Olivia Collet

Estimating azimuthal stress-induced P-wave anisotropy from S-wave anisotropy using sonic log or vertical seismic profile data

Most sedimentary rocks are anisotropic, yet it is often difficult to accurately incorporate anisotropy into seismic workflows because analysis of anisotropy requires knowledge of a number of parameters that are difficult to estimate from standard seismic data. In this study, we provide a methodology to infer azimuthal P-wave anisotropy from S-wave anisotropy calculated from log or vertical seismic profile data. This methodology involves a number of steps. First, we compute the azimuthal P-wave anisotropy in the dry medium as a function of the azimuthal S-wave anisotropy using a rock physics model, which accounts for the stress dependency of seismic wave velocities in dry isotropic elastic media subjected to triaxial compression. Once the P-wave anisotropy in the dry medium is known, we use the anisotropic Gassmann equations to estimate the anisotropy of the saturated medium. We test this workflow on the log data acquired in the North West Shelf of Australia, where azimuthal anisotropy is likely caused by large differences between minimum and maximum horizontal stresses. The obtained results are compared to azimuthal P-wave anisotropy obtained via orthorhombic tomography in the same area. In the clean sandstone layers, anisotropy parameters obtained by both methods are fairly consistent. In the shale and shaly sandstone layers, however, there is a significant discrepancy between results since the stress-induced anisotropy model we use is not applicable to rocks exhibiting intrinsic anisotropy. This methodology could be useful for building the initial anisotropic velocity model for imaging, which is to be refined through migration velocity analysis.

Frequency dependence of anisotropy in fluid saturated rocks - Part II: Stress-induced anisotropy case

A major cause of attenuation in fluid-saturated media is the local fluid flow (or squirt flow) induced by a passing wave between pores of different shapes and sizes. Several squirt flow models have been derived for isotropic media. For anisotropic media, however, most of the existing squirt flow models only provide the low- and high-frequency limits of the saturated elastic properties. We develop a new squirt flow model to account for the frequency dependence of elastic properties and thus gain some insight into velocity dispersion and attenuation in anisotropic media. In a companion paper, we focused on media containing aligned compliant pores embedded in an isotropic background matrix. In this paper, we investigate the case for which anisotropy results from the presence of cracks with an ellipsoidal distribution of orientations due to the application of anisotropic stress. The low- and high-frequency limits of the predicted fluid-saturated elastic properties are respectively consistent with Gassmann theory and Mukerji–Mavko squirt flow model. In the most important case of liquid saturation, analytical expressions are derived for elastic properties and Thomsen anisotropy parameters. The main observations drawn from this model are as follows. Crack closure perpendicular to the applied stress leads to an increase in seismic velocities as a function of stress in the direction of applied stress and a decrease in squirt-flow-induced dispersion and attenuation in this direction. The anisotropy of squirt flow dispersion engenders a decrease in the degree of anisotropy with frequency. The stress-induced anisotropy remains elliptical, even in saturated media, for all frequency ranges.

Estimating Azimuthal Stress-induced P-wave Anisotropy from S-wave Anisotropy Measured by VSP

Anisotropy is sometimes hard to characterize as it requires a number of parameters which are seldom measured on the field. In this study, we present a way to infer azimuthal P-wave anisotropy from measured S-wave anisotropy in areas where anisotropy is known to be caused by the presence of differential stresses acting on the rock. This methodology involves two key steps. In the first one, we estimate the P-wave anisotropy in the dry medium from S-wave anisotropy using a rock physics model which accounts for the stress dependency of seismic velocities in dry elastic isotropic media submitted to triaxial compression. The second major step then consists in calculating the P-wave anisotropy in the saturated medium using the anisotropic Gassmann equations. This workflow is applied to data acquired off North West Australia, in an area where anisotropy is likely to be stress-induced. Such analysis could help constraining orthorhombic tomography or azimuthal AVO analysis.

Effect of fluid on wave propagation in weakly anisotropic porous media

Predicting seismic velocities in isotropic fluid-saturated rocks is commonly done using the isotropic Biot-Gassmann theory. For anisotropic media, the BiotGassmann solution is expressed in terms of stiffness or compliance, which does not provide an intuitive understanding on the impact of fluid on anisotropy. To analyse how the pore fluid affects wave propagation in weakly anisotropic media, we rederived these expressions in terms of dimensionless anisotropy parameters. Besides, we study the effect of fluid on two anisotropy patterns, the one caused by aligned fractures embedded in an isotropic porous background and the stress-induced anisotropy pattern. By deriving an approximation of the anellipticity parameter η, we show that if the dry medium is elliptical, the saturated medium is also elliptical but only if the porosity is not too small. This result can provide a way of differentiating between stress- and fracture-induced anisotropy.