Pore morphology and distribution in the Shaly facies of Opalinus Clay (Mont Terri, Switzerland): Insights from representative 2D BIB–SEM investigations on mm to nm scale

Opalinus clay is a candidate host formation for the geological disposal of nuclear wastes in Switzerland. The understanding of its long-term mechanical (M) and hydraulic (H) behaviour is an essential requirement for the assessment of its performance as a barrier against radionuclide transport. To study the HM response of Opalinus clay, a microtunnel, 13 m in length and 1 m in diameter, was excavated in that formation at the Mont Terri Underground Research Facility. The rock mass was equipped with sensors to measure the deformation and pore pressure in the rock mass during and after the excavation. A mathematical model that couples the equations of flow and mechanical equilibrium was developed to simulate the HM response of the rock mass. An anisotropic plastic constitutive relationship, based on a microstructure tensor approach, was incorporated in the model. Creep was also considered, as well as the anisotropy of permeability. It is shown that the model satisfactorily predicts the shape and extent of the excavation damage zone (EDZ), deformation, and pore pressure in the rock mass. It is also shown that anisotropy and creep play an important role in the HM response of the rock mass to excavation. The model was further used to simulate water injection tests performed at the test section in the microtunnel. The results show that EDZ, due to its high permeability, is a preferential groundwater flow path along the microtunnel.

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Study of the permeability in the Opalinus clay series (Mont Terri - Switzerland) using the steady state method in Hassler cell

Journal of Petroleum Science and Engineering ◽

10.1016/j.petrol.2019.106457 ◽

2020 ◽

Vol 184 ◽

pp. 106457 ◽

Cited By ~ 1

Author(s):

Stéphane M. Al Reda ◽

Catherine Yu ◽

Guillaume Berthe ◽

Jean-Michel Matray

Keyword(s):

Steady State ◽

Opalinus Clay ◽

Steady State Method ◽

State Method ◽

Mont Terri

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Estimating perturbed stress from 3-D borehole displacements induced by fluid injection in fractured or faulted shales

Geophysical Journal International ◽

10.1093/gji/ggaa103 ◽

2020 ◽

Vol 221 (3) ◽

pp. 1684-1695 ◽

Cited By ~ 2

Author(s):

Yves Guglielmi ◽

Christophe Nussbaum ◽

Jonny Rutqvist ◽

Frédéric Cappa ◽

Pierre Jeanne ◽

...

Keyword(s):

Principal Stress ◽

Fluid Pressure ◽

Injection Pressure ◽

Local Stress ◽

Opalinus Clay ◽

Step Rate ◽

Pressure Step ◽

Heterogeneous Rock ◽

Magnitude Variation ◽

Mont Terri

SUMMARY Hydrofracturing stress measurements in fractured and anisotropic shales are notoriously difficult, because opening of existing geological features tends to prevent the creation of a pure hydraulic fracture perpendicular to the least compressive principal stress. Here we show how adding 3-D borehole-displacement measurements while conducting the hydraulic injection test helps to better constrain the principal stress orientations and magnitudes. We developed a 3-D fully coupled hydromechanical numerical model to analyse the displacement, fluid pressure and injection flow-rate data measured during an injection pressure-step-rate test conducted to activate a faulted borehole interval in the Mont Terri Opalinus Clay (Switzerland). We find that injected fluids can only penetrate the fault when it is at or above the Coulomb failure pressure. Borehole displacement orientations are sensitive to a ∼15° variation in the stress–tensor orientation and a 1 MPa stress magnitude variation. Although some dispersion occurs while rupture is propagating along the fault plane ∼4 m away from the borehole, the maximum density of displacement orientations consistently informs about the stress orientation. Thus, an extended injection step-rate approach coupled with an accurate in situ measurement of the borehole wall displacements can be used to better constrain the local stress field perturbations in fractured shales and in heterogeneous rock in general.

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