Stress versus damage-induced permeability anisotropy under true triaxial stress states in Etna Basalt

Fluids within low-porosity rocks are transported through networks of interconnected microcracks and fractures. Under crustal conditions, rocks are subjected to true triaxial stress states characterized by three unequal principal stresses, where σ₁>σ₂>σ₃. These triaxial stress states can influence the magnitude and direction of fluid flow in two ways. First, cracks may open, close, and/or slip depending on their orientation with respect to the anisotropic stress field, and thereby potentially reducing fluid flow in certain directions while enhancing flow in others. Second, once the magnitude of differential stresses surpasses the onset of dilatancy in the rock, new cracks form, providing additional pathways for fluid transport. The geometry of these new fractures, and therefore the direction of fluid flow enhancement, is also controlled by the anisotropic stress field.

Despite the fundamental role of triaxial stresses in controlling the magnitude and direction of fluid flow through the crust, very little is known regarding the anisotropy of permeability under true triaxial stress states. This knowledge gap exists primarily because experimental permeability measurements are typically conducted under axisymmetric stress states (σ₁>σ₂=σ₃​) with fluid flow and permeability usually measured only parallel to the σ₁​-direction. To address this, we have developed a new True Triaxial Apparatus (TTA) at UCL equipped with a pore fluid system to deform cubic, saturated rock samples under true triaxial loading while contemporaneously measuring permeability along all three loading axes and recording the output of acoustic emissions (AEs).

Results from tests conducted on 50 mm cubes of initially isotropic Etna basalt under true triaxial loading indicate that, under relatively low differential stresses (σ₁-σ₃<180 MPa), fluid flow is reduced by over one order of magnitude in the direction parallel to σ₁. Increasing the magnitude of stress along the σ₂ axis also results in a decrease in permeability along the same axis. The increase of differential stress eventually leads to an increase in AE hits, which further coincides with a sudden increase in permeability along the σ₂​-axis. Our results revealed two completely different anisotropic permeability behaviours during the progressive deformation of the rock. At lower differential stresses, permeability is stress-controlled and is characterised by the reduction of permeability parallel to σ_1. Increasing differential stresses beyond the onset of dilatancy in the rock results in the creation of new cracks that creates pathways for fluids parallel to the σ₂ axis. Further experiments and analysis are in progress to fully quantify and characterise these behaviours.