The Impact of Pore Geometry and Orientation on Permeability Evolution and Compaction Band Formation in Volcanic Rocks

Compaction band formation and permeability evolution in volcanic rocks are key to understanding fluid transport and the potential for pore fluid pressurisation, impacting volcano eruption dynamics and volcanic hazards, geothermal energy extraction, and CO₂ sequestration. Compaction banding and permeability evolution are influenced by the geometry and alignment of pores. Laboratory studies on volcanic rocks have provided valuable insights, yet the heterogeneity of volcanic rock microstructures—particularly in pore geometry and distribution—presents challenges in predicting deformation and permeability changes across varied geological settings. This study systematically investigates the role of pore geometry on compaction band formation and permeability evolution in a porous lava.A porous lava, a trachyandesite from a quarry near Volvic, France, known as "Volvic Bulleuse" (VB), was studied to explore the factors influencing compaction band and permeability evolution. The pores in VB with an average aspect ratio of 0.44, exhibit elongation along a preferred orientation within a groundmass dominated by plagioclase microlites. To investigate the effects of pore geometry, cylindrical samples were drilled along two orientations—parallel (VBY) and perpendicular (VBZ) to the pore major axis—such that in VBY samples, the pore major axis aligns with the cylinder’s long axis, while in VBZ samples, the axes are perpendicular. Both VBY and VBZ exhibited porosities ranging from 23–27%, as determined by gas pycnometry. In terms of permeability, measured along the cylinder’s long axis, VBY samples showed a value of approximately 10⁻¹⁴ m², while VBZ samples exhibited a lower permeability of around 10⁻¹⁵ m².Triaxial deformation experiments demonstrated that VBZ samples—featuring pores perpendicular to the cylinder axis—are approximately twice weaker than VBY samples deformed at the same pressure. Microstructural analysis of deformed samples revealed that pore geometry has minimal influence on compaction band orientation at lower effective pressures, where compaction bands typically formed sub-perpendicular to the major principal stress, as is commonly observed. However, at higher pressures, compaction bands preferentially formed at angles of 45–50° to the loading direction in VBY samples, a development that is closely linked to the preferred orientation of the pores.Additionally, we measure permeability during triaxial deformation under an effective pressure in the ductile regime (75 MPa), revealing significant changes in permeability due to deformation and pore orientation. Our analysis emphasizes pore structure's role in deformation and permeability evolution, with applications ranging from geothermal energy extraction to various subsurface fluid transport processes.