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Static vs. dynamic stiffness of shales: Frequency and stress effects

Abstract

In this work, an experimental study was carried out with the aim of linking static and dynamic stiffness of shales. Four fully saturated field shales were tested. The experiments were carried out in SINTEF's low-frequency cell. The measurements included undrained quasi-static loading cycles from which the static stiffness was derived, dynamic stiffness measurement at seismic frequencies (1 – 150 Hz), and ultrasonic velocity measurements (500 kHz). The obtained results demonstrate that the difference between static and dynamic stiffness is due to both dispersion and non-elastic effects: All tested shales exhibited P-wave velocity dispersion of the order of 10-20% between seismic and ultrasonic frequencies. Much larger dispersion of up to 150% was observed for Young's moduli. In static tests, non-elastic deformations increase with increasing stress change, resulting in a reduction of the rock stiffness. The largest stress-amplitude effects were observed for Opalinus Clay: the static undrained average Young modulus, measured for a stress amplitude of 3 MPa is 50% lower than that measured for a 1 MPa stress cycle. The zero-stress extrapolated static undrained stiffness, however, reflects the purely elastic response and agrees well with the dynamic stiffness measured at low frequency. One shale exhibited nearly perfect elastic behavior for stress amplitudes up to 5 MPa.

Category

Academic lecture

Client

  • Research Council of Norway (RCN) / 234074

Language

English

Author(s)

Affiliation

  • Norwegian University of Science and Technology
  • SINTEF Industry / Applied Geoscience
  • Switzerland
  • USA

Presented at

52nd US Rock Mechanics/Geomechanics Symposium

Place

Seattle

Date

17.06.2018 - 20.06.2018

Organizer

American Rock Mechanics Association

Year

2018

View this publication at Cristin