Development of Permeability Heterogeneity During Compaction of Porous Sandstone

Abstract

We aimed to establish how permeability heterogeneities develop in relation to compaction deformation in sandstone. Three sandstones were tested in the compactant regime: Locharbriggs sandstone, which is initially heterogeneous with beds of lower initial permeability; a low porosity (22%) Bleurswiller sandstone, which is initially homogeneous and produces localized compaction bands; a high porosity (24%) Bleurswiller sandstone, also homogeneous but producing compaction in a more diffused pattern. We monitored acoustic emission locations and elastic wave speed variations throughout deformation. In addition, at regular stages during each test, a constant pore pressure difference was imposed at the boundaries of the samples, and steady-state flow was established. Internal pore pressure measurements at four locations allowed us to derive local permeability estimates. In all samples, progressive compaction produced overall reductions in permeability. In addition, localized compaction also produced internal reorganization of the permeability structure. Strong permeability reductions in the direction perpendicular to flow, by up to two orders of magnitude, are only observed when fully connected compaction bands grow across samples. Compaction and permeability reduction preferentially impacted the more porous and permeable regions of the samples, which lead to an overall homogenization of the transport properties of the samples during deformation. Compaction results from grain crushing, and is directly linked to progressive reductions in elastic wave speed. However, the impact of compaction on permeability depends strongly on the spatial connectivity of the compacted regions.