearest‐neighbor pairs of inclusions and (2) flow through a network of inclusions that communicates fluid pressure throughout a rock sample. In both models, we assume that pore pressure gradients induce laminar flow through narrow ducts, and we give expressions for the effective bulk moduli of inclusions. We compute P‐wave velocities and attenuation in a model sandstone and illustrate that the dependence on frequency and water‐saturation agrees qualitatively with laboratory data. We consider levels of water saturation from 0 to 100% and all wavelengths much larger than the scale of material heterogeneity, obtaining near‐exact agreement with Gassmann theory at low frequencies and exact agreement with inclusion‐based models at high frequencies.

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Changes in dynamic shear moduli of carbonate rocks with fluid substitution

Geophysics ◽

10.1190/1.3111063 ◽

2009 ◽

Vol 74 (3) ◽

pp. E135-E147 ◽

Cited By ~ 36

Author(s):

Gregor T. Baechle ◽

Gregor P. Eberli ◽

Ralf J. Weger ◽

Jose Luis Massaferro

Keyword(s):

Shear Modulus ◽

Shear Wave ◽

Wave Velocity ◽

Shear Wave Velocity ◽

Water Saturation ◽

Acoustic Properties ◽

P Wave ◽

Shear Moduli ◽

Water Saturated ◽

Fluid Interaction

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.

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