Oomoldic carbonates: Pore structure and fluid effects on sonic velocity

Gregor T. Baechle, Gregor P. Eberli, Austin Boyd, Jean Marie DeGrange, Layaan Al-Kharusi

Research output: Contribution to journalArticlepeer-review

8 Scopus citations

Abstract

In order to model the effect of oil/gas production or CO2 injection at the seismic scale, we have to understand the effects of pore structure, pressure and fluid changes on velocity at the laboratory scale. To reach this goal, we measured carbonate rocks with a suite of miscible fluids, simulating the entire range of reservoir fluid moduli from light to heavy oils. In our experiments, compressional velocity (Vp) and shear wave velocity (Vs) are 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 2000m/s difference at a given porosity. The velocity increases between 250 and 750m/s as pressure incresases from 3 to 30MPa. The bulk of the samples show increasing Vp/Vs ratios with pressurization, up to values between 1.7 and 1.84. The ratio of normalized bulk versus shear modulus ranges from 0.7 to 0.9. Twenty-one oomoldic carbonate samples with nearly spherical pores show a weak correlation between velocity and porosity under dry conditions. We attribute the weak correlation between velocity and porosity in rocks with similar pore geometry to variations in inter-crystalline porosity in the rock frame. This finding questions the assumption that spherical pores have a dominant effect on velocity. Four oomoldic samples were chosen for fluid substitution and saturated "in-situ" with seven different pore fluids. Significant effects of fluid changes on velocity are observed. A linear correlation exists between bulk modulus and fluid modulus (r2 > 0.97). In contrast, shear modulus changes correlated with the viscosity of the fluids: the lower the fluid viscosity, the lower the shear modulus. Ourresults question common hypotheses for modeling pore-structure effects on acoustic properties in carbonates; (a) P-wave velocity is controlled by the percentage of spherical porosity, and (b) the P-wave velocity in oomoldic rocks is insensitive to fluid and pressure changes because of high stiffness of the rock frame. These findings imply that one has to be cautious in relating rock-physics model parameters to volumetric dominant pore types.

Original languageEnglish (US)
Pages (from-to)1660-1664
Number of pages5
JournalSEG Technical Program Expanded Abstracts
Volume27
Issue number1
DOIs
StatePublished - Jan 1 2008

Keywords

  • Carbonate
  • Porosity
  • Rock Physics
  • Shear modulus
  • Velocity

ASJC Scopus subject areas

  • Geotechnical Engineering and Engineering Geology
  • Geophysics

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