Thermo-hydro-mechanical properties of Cobourg limestone

The results of hydro-mechanical and thermo-hydro-mechanical experiments on Cobourg limestone are presented here. Experiments were performed within geophysically instrumented Hoek-type cell under hydrostatic and deviatoric stresses, controlled at ambient temperature (25 °C) and elevated in-situ temperatures up to 150 °C. Variations and differences in the evolution of VP and VS during various stages of each experiment correspond to the heterogeneous and anisotropic nature of tested specimens. VS is influenced by micro-thermal damages caused by the heating stage more than VP. Whereas the latter proves to be more sensitive to the progressive fracture growth leading to rock failure during deviatoric loading stage. Both the elastic wave velocities are a better means to recognize earlier effect of dilatancy than what is shown by volumetric strain curve. The permeability decreases after heating stage and starts to increase beyond compaction and after emergence of axial cracking. This study shows that there exists a self-consistent behavior among the evolution of seismic wave velocities with the hydrostatic stresses, heating stages, deviatoric stress increments, deformational patterns, and transport properties of tested specimens. These complicated experiments are crucial to model various stages of HM and THM conditions and understand the relationships between various types of stress in relation to the deformational responses of argillaceous rocks.

► VP and VS decrease with increase in thermal expansion, 150 °C sample shows highest thermal expansion. ► Permeability (k) decreases after heating and during compaction and increase prior to failure. ► A constant thermal stress rate of 0.21 MPa/°C measured during heating of samples. ► VS is sensitive to thermal expansion and VP is sensitive to induced fractures prior to failure. ► Lateral monitoring of VP and VS are better indicator of earlier weakening than volumetric strains.