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Changes in dynamic shear moduli of carbonate rocks with fluid substitution

Gregor T. Baechle, Gregor P. Eberli, Ralf J. Weger and Jose Luis Massaferro
Changes in dynamic shear moduli of carbonate rocks with fluid substitution
Geophysics (May 2009) 74 (3): E135-E147

Abstract

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 500 m/s higher than the velocities of the dry samples. Some show no change, and a few even show a decrease in velocity. Shear-wave velocity (V (sub s) ) generally decreases, but nine samples show an increase of up to 230 m/s. Water saturation decreases the shear modulus by up to 2 GPa in some samples and increases it by up to 3 GPa in others. The average increase in the shear modulus with water saturation is 1.23 GPa; the average decrease is 0.75 GPa. The V (sub P) /V (sub S) ratio shows an overall increase with water saturation. In particular, rocks displaying shear weakening have distinctly higher V (sub P) /V (sub S) 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 400 m/s for samples exhibiting shear weakening. In samples showing shear strengthening, Gassmann-predicted velocity values are as much as 600 m/s 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.


ISSN: 0016-8033
EISSN: 1942-2156
Coden: GPYSA7
Serial Title: Geophysics
Serial Volume: 74
Serial Issue: 3
Title: Changes in dynamic shear moduli of carbonate rocks with fluid substitution
Affiliation: ExxonMobil Upstream Research Company, Houston, TX, United States
Pages: E135-E147
Published: 200905
Text Language: English
Publisher: Society of Exploration Geophysicists, Tulsa, OK, United States
References: 45
Accession Number: 2009-074585
Categories: Engineering geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
Secondary Affiliation: University of Miami, USA, United StatesRepsol-YPF, ARG, Argentina
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute. Reference includes data supplied by Society of Exploration Geophysicists, Tulsa, OK, United States
Update Code: 200940
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