Multiphysical behaviour of shales from Northern Switzerland

Shales are involved in energy-related applications such as the extraction of shale gas, the CO2 storage and the nuclear waste disposal. In these contexts, fundamental issues come along with the multiphysical conditions in which shales are found, where mechanics, temperature, chemistry and unsaturated conditions play a relevant role. This thesis analyses, theoretically and experimentally, the multiphysical behaviour of Swiss shales: retention capacity, hydro-mechanical behaviour, thermo-mechanical response and chemo-mechanical processes are investigated. An advanced experimental methodology to analyse the retention behaviour of shales is presented. The existence of main wetting, drying and scanning paths is investigated, together with the hysteresis domain, the volume change induced by suction variations, and the dependency of the air entry value on porosity. The generation of excess pore water pressure due to changes in stress is a concern during tunnel excavations and drilling operations, therefore, the consolidation of shales must be analysed. This thesis describes an apparatus designed to perform high-pressure oedometric tests in multiphysical conditions, and an analytical model which allows to gather information on the coefficient of consolidation, stiffness, poroelastic properties, secondary compression and permeability. Remoulded shales behaviour is compared to the intact shales response, in order to analyse the impact of diagenesis. The results highlight greater stiffness, lower porosity, lower swelling sensitivity and less creep for the intact shale with respect to the remoulded one: this response is considered to be the result of diagenesis. The thermo-mechanical behaviour of Opalinus Clay is investigated at different overconsolidation ratio (OCR) values. Thermal expansion is found when heating is carried out at high OCR, while irreversible thermal compaction is observed when heat is applied at low OCR. A decrease of the yield threshold with increasing temperature is observed. Compressibility and swelling indexes are not significantly influenced by thermal changes, whereas consolidation processes occur faster at high temperature. The research on chemo-mechanical processes in Opalinus Clay highlights that the volume change of the material upon wetting is related to the osmotic suction of the pore fluid. Moreover, an increase in osmotic suction at constant stress leads to development of irreversible settlements, whose magnitude depends on the stress history. The compressibility and consolidation coefficient of Opalinus Clay are not significantly affected by osmotic suction. An elasto-plastic chemo-mechanical framework is formulated in order to model the chemo-mechanical behaviour of Opalinus Clay. The strength and stiffness of Opalinus Clay are investigated through the performance of triaxial tests. The results allow to gain information on the anisotropic and stress-dependent behaviour of the material. Based on the experimental evidences, a constitutive framework is formulated. Transversely isotropic elasticity is considered in order to reproduce the anisotropic elastic response of the material. Moreover, the elastic parameters are defined in order to account for stress dependency. The possibility to consider anisotropic strength response is also included in the model. The constitutive approach includes the introduction of the disturbed state concept (DSC) in order to reproduce softening.

Hydromechanical Aspects of CO2 Breakthrough into Clay-rich Caprock

Lyesse Laloui, Roman Makhnenko, Danila Mylnikov, Victor Vilarrasa Riano

Caprock formations are intended to prevent upwards carbon dioxide (CO2) migration to the surface during CO2 geological storage. Caprock interaction with CO2, as well as its potential consequences, requires to be predicted, and thus, need to be studied experimentally. Laboratory investigations of caprock behavior are complex due to its low permeability, and the scarcity of experimental studies involving high-pressure CO2 injection into caprock representatives puts this difficulty into manifest. In this study, we perform laboratory experiments in an oedometric cell on intact and remolded Opalinus clay (Jurassic shale), evaluating the breakthrough pressure and permeability for liquid and supercritical CO2. Intact and remolded shale specimens present intrinsic permeabilities of 10-21 m2 to 10-20 m2, respectively. Applied axial stress ranges from 27 MPa to 42 MPa and the pressure and temperature conditions are representative of a caprock at a depth of 800 m. We found that the microstructure of the caprock has a great effect on the material properties. The intrinsic permeability of a more tight material (intact Opalinus clay) is around two times lower than that of remolded shale, which has a more open microstructure. Additionally, the intact rock becomes 30 times less permeable to CO2 than the remolded shale, which implies that the CO2 relative permeability is 15 times smaller for intact rock than for remolded shale. On the other hand, CO2 breakthrough pressure for the tighter material is almost three times lower than for the more permeable remolded shale. Breakthrough pressure of the remolded shale ranges from 3.9 MPa to 5.0 MPa for liquid CO2 and from 2.8 MPa to 4.6 MPa for supercritical CO2. For the intact shale, breakthrough pressure is 0.9 MPa for liquid CO2 and 1.6 MPa for supercritical CO2. Thus, the breakthrough pressure cannot be correlated with the intrinsic permeability of the caprock.

Elsevier2017

Quantification of Viscous Creep Influence on Storage Capacity of Caprock

Lyesse Laloui, Roman Makhnenko, Ludovic Räss

In the light of growing concerns for the climate change, it is of particular interest for governments to encourage efficient capture and safe storage of large amounts of carbon dioxide in the subsurface. In this perspective and in order to accurately predict the short and long-term response of the reservoir, a precise characterization of the geomechanical properties has to be carried out. In addition to the classical poroelastic properties, time-dependent deformation, such as viscous creep should also be considered. Storage capacity of a caprock may be seriously affected by local creep deformation allowing fast vertical fluid flow through an a priori very impermeable formation. Within this study, we investigate the ease to creep of Opalinus clay (Jurassic shale) under shallow geological storage conditions and predict the propagation of high porosity channels at operational time scales. The effective poroviscoelastic parameters of rock are inferred from the novel laboratory experiments that allow evaluation of time-dependent deformation. The bulk viscosity of the shale is found to be ∼ 1014 - 1015 Pa·s and it decreases with rise of temperature and pore fluid pressure to total mean stress ratio. Furthermore, the propagation speed of high porosity channels (porosity waves) is calculated to be on the order of the centimeters per year.

2017

Hydromechanical Aspects of CO2 Breakthrough into Clay-rich Caprock

Lyesse Laloui, Roman Makhnenko, Danila Mylnikov, Victor Vilarrasa Riano

Elsevier2017