Estimation of anisotropy parameters using the P-wave velocities on a cylindrical shale sample

In this paper we present a new approach to the estimation of the Thomsen anisotropy parameters and symmetry axis coordinates from the P-wave traveltime measurements on cylindrical shale samples. Using the tomography-style array of transducers, we measure the ultrasonic P-wave ray velocities to estimate the Thomsen anisotropy parameters for a transversely isotropic shale sample. This approach can be used for core samples cut in any direction with regard to the bedding plane, since we make no assumption about the symmetry axis directions and will estimate it simultaneously with the anisotropy parameters. We use the very fast simulated re-annealing to search for the best possible estimate of the model parameters. The methodology was applied to a synthetic model and an anisotropic shale sample

Estimation of stress-dependent anisotropy from P-wave measurements on a spherical sample

Our aim is to understand the stress-dependent seismic anisotropy of the overburden shale in an oil field in the North West Shelf of Western Australia. We analyze data from measurements of ultrasonic P-wave velocities in 132 directions for confining pressures of 0.1–400 MPa on a spherical shale sample. First, we find the orientation of the symmetry axis, assuming that the sample is transversely isotropic, and then transform the ray velocities to the symmetry axis coordinates. We use two parameterizations of the phase velocity; one, in terms of the Thomsen anisotropy parameters α, β, ɛ, δ as the main approach, and the other in terms of α, β, η, δ. We invert the ray velocities to estimate the anisotropy parameters α, ɛ, δ, and η using a very fast simulated reannealing algorithm. Both approaches result in the same estimation for the anisotropy parameters but with different uncertainties. The main approach is robust but produces higher uncertainties, in particular for η, whereas the alternative approach is unstable but gives lower uncertainties. These approaches are used to find the anisotropy parameters for the different confining pressures. The dependency of P-wave velocity, α, on pressure has exponential and linear components, which can be contributed to the compliant and stiff porosities. The exponential dependence at lower pressures up to 100 MPa corresponds to the closure of compliant pores and microcracks, whereas the linear dependence at higher pressures corresponds to contraction of the stiff pores. The anisotropy parameters ɛ and δ are quite large at lower pressures but decrease exponentially with pressure. For lower pressures up to 10 MPa, δ always is larger than ɛ; this trend is reversed for higher pressures. Despite the hydrostatic pressure, the symmetry axis orientation changes noticeably, in particular at lower pressures. </jats:p

Stress induced azimuthally anisotropic reservoir - AVO modeling

The analysis of rock anisotropy in terms of seismic velocities and within the context of rock physics (Biot- Gassmann theory of poroelasticity) provides important information for the evaluation of the stress state (tensors) of rocks, detection of the directions of formation weaknesses, helps in the estimation of overall permeability and failure prediction. Understanding the influence of stress and pore pressure on seismic velocities is important for 4-D reflection seismic interpretation, AVO analysis and reservoir modeling. Laboratory measurements were carried out on spherical shale samples from the overburden under confining stress up to 400 MPa, by means of ultrasonic soundings in 132 independent directions. Such an approach enables the estimation of 3-D elastic anisotropy. Since the sandstones were partly unconsolidated, it was not possible to take ultrasonic measurements. To overcome this, we developed a method for stress induced azimuthal anisotropy estimation using only cross-dipole logging data. These results give the possibility for anisotropic correction in AVO analysis

An estimation of sonic velocities in shale using clay and silt fractions from the elemental capture spectroscopy log

Anisotropic differential effective medium approach is used to simulate elastic properties of shales from elastic properties and volume fractions of clay and silt constituents. Anisotropic elastic coefficients of the wet clay pack are assumed to be independent of mineralogy and to be linearly dependent on clay packing density (CPD), a fraction of clay in an individual wet clay pack. Simulated compressional and shear velocities normal to the bedding plane and are shown to be in a good agreement with measured sonic velocities. Further, elastic coefficients of shales, and, calculated from the log sonic velocities, calibrated porosity and clay fraction obtained from the mineralogy tool are used to invert for elastic constants of clays, C33 and C44. The obtained elastic coefficients of clays show lower scatter than the original elastic coefficients of shales. The noticeable increase of the clay elastic coefficients with the depth increase is shown to result from the positive trend of the CPD with depth. Being interpolated to the same CPD = 0.8, elastic coefficients of clays show no depth dependency. Our findings show that the CPD and silt fraction are the key parameters that can be used for successful modelling of elastic properties of shales

Estimation of stress induced azimuthal anisotropy - AVO modeling

The analysis of rock anisotropy in terms of seismic velocities and within the context of rock-physics (Biot-Gassmann theory of poroelasticity) provides important information for the evaluation of the stress state (tensors) of rocks, detection of the directions of formation weaknesses, helps in the estimation of overall permeability and failure prediction. Understanding the influence of stress and pore pressure on seismic velocities is important for 4-D reflection seismic interpretation, AVO analysis and reservoir modeling. Laboratory measurements were carried out on spherical shale samples from the overburden under confining stress up to 400 MPa, by means of ultrasonic soundings in 132 independent directions. Such an approach enables the estimation of 3-D elastic anisotropy. Assuming VTI symmetry approximation, from the measured velocities the stiffness tensor was inverted. Since the sandstones were partly unconsolidated, it was not possible to take ultrasonic measurements . To overcome this, we developed a method for stress induced azimuthal anisotropy estimation using only cross-dipole logging data. These results give the possibility for anisotropic correction in AVO analysis

ORAL LESIONS OF HIV-INFECTED CHILDREN IN WEST AFRICA IN THE ERA OF ANTIRETROVIRAL TREATMENTS

Oral Communication presented at the "Forum des Jeunes Chercheurs", Brest (France) 2011