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Effect of Spherical Pore Shapes on Acoustic Properties in Carbonates*

By

Gregor T. Baechle1, Layaan Al-Kharusi1, Gregor P. Eberli1, Austin Boyd2,

and Alan Byrnes3

 

Search and Discovery Article #50047 (2007)

Posted July 30, 2007

 

*Adapted from oral presentation at AAPG Annual Convention, Long Beach, April 1-4, 2007

 

1University of Miami, Miami, FL ([email protected])

2Schlumberger, Ridgefield, CT

3Kansas Geological Survey, Lawrence, KS

 

Abstract 

Twenty-eight mono-mineral oomoldic carbonate samples with near-spherical pores show a large scatter in a velocities – porosity plot that is caused by inter-crystalline porosity in the re-crystallized rock frame. This finding questions the assumption that spherical pores have a dominant effect on the p-wave velocity. Vp and Vs is simultaneously measured at a frequency of 1MHz and under increasing effective stress from 3 MPa to 30 MPa. We observe large variations in velocities between 3200 m/s and 6500 m/s and a large scatter in the p-wave velocity – porosity relationship. The p-wave velocity shows up to 2500m/s difference at a given porosity. The velocity increases between 250 and 750m/s with pressures from 3 to 30MPa. The Previous HitbulkNext Hit of the samples show increasing Vp/Vs ratios with pressurization, up to values between 1.7 and 1.84. The ratio of normalized Previous HitbulkNext Hit versus shear Previous HitmodulusNext Hit is ranging from 0.7 to 0.9. Several samples have been chosen for fluid substitution and saturated “in-situ” with 7 different pore fluids. Significant effect of fluid changes on velocity is observed. A linear correlation exists between Previous HitbulkNext Hit Previous HitmodulusNext Hit and fluid Previous HitmodulusNext Hit (r2 > 0.97). In contrast, shear Previous HitmodulusNext Hit changes correlated with the viscosity of the fluids: the lower the fluid viscosity, the lower the shear Previous HitmodulusNext Hit. Our results question common hypothesizes for modeling pore structure effects on acoustic properties in carbonates; (a) P-wave velocity is controlled by the amount of spherical pores, and (b) the velocity in oomoldic rocks is insensitive to fluid and pressure changes because of high aspect ratio pores.

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uAbstract

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Key Points 

  • Large p-wave velocity scatter (2500 m/s) at given porosity.

  • Our results question two major assumptions in respect to moldic, spherical pores:

    • Moldic pores are always associated with strong rock-frame

    • Amount of spherical porosity is related to velocity variations at a given porosity

Not dominant pore type, but minor inter-crystalline pore type in rock frame causes slow velocity at given porosity.

 

  • P-wave velocity of rocks with oomoldic porosity changes significantly with different pore fluids.

  • Previous HitBulkNext Hit Previous HitmodulusNext Hit shows linear relationship with changes in fluid Previous HitmodulusNext Hit,

  • Fluid saturation effects on Previous HitbulkNext Hit Previous HitmodulusTop is function of porosity & pore types

 

 

Conclusions and Implications 

  • Large velocity scatter (2500 m/s) at given porosity observed in rocks dominated by oomoldic porosity.

  • Minor inter-crystalline microporosity causes slow velocity at given porosity.

  • The rock frame affects velocity at high frequencies and not the dominant pore shape or pore type!

  • Ground truth of theoretical models is necessary to obtain meaningful acoustic characterization of carbonates rock/pore types

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