Rune M. Holt

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.

Mechanical Anisotropies of Shale

Laboratory experiments have been performed with a number of shale samples, including commonly studied outcrops like Pierre and Mancos Shale. The angular dependences of unconfined strength, friction angle, static E-modulus and P-wave velocity have been measured and modelled. The anisotropy of the ratio between dynamic and static moduli is found to represent plastic / brittleness anisotropy. Stress and stress path dependent velocity anisotropy will be shown and related to possible 4D seismic applications. Implications for field applications to borehole stability and fracturing will be discussed.

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.