Heterogeneity of pore geometry and directional fluid flow behavior in vesicular basalts

Characterizing pore volume, connectivity, and multi-phase flow properties in subsurface rocks is fundamental to understanding fluid transport mechanisms. The latter is crucially important for the effective development of strategies for the utilization of subsurface resources such as groundwater aquifers, hydrocarbon and CO 2 storage reservoirs. This study investigates morphological and flow attributes in vesicular basalts through an analysis of pore-scale architecture and directional fluid flow utilizing processed and segmented micro-CT images. Incorporating pore shape clustering, shape factor analysis and direct numerical simulations, we characterize the internal anisotropy and heterogeneity of these basalts and compare their properties with the commonly studied sedimentary lithologies. The investigation of morphological heterogeneity revealed a size-dependent transition in pore geometries. Following an increase in the longest Feret diameter, a convergence toward cubic and cuboidal voids occurs, suggesting a geometric limit of complexity within the volcanic matrix, defining a unique structural signature of these rocks. This plateau limit value varies over different samples, highlighting the existing heterogeneity. Permeability anisotropy is assessed through center-based fluid injection simulations in micro-CT images, designed to replicate radial flow conditions representative of subsurface reservoir scenarios, which are validated against pore network-derived directional metrics. Analysis of azimuthal and polar distributions of both velocity vectors and pore-center-to-pore-center vectors shows well-defined principal flow orientations across all investigated sub-volumes. Furthermore, the full permeability tensors resolved for each sub-volume demonstrate extreme transverse hydrodynamic coupling, with off-diagonal components exceeding principal diagonal entries by several orders of magnitude in select sub-volumes, confirming the existence of oriented hydraulic conduits in vesicular basalts. In contrast, such behavior was not observed in sandstone and limestone. The substantial variability in permeability and principal flow axes across and within samples represents extreme hydraulic heterogeneity and anisotropy in vesicular basalts with significant implications for fluid flow.