Velocity anisotropy in shales: A petrophysical study

Abstract

Using ultrasonic velocity and anisotropy measurements on a variety of shales with different clay and kerogen content, clay mineralogy, and porosity at a wide range of effective pressure, we find that elastic anisotropy of shales increases substantially with compaction. The effect is attributed to both porosity reduction and smectite‐ to‐illite transformation with diagenesis. A means of kerogen content mapping using velocity versus porosity crossplot for shales is shown. Matrix anisotropy of shales dramatically increases with kerogen reaching the maximum values of about 0.4 at total organic carbon (TOC)=15–20%. A strong chemical softening effect was found in shales containing even minor amounts of swelling (smectite) clay when saturated with aqueous solution. This effect results in a significant P‐wave anisotropy reduction as compared to dry and oil‐saturated shales. Since mature black shales are normally oil wet, this effect can only have a local significance restricted to the wellbore wall. Accurate measurements of phase velocities, including velocities at a 45° direction to the bedding plane, allow us to immediately calculate elastic stiffnesses and anisotropic parameters. Intrinsic (high pressure) properties of shales display an ε > δ > 0 relation. Introduction of the bedding‐parallel microcracks in overpressured shales results in a δ decrease when fully fluid saturated and a δ increase when partially gas saturated, with a characteristic effect on the shape of the P‐wave velocity surface at small angles of incidence. Filtering the contribution of the intrinsic anisotropy of shales, it is possible to estimate the pore fluid phase, microcrack density, and aspect ratio parameters using seismic anisotropy measurements.

Moment tensor decompositions revisited

“…3a) or shear-tensile ( Fig. 3b) faulting in the Bazhenov shale (Vernik and Liu 1997). This complicated behaviour prevents a straightforward interpretation of moment tensors in terms of the physical faulting parameters.…”

Seismic and Rockphysics Diagnostics of Multiscale Reservoir Textures

“…P-and S-wave velocities (Vp and Vs) were measured on cm-sized core plugs at 1 MHz frequency and 25 MPa pressure. The fast-direction kerogen shale velocities are taken from Vernik and Nur (1994) and Vernik and Liu (1997 …”

Anisotropic Elastic Modelling for Organic Shales

“…These core samples came from a single well located in the northeastern part of the West Siberian basin at depths from 3784m to 3842m. Vernik and Liu (1997) further provided the ultrasonic velocities of the 8 samples under dry condition and 5 samples under brine-saturated condition. Table 1 shows four mineral groups that dominate the mineralogy.…”

“…The volume percentage of each mineral was given on a kerogen-free basis. We take the average mineralogy as an example, and assume that the volume percentage of kerogen is 16.8%, the porosity is 4.12% (referring to No.3 sample of Bazhenov in appendix A, Vernik and Liu, 1997). The elastic moduli of clay are cited from Hornby et al (1994).…”

“…SEM observation of these core samples (Vernik and Landis, 1996) indicated that kerogen forms a continuous network in organic-rich shales (Total Organic Carbon, TOC >5%), and discontinued the inorganic minerals into lenticular laminae. A modified Backus average with an empirical constant to control the textural discontinuity was used to model the anisotropy of Bakken shales (Vernik and Landis, 1996;Vernik and Liu, 1997). Bandyopadhyay (2009) showed that the same data can be predicted using the anisotropic Differential Effective Medium (DEM) model with kerogen as the background matrix.…”