Shales and applications in nuclear waste storage and petroleum industry

Shales are fine-grained sedimentary geomaterials composed of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. Shales are characterized by the presence of lamination and fissility which contribute in the development of a marked anisotropic behaviour. In addition, they are characterized by a very low permeability and good self-sealing potential; these properties make shales a suitable host material for many engineering applications. In this sense, the geomechanical behaviour of shales is quickly becoming one of the most important issues in modern geomechanics, largely driven by the nuclear waste geological storage and petroleum industries (i.e., the extraction of gas shale and the sequestration of CO2). In any such application, a deep understanding of the hydro-mechanical behaviour of the involved materials is of primary significance and it is indeed an exciting subject of research for the civil and environmental engineering society.

The water retention mechanisms play a major role in either fluid trapping due to the capillary forces present in low permeability reservoirs or in the resaturation of shale formations after ventilation, as in the case of deep geological repositories. The swelling/shrinkage of shale is related to suction, or the degree of saturation variations. New experimental methodologies have been recently developed at LMS for the analysis of the retention behaviour of shales (Laloui et al. 2012; Ferrari and Laloui, 2012); they involve the direct control of the shale water content and the subsequent measurement of the total suction by a psychrometer. Different techniques have also been considered at LMS for the volume measurement of the shale samples thus allowing the computation of the degree of saturation of the material.

The effects of a reduction of earth pressure on the surrounding material during the construction of a nuclear waste repository or the determination of the production capacity of shale gas reservoirs constitute some of the valuable geomechanical issues in this field of research; as a consequence, the mechanical properties and the permeability are challenging aspects to be investigated. Nonetheless, advanced testing devices have been recently developed at LMS thus providing the adequate equipment to face the mentioned challenges. In particular a high-pressure oedometer cell has been developed to analyse the hydro-mechanical behaviour of the geomaterials at high confining stresses (Salager et al. 2010; Ferrari and Laloui, 2012). The device can apply pressures up to 100 MPa; the high level of applicable vertical stress is required in order to observe the transition from the pre- to the post-yield behaviour of the material thus allowing for a mechanical characterization of the shales using methods similar to those applied to conventional soils. In order to analyse the settlement versus time curve, an analytical solution has been developed at LMS; it considers the time requested by the controller for applying the target pressure, the deformation of the apparatus, the primary consolidation settlement for a non-instantaneous loading and the secondary consolidation settlements.

A second aspect inspiring the research on shales for nuclear waste storage, shale gas and/or oil exploitation is the occurrence of high temperatures. At high depths and under high pressures, temperatures up to 100°C can be reached. The high-pressure oedometer cell presented above allows for the performance of tests in non-isothermal and controlled suction conditions. In addition, LMS is equipped with an innovative triaxial cell that can reproduce such temperatures, as well as extreme mechanical conditions, with an option for partial saturation (Seiphoori et al. 2010). LMS’s thermo-hydro-mechanical (THM) triaxial cell is especially designed for this kind of situations. Bore companies always search for efficient ways of exploitation. This affects also the engineers. One example of borehole improvement is the use of a drilling fluid, which is often water with dissolved salts. This fluid interacts with the soil with which it is in contact. In the case of partial saturation, the fluid can enter the pores of the material, for example. If saturated, a chemical gradient can result in osmotic pressures and flow of water, or diffusion of ions. All cases involve a change in the local chemistry of the pore fluid. LMS is currently researching the influence of salts on saturated soils and shales. Previous studies in the field of shale characterization focused mainly on (chemo-) poro-elasticity. LMS’s approach is developed in such a way that it provides the opportunity to consider irreversible strains as well (Witteveen et al. 2013). The mentioned points altogether form an advanced, unitary framework for the analysis of the thermo-hydro-chemo-mechanical behaviour of shales. LMS is convinced that its research adds valuable knowledge to the civil and environmental engineering community by profoundly researching the elasto-plasticity of this material.