Permeability anisotropy under true triaxial stress states: strong flow reduction parallel to the maximum principal stress.

Rocks in the upper crust are generally subjected to true triaxial stress conditions, in which all three principal stresses are unequal (σ₁ > σ₂ > σ₃). Pore and fracture networks respond to anisotropic loading by opening in certain directions while closing in others, potentially inducing strong permeability anisotropy. However, most experimental constraints on stress-dependent permeability are derived from conventional triaxial tests, where two principal stresses are equal and permeability is measured in only one direction.

Here, we use a true triaxial apparatus equipped with a pore-fluid system to measure permeability parallel to all three principal stress axes in cubic samples of Etna basalt (EB) and Crab Orchard sandstone (COS) subjected to anisotropic loading.

For an initially isotropic EB sample, increasing stress along a single axis results in a sharp permeability decrease in the corresponding maximum stress direction, reaching up to two orders of magnitude (from ~10⁻¹⁶ to ~10⁻¹⁸ m²) at a differential stress (σ₁ − σ₃) of 215 MPa. In contrast, permeability parallel to σ₂ decreases mildly when stresses are increased up to ~75 MPa while permeability parallel to σ₃ remains largely unchanged. During unloading, permeability parallel to σ₁ recovers by approximately 1.5 orders of magnitude once σ₁ is reduced to 75 MPa.

Similarly, samples of the initially anisotropic COS also experience a decrease in permeability of two orders of magnitude (from ~10⁻¹⁷ to ~10⁻¹⁹ m²) along the maximum compressive stress at (σ₁ − σ₃) of just 100 MPa. Permeability along σ₁ recovers only partially after unloading, up to 10⁻¹⁸ m², indicating that some permanent compaction occurred along the maximum compression.

These results demonstrate that true triaxial stress conditions can induce pronounced permeability anisotropy through directional closure of pores and microcracks, with important implications for fluid transport in the upper crust, including fault zones, geothermal systems, and stressed reservoirs.