24 The Leading Edge January 2009 SPECIAL SECTION: R o c k p h y s i c s Direct laboratory observation of patchy saturation and its effects on ultrasonic velocities M aximizing the recovery of known hydrocarbon reserves is one of the biggest challenges facing the petroleum industry today. Optimal production strategies require accurate monitoring of production-induced changes of reservoir saturation and pressure over the life of the field. Time-lapse seismic technology is increasingly used to map these changes in space and time. However, until now, interpretation of time-lapse seismic data has been mostly qualitative. In order to allow accurate estimation of the saturation, it is necessary to know the quantitative relationship between fluid saturation and seismic characteristics (elastic moduli, velocity dispersion, and attenuation). Te problem of calculating acoustic properties of rocks saturated with a mixture of two fluids has attracted considerable interest (Gist, 1994; Mavko and Nolen-Hoeksema, 1994; Knight et al., 1998. For a comprehensive review of theoretical and experimental studies of the patchy saturation problem see Toms et al., 2006). For a porous rock whose matrix is elastically homoge- neous and inhomogeneities caused only by spatial variations in fluid properties, two theoretical bounds for the P-velocity are known (Mavko and Mukerji, 1998; Mavko et al., 1998). In the static (or low-frequency) limit, saturation can be con- sidered homogeneous, and, hence, the rock may be looked at as saturated with a homogeneous mixture of the fluids. In this case, the bulk modulus of the rock is defined by the Gassmann equation with the fluid bulk modulus given by Wood's formu- la, i.e., the saturation-weighted harmonic average of the bulk moduli of fluids. Te Gassmann-Wood bound is valid when the characteristic patch size is small compared to the fluid dif- fusion length. Te diffusion length is primarily controlled by rock permeability, fluid viscosity, and wave frequency. In the opposite case, when the patch size is much larger than the dif- fusion length, there is no pressure communication between fluid pockets, and, consequently, no fluid flow occurs. In this no-flow (or high-frequency) limit, the overall rock behaves like an elastic composite consisting of homogeneous patches whose elastic moduli are given by Gassmann's theory. Since all these patches have the same shear modulus, the effective P- wave modulus can be obtained using Hill's equation, i.e, the saturation-weighted harmonic average of the P-wave moduli. Te Gassmann-Wood and Gassmann-Hill bounds apply in the low- and high-frequency limits, respectively. For inter- mediate frequencies, uneven deformation of fluid patches by the passing wave results in local pressure gradients and hence, in the wave-induced fluid flow, which in turn causes wave attenuation and velocity dispersion. For regular distributions of fluid patches of simple geometry (spheres, flat slabs), these effects have been first studied by White (1975), White et al. MAXIM LEBEDEV and JULIANNA T OMS-STEWART, Curtin University BEN CLENNELL, MARINA PERVUKHINA, VALERIYA SHULAKOVA, LINCOLN PATERSON and T OBIAS M. MÜLLER , CSIRO Petroleum BORIS GUREVICH, Curtin University and CSIRO Petroleum FABIAN WENZLAU, Karlsruhe University (1975), and Dutta and Ode (1979). More recently, the ef- fect of regularly distributed patches of more general shape was modeled by Johnson (2001) and Pride et al. (2004). For ran- domly distributed fluid patches Müller and Gurevich (2004) and Müller et al. (2008) showed how the effect of wave-in- duced flow controls the transition from the Gassmann-Wood to the Gassmann-Hill bounds. While theoretical poroelastic models can predict the acoustic response for a given spatial distribution of fluid patches, the factors controlling the formation of the patches are less understood. Tese factors can be studied using fluid- injection experiments in the laboratory. Previously reported laboratory observations demonstrate a qualitative link be- tween fluid patch distribution and acoustic velocities (Cado- ret et al., 1995, 1998; Monsen and Johnstad, 2005). In order to get a deeper insight into the factors influencing the patch distribution and the associated wave response, we perform simultaneous measurements of P-wave velocities and rock sample X-ray computer tomography (CT) imaging. Te CT imaging allows us to infer the fluid distribution inside the rock sample during saturation (water imbibition). We then show that the experimental results are consistent with theo- retical predictions and numerical simulations. Experimental setup Experiments are performed on a cylindrical sample (38 mm in diameter and 60 mm long) cut from a Casino sandstone (Otway Basin, Australia). Te sample is dried at 100 ºC under reduced pressure for 24 hours. Te petrophysical properties are measured using a Coretest AP-608 automatic permeame- ter/porosimeter (Table 1). Ten, the sample is sealed with a thin epoxy layer in order to prevent fluid leakage through the surface. Longitudinal ( V P ) and shear-wave ( V S ) velocities at 1 MHz are measured in the direction across to the core axis (perpendicular to the fluid flow) using broadband ultrasonic transducers. Intermediate aluminum "guide pins" are placed between the sample and transducers to secure sufficient and constant coupling, as well as to provide transparency for X- ray radiation. SPECIAL SECTION: R o c k p h y s i c s Casino Otway Bulk density, g/cm 3 2.2 Grain density, g/cm 3 2.65 Porosity % 16.7 Permeability, mD 7.26 Table 1: Petrophysical properties of the dry rock sample.