Audun Bakk

Stress-induced versus lithological anisotropy in compacted claystones and soft shales

Shales are anisotropic. Most definitions of shale in-corporate this attribute, either by referring to fissility and existence of cleavage planes, or to anisotropic texture resulting in anisotropy of physical properties on many length scales. Definitions of shale scatter though; some focus on a high content of clay minerals as characteristic of a shale, while others consider a large amount of fine grains (< 2μm) as sufficient. In a rock mechanical context, it is natural to define shale as a rock in which clay minerals constitute the load-bearing framework. This means that “gas shales” in oil-field terminology are, strictly speaking, not shales according to a geological or a geomechanical perspective. Still, these materials have a lot in common with classically defined shales (e.g., low permeability) and anisotropy.

The Importance of Overburden Stress Path in Assessment of Stress Dependence for 4D Applications

This work demonstrates that for stress changes around the in situ stress, experimental stress sensitivity of a field shale core fits the linear stress path dependence that is expected on the basis of an assumed linear relationship between velocity changes and stress / pore pressure changes. This is based on ultrasonic laboratory measurements on a field shale core tested in undrained conditions along four different stress paths:Constant mean stress, isostatic, uniaxial strain, and triaxial. The in situ stress path depends on reservoir geometry, tilt, elastic contrast between reservoir and overburden, and possible non-elastic effects. Most often laboratory measurements are only carried out along one stress path, so the results from this work underline the significant importance of stress path for stress (and strain) sensitivity.

Static and Dynamic Characterization of a Deep Overburden Shale

Since shales are the far most abundant overburden formation, understanding the geomechanical and rock physics behavior of shales is essential. We discuss a particular deep overburden shale in light of a thorough static and dynamic multi-directional data acquisition, as a basis for complete static and dynamic characterization assuming transverse isotropy. The stress-and strain-path dependence of the principal P- and S-wave velocities are also analyzed. These quantities are of great interest for interpretation of seismic or sonic-log overburden signatures upon reservoir depletion (or injection), as geomechanical simulations show significant variations of stress and strain changes throughout the overburden.

Experimental Studies of Stress Dependence,Static vs Dynamic behaviour and Mechanical Anisotropies of Shale

Laboratory data from controlled rock mechanical tests with various shales have been used to understand and quantify stress effects on wave velocities of relevance for 4D seismics, and to quantify the difference between static and dynamic mechanical properties and their anisotropies.

Temperature Dependence of Ultrasonic Velocities in Shales – Can We Use It For Interpreting Time-lapse Seismic?

The temperature dependence of ultrasonic velocities in shales show significantly larger temperature sensitivities than predicted by the Gassmann fluid-substitution model, which can be attributed to temperature dependent velocity dispersion. We expect the temperature dependence of velocities to be frequency dependent, resulting in different temperature sensitivities at seismic, sonic and ultrasonic frequencies.