2405_0507_baechle.qxd

Abstract

In laboratory experiments we measured the saturation effects on the acoustic properties in carbonates and the results question some theoretical assumptions. In particular, these laboratory experiments under dry and wet conditions show that shear moduli do not remain constant during saturation. This change in shear modulus puts Gassmann's assumption of a constant shear modulus into question and also explains why velocities predicted with the Gassmann equation can be lower or higher than measured velocities. Background and experimental setup. Porosity is the most important factor controlling sonic velocity but our data document that pore type, pore fluid compressibility and variations in shear modulus due to saturation are also important factors for velocities in carbonate rocks. Most laboratory research on saturation effects has been carried out in sandstone, despite the fact that about half of the world's oil and gas reserves are in carbonates. Only a few studies had investigated the effect of saturation on velocity in carbonate rocks. Rafavich et al. (1984) conclude that porosity is the major factor influencing velocity and that pore-fluid type has no statistically relevant influence. In contrast, Japsen et al. The aim of the study described in this article was to further investigate the effect of saturation on different carbonates. We selected 30 limestone samples from Cretaceous and Miocene reservoirs with porosities from 3% to over 30%, and having different texture and pore types. Measuring sonic velocity under dry and saturated conditions on a single sample under variable confining pressure is an experimental challenge, because the sample might be altered during pressurization. To overcome this experimental dilemma we selected macroscopically homogeneous samples and cut them in half. Samples with nearly identical porosities (less the 2% variation) in both halves were used for this experiment. One half of each sample was measured first under dry conditions using variable hydrostatic confining pressure in steps from 2 MPa up to a maximum of 80 MPa and back down to 2 MPa. 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. The samples were saturated with degassed, distilled water for at least 12 hours under vacuum conditions to assure complete saturation. 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. Effect of saturation on V P , V S , and V P /V S ratio. Gas-water substitution causes an increase in bulk modulus and in compressional-wave velocity (V P ). In we exclude the bulk moduli stiffening effect, then the increased bulk density due to water-filled pores slightly reduces both the shear-wave velocity (V S ) and V P . This density effect solely does not change the V P /V S ratio Many compressional velocities of our water-saturated samples are higher than the dry samples, and most shearwave velocities decrease in the saturated samples