Julianna Toms

A Simple Approximation For Seismic Attenuation And Dispersion In a Fluid-saturated Porous Rock With Aligned Fractures

Physical properties of many reservoir rocks can be modelled using the concept of poroelasticity. Many reservoir rocks, in addition to the network of pores, contain larger fractures or cracks. Galvin and Gurevich (2006) solved the single scattering problem for a crack in a poroelastic medium and then estimated the effective properties for a distribution of cracks. However this problem requires the solution of a Fredholm integral equation of the 2 kind which in general has no analytical solution for intermediate frequencies. We propose a simple analytical approximation of this solution using the branching function approach. Quantitative comparison shows good agreement between the two solutions. Our analytical solution exhibits a relaxation peak at a frequency where the fluid diffusion length is of the order of the crack

Velocity-Saturation Relation for Rocks with Fractal Distribution of the Pore Fluids

F020 Velocity-Saturation Relation for Rocks with Fractal Distribution of the Pore Fluids T.M. Mueller* (University of Karlsruhe) J. Toms (Curtin University) & G. Quiroga Goode (Universidad Autonoma de Tamaulipas) SUMMARY Seismic attributes like attenuation and velocity dispersion are sensitive to the pore fluid distribution in rocks. That is because seismic waves induce local pressure gradients between fluid patches of different elastic properties and consequently induce local fluid flows that are accompanied by internal friction. This effect is known as wave-induced-flow and it has the potential to characterize the heterogeneous pore fluid distributions. In particular velocity-saturation relationships are of practical importance

Attenuation and Dispersion in Partially Saturated Porous Rock - Random vs Periodic Models

Mesoscopic heterogeneities often occur on a mesoscopic scale, that is scale which is greater than pore-scale but less than wavelength scale. Presence of mesoscopic fluid patches in a porous rock may cause a substantial phase velocity dispersion and attenuation. This is a result of wave induced fluid flow, which arises when a passing wave induces spatial gradients in fluid pressure. Attenuation and dispersion arising from mesoscopic heterogeneities is affected by the spatial distribution of saturating fluids. Here we compare theoretical models for attenuation and dispersion which utilize a 3D random and periodic distribution of fluid heterogeneities. In particular, the periodic model proposed by Johnson (2001) is reinterpreted within the context of the random model. Good agreement between estimates of attenuation and phase velocity is obtained showing that with the right choice of parameters Johnson’s model can describe random as well as periodic distribution of fluid patches.

Direct laboratory observation on velocity-saturation relation transition during rocks saturation

Ultrasonic velocities and fluid saturations are measured simultaneously during water injection into sandstone core samples. The experimental results obtained on lowpermeability samples show that at low saturation values the velocity-saturation dependence can be described by the Gassmann-Wood relationship. However, with increasing saturation a sharp increase of P-wave velocity is observed, eventually approaching the Gassmann-Hill relationship. We relate this transition behavior to the change of the fluid distribution characteristics inferred from CT scans. In particular, we show that for relatively large fluid injection rate this transition occurs at smaller degrees of saturation as compared with high injection rate.

Modelling P-Wave Velocities from X-Ray Tomographic Images of Partially Saturated Porous Rock

F021 Modelling P-Wave Velocities from X-Ray Tomographic Images of Partially Saturated Porous Rock J. Toms* (Curtin University of Technology) T.M. Mueller (University of Karlsruhe) & B. Gurevich (Curtin University and CSIRO) SUMMARY It is well known that P wave velocities measured from drainage and imbibition experiments are often different at the same percentage fluid saturation. Specifically in drainage experiments P-wave velocities change considerably over a wide range of saturations; where as in imbibition experiments considerable changes occur only at very large saturations of the wetting fluid. These discrepancies have been attributed to differences existing between partial fluid distributions obtained from

Acoustics of Random Patchy Saturation

B048 Acoustics of Random Patchy Saturation J. Toms* (Curtin University of Technology) B. Gurevich (Curtin University and CSIRO Petroleum) T.M. Mueller (University of Karlsruhe) & D.L. Johnson (Schlumberger-Doll Research) SUMMARY Mesoscale heterogeneities occur on a spatial scale which is greater than pore-scale but less than wavelength scale. The presence of mesoscale heterogeneities in saturating fluids within porous rock causes significant attenuation and phase velocity dispersion. In particular both contrast in fluid properties and spatial distribution of fluids significantly affects attenuation and dispersion. Thus patchy saturation models need to be flexible in order to account for both contrast and distribution effects.