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Effect of Pore Structure from Dynamic Shear Moduli in Carbonates

Verwer, Klaas 1; Eberli, Gregor P.1; Weger, Ralf 2; Fabricius, Ida L.3; Baechle, Gregor T.4
1 Comparative Sedimentology Laboratory, University of Miami, Miami, FL.
2 Repsol-YPF, Buenos Aires, Argentina.
3 Technical University of Denmark, Lygnby, Denmark.
4 ExxonMobil Upstreach Research, Houston, TX.

Pore structure is known to exert a strong control on both the acoustic properties and the permeability of carbonate rocks. Recent studies also document that carbonates show changes in shear modulus upon fluid saturation whereby shear moduli either increase (strengthening) or decrease (weakening) when the dry rock is saturated with a brine. These changes can be partly explained by varying pore structure and can be related to permeability.The effect of pore structure on dynamic shear moduli in a sample set of carbonate rocks was evaluated using ultrasonic acoustic measurements combined with digital image analysis (DIA) of thin sections. Two DIA parameters, dominant pore size (DOMsize) and perimeter-over-area (PoA), quantitively describe the pore structure of the investigated samples and demonstrate how physical properties relate to the geological make up of the rock. Samples that exhibit shear weakening display a high PoA and small DOMsize, indicating an intricate pore system (high specific surface). The decrease of the shear modulus is attributed to the large surface area available for efficient matrix-fluid interaction at grain contacts. In contrast, samples that exhibit shear strengthening correlate to low PoA and large DOMsize, indicative of a pore structure with larger, simple pores combined with higher permeability.This finding is corroborated by a parallel study that documents that water weakening is related to permeability. Samples with permeability up to 1 mD have relatively high water weakening. Samples with permeability above 100 mD show a strenghtening of saturated samples. This shear strengthening may be a dispersion effect as high permeability may cause the fluid in the saturated samples to move out of phase with the solid during propagation of the acoustic wave and thus cause a stiffening effect. All above observed effects are attributed to matrix-fluid interactions in relation to the pore structure, i.e., the geological make-up of the rock. The results are of significance for lithologic discrimination, amplitude versus offset and time-lapse seismic analyses.

 

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