Permeability evolution during cyclic loading at different temperatures using the Thermo-Triaxial device

The study of the long-term evolution of permeability of crystalline rocks under different high-pressure/temperature conditions is important for various geological underground projects, including the safety disposal of nuclear waste. As one of the specific objectives of the AMPEDEK project, this work focuses not only on studying the thermo-hydro-mechanical response of a representative granitic rock sample from the German crystalline basement under different high-pressure/temperature conditions, but also on analyzing the dynamics of permeability through long-term flow-through experiments.

The experiments were conducted using the Thermo-Triaxial device, which was designed to perform triaxial tests under high pressure/temperature. Cylindrical plugs were tested under different combinations of vertical and lateral stresses at three temperatures (30, 60, and 90 °C). Matrix permeability and the resulting deformation of the plugs were continuously monitored throughout the entire experiment (12 days).

At 30 °C and 60 °C, permeability exhibited minor and reversible changes during one or two load–unload cycles, indicating limited alteration of the rock structure under these conditions. In contrast, at 90 °C, a pronounced and irreversible decrease in permeability—by more than two orders of magnitude—was observed during the first load cycle, suggesting permanent structural changes or clogging effects within the pore network. During the second cycle at 90 °C, the permeability behavior resembled the reversible trends observed at the lower temperatures.

Permeability changes can be categorized into two types based on the prevailing stress conditions: (1) instantaneous changes resulting from the modification of the mean stress, and (2) gradual changes attributed to time-dependent deformation (creep-effect).