Experimental fluid substitution studies question applicability of Gassmann's theory in carbonate rocks

BAECHLE, GREGOR T., RALF J. WEGER and GREGOR P. EBERLI, Comparative Sedimentology Laboratory, University of Miami, 4600 Rickenbacker Cswy, Miami, FL 33149, [email protected]

    Interpretation of amplitude versus offset analysis and 4-D seismic surveys in carbonate reservoirs rely on the effects of saturation on velocities. In order to assess these saturation effects in carbonates, we measured ultrasonic velocity on 30 Cretaceous and Miocene limestones samples under dry and saturated conditions at confining pressures from 2 MPa to 80 MPa with porosity ranging from 5% to over 30%. We observe variations in the shear modulus due to saturation. This variation of the shear modulus questions the basic assumption of constant shear modulus with saturation in Gassmann's theory and its applicability in carbonates.
    In one group of samples the shear modulus of the rock decreases up to 2 GPa, while in another group it increases up to 3 GPa. As a result, velocities predicted with the Gassmann equation for the group with shear weakening overestimate the measured values by as much as 400 m/s; in the group with shear strengthening the values are underestimated by as much 600 m/s. In addition, the Vp/Vs ratio shows an overall increase with saturation. Rocks displaying shear weakening have distinct higher Vp/Vs ratios than those with shear strengthening. Samples with shear weakening and high Vp/Vs ratio consist mostly of rocks with interparticle/intercrystalline porosity. In contrast, carbonate rocks with dominant microporosity and moldic pore types tend to show less effects of fluid saturation on velocity.
    We conclude that a) shear moduli do not remain constant during saturation, b) the rock-fluid interaction causes errors in Gassmann-predicted velocities, and c) pore types influence elastic moduli. These findings imply that AVO analyses and velocity prediction using Gassmann’s equation contain a large degree of uncertainty in carbonates.