Fluid Content and Pore Type Effects on Acoustic Velocities in Carbonates

Abstract

Most rock physics models were focused on clastics, and were shown to be challenging in carbonates. In order to have better models, the effects of fluid substitution and pore types on velocities need to be investigated. We study the effects of pore types and saturating fluids on acoustic velocities (Vp and Vs) and rock properties (porosity and permeability) dependence on a set of 33 carbonate samples from a Middle East reservoir. We found that Vp versus porosity and Vs versus porosity show a clear inverse trend. We also found a linear inverse trend of Vs versus permeability in samples with predominant primary porosity. However, Vp, Vs-porosity and Vs-permeability correlations become scattered upon the introduction of brine into the pore space due to possible rock-brine chemical interactions at high confining pressures. We also found that fluid substitution effects seem to be related to dominant pore types; rocks with predominant primary porosity show a clear separation of Vp between the different fluids. This was not shown in rocks with predominant secondary pore space. Further, changes of the shear modulus upon saturation were observed which imply that Gassmann’s premise of constant shear modulus upon fluid substitution is not valid.