Effective Stress Coefficient for Seismic Velocities in Carbonate Rocks: Effects of Pore Characteristics and Fluid Types

The concept of effective stress is key for understanding the dependence of rock elastic and compaction behaviors on stress and pore-fluid pressure. Previous studies on the concept largely used data acquired on siliciclastic rocks. Carbonate rocks, however, display elastic and compaction behaviors that can be very different than those of siliciclastic rocks. For example, applying most velocity-to-pore-pressure transforms in the context of carbonate reservoirs can be quite challenging. Our study used an experimental approach (a disequilibrium compaction scenario) to assess effective stress coefficient (n) for velocities in three carbonate samples displaying comparable porosities but different dominant pore types (in terms of shape and compliance). Different saturating fluids (nitrogen and distilled water) were used, one at a time, which allowed us to compare both pore and fluid type effects on the coefficient n between these rocks. We found that n is generally bounded by unity. The exception is with n(Vs) (n derived from shear wave velocities) obtained under nitrogen-saturated conditions; n(Vs) is higher than 1 on the three studied samples. Under nitrogen-saturated conditions, the less compliant the main pore types in a given rock are, the higher the value of n(Vs) is. Higher-than-unity values of n(Vs) indicate a deviation from the behavior predicted by existing theories. This could stem from (i) the fact that theoretical analyses assume a pore fluid whose properties are not comparable to those of nitrogen and/or (ii) the way the bulk volumetric strain (a main factor in elastic wave propagation) is incorporated into those theories.