Triaxial Strain-Rate Experiments in Smectitic Materials. Analogue Experiments for Deformation Characterization from Granular Materials to Mudrock (Shales)

For this study, samples of clayey granular materials with a large content of smectite minerals were extracted from the lacustrine plain of the Queretaro Valley (Mexico) at a depth of 2.15 m. The analysis of these materials, used as analogue to mudrocks (shales), allowed a better understanding of the mechanical behavior of shales under pressure conditions (≈5-100 MPa) and reservoir-scale temperatures >100°C. Petrophysical properties and mineralogical characterization were used to evaluate the particle structure response during deformation. The main anisotropy surfaces were identified in the clayey samples that had field unsaturated conditions (30.69 w%) and porosity values varying from 47.6 to 49.2 %. Triaxial undrained experiments with confining pressures of 100, 200 and 300 kPa were carried out. The influence of the smectitic minerals content was revealed by a softening behavior beyond the failure point in the stress-strain curve. The average values for the deviator stress at the moment of failure σ_d represents about 55.55% of the effective vertical main stress value σ₁. Axial deformation at failure varied between 1.95% and 3.65%. The strain rate applied was 0.005 mm/s at room temperature. For shale samples, Rybacki et al. (2015) reported similar deformation rates (0.003 mm/s) at 100 MPa confining pressure and 200°C. Similarly, Dewhurst (2003) reported values for σ_d that represents about 45.94% of σ₁ for confining pressures between 5 and 40 MPa at room temperature. Wensaas (1998) indicates values for σ_d about 41.5% of σ₁ with confining pressures between 1.1 and 25.9 MPa for samples with porosity ranges similar to those of the present study (36-57%). Those samples reached the point of failure at axial deformation values between 2.5-3.5% at room temperature. The comparison of the mechanical behavior of both materials, shallow clays and unsaturated shales, with similar characteristics of anisotropy, mineralogy and porosity, allowed to assess the transition between the elasto-plastic behavior and ductile behavior. Smectitic-rich materials submitted to similar deformation rates and stress states show similar failure conditions at comparable values of axial deformation. The mechanical transition is influenced mainly by the water content in shallow clays and by high pressure and temperature conditions in shales