Influence of 3D pore-scale heterogeneity on the physical, water transport, and mechanical properties of vesicular lavas from S. Miguel Island

Abstract

Abstract This study investigates how 3D microstructural variability influences vesicular lavas, which are globally widespread and important geo-resources. A lava block from S. Miguel Island (Azores, Portugal) was oriented along three orthogonal directions defined by pore elongation and analysed using laboratory methods. Mercury intrusion porosimetry and X-ray microtomography (µCT) characterised pore networks, and mechanical properties were measured through time-resolved in situ µCT tests. The lava sample has a heterogeneous, bimodal pore size distribution, comprising coalescent pores formed during magma ascent and lava emplacement, producing variability in 3D rock properties. Pore and throat size and heterogeneity distributions govern connected porosity and permeability across orientations, whereas directional variations in strength are influenced by the distribution of large, edge-proximal pores. Ultrasonic velocities and capillary absorption show limited directional variability at the mesoscale. Capillary water absorption is moderate and follows the Sharp Front model; pores larger than 1 mm contribute little to absorption, promoting predominantly gravity-driven flow despite low permeability. Our investigation demonstrates that strength variability in vesicular lavas is controlled by pore spatial distribution (rather than orientation alone), while large (> 1 mm) pores suppress directional capillary effects. This study also shows that similar multiscale approaches allow efficient exploration of pore-scale heterogeneity in volcanic rocks.