A good knowledge of the simultaneous effect of fluid substitution and pressures on seismic properties is a key point in exploration for overpressure prediction from seismic data, and in exploitation for the seismic monitoring of reservoirs.The aim of this presentation is to find the simplest theoretical model that can correctly describe the complete pressure and fluid dependence of elastic velocities of rocks measured in the laboratory. We find that, for a given rock, a three-parameter model for the pressure dependence of bulk and shear moduli of the rock skeleton injected in the poroelastic equations of Gassmann (1951) type is sufficient to completely and accurately describe the pressure and fluid dependence of the fluid saturated rock, with various types of fluids. The pressure dependence of the dry rock frame moduli is either of the type M(Pdiff) = A - C exp(-D Pdiff) or of the type M(Pdiff) = A/[1 +C exp(-D Pdiff)] (e.g. Zimmermann, 1984), where Pdiff = Pconf - Ppore is the difference between confining pressure Pconf and pore pressure Ppore. As previously noted (Rasolofosaon and Zinszner, 2005) taking also into account the pressure dependence of the saturating fluid substantially improve the agreement between experimental results and theoretical predictions.


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