Changes of shear moduli in carbonate rocks: Implications for Gassmann applicability

The effect of saturation on different carbonates was investigated. For this study, 30 limestone samples from Cretaceous and Miocene reservoirs with porosities from 3% to over 30%, and having different texture and pore types, were selected. One half of each sample was measured first under dry conditions. If, upon completion of the pressure cycle, no hysteresis effect was detected, the sample was considered unaltered and measured under saturated conditions. Otherwise, the second half of the sample was used for measurements under saturated conditions. During the measurement, the pore fluid pressure was kept constant at 2 MPa. The same effective pressure steps as used in the dry sequence were measured. A single compressional wave and two orthogonally polarized shear waves were simultaneously measured at center frequency of 1 MHz. Shear weakening and shear strengthening that affect indirectly the velocity of the compressional wave were observed. Preliminary results show that the change of the shear modulus is related to pore types.