Abstract:

This paper compares the microfabric characteristics of natural and reconstituted Opalinus clay shales. Reconstituted specimens are prepared using a resedimentation technique which enables the production of uniform specimens with replicable microstructures and engineering properties. The Authors investigated the evolution of porosity and microstructure using Scanning Electron Microscopy (SEM), Mercury Intrusion Porosimetry (MIP) and P-wave velocity measurements. Intact specimens of Opalinus shale exhibit highly heterogeneous microstructures with significant concentrations of micron-scale, foraminifera fossils, pyrite crystals and well-defined bedding planes, all of which are absent in the reconstituted specimens. Porosimetry shows that the natural specimens contain a single dominant pore size ~18 nm, while the reconstituted specimens have a bi-modal pore distribution, with inter-aggregate pores O[102] nm that decrease in size with the applied consolidation stress. There is no directional dependence in P-wave velocity for the reconstituted specimens, while P-wave measurements parallel to the bedding planes of intact specimens are characteristically 2.5x larger than those normal to the bedding planes.

Introduction

The understanding of geomechanical behavior of shales is critical for their wide range of applications. Shales are used as caprocks in geological systems for carbon sequestration (Rutqvist, 2012). They are also under consideration as host rock in geological repositories for disposal of radioactive wastes (Marschall et al., 2005). To evaluate the sealing capacity of shales for such applications, the porosity and mass transport process need to be well investigated. Shales can be also the rich sources of ‘unconventional’ shale gas and the evaluation of their hydrocarbon deliverability requires a comprehensive understanding of the porosity and permeability (Sondergeld et al., 2010).

Insight into the microstructural features of shales is of primary importance to fully understand and interpret their macroscopic response particularly the mass transport properties (Keller et al., 2011; Keller et al., 2013; Sondergeld et al., 2010). Most of the available data on macroscopic behavior of shales are based on the characterization of a limited number of natural materials that feature significant variability related to the heterogeneity of diagenetic bonding and cracks, while bedding features result in strongly anisotropic mechanical properties. The evolution of porosity and microfabric during sedimentation, the effects of diagenetic bonding and the bedding planes are important for analyzing the mass transport process in shales. A more reliable basis for predicting the geomechanical behavior of these sedimentary rocks may be through preparation of reconstituted samples based on resedimentation of the natural material. In the resedimentation technique, the sediment slurry is mixed and then incrementally consolidated to a target axial effective stress. This procedure results in production of uniform, repeatable and saturated specimens (Abdulhadi et al., 2010; Adams et al., 2013; Schneider et al., 2011). The proposed technique enables us to fundamentally understand the shale behavior by comparing the characteristics of natural and reconstituted materials.

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