Permeability heterogeneity during sandstone compaction

Compaction of porous sandstones is generally associated with a reduction in permeability. Depending on porosity and other microstructural characteristics, compaction may be diffuse or localised in bands. Compaction bands have been shown to act as barriers to fluid flow and therefore reduce permeability perpendicular to the band orentiation, and thus also introduce permeability anisotropy. Additionally, the localised nature of compaction bands should also introduce strong permeability heterogeneity. We present new experimental data on sandstone compaction combining acoustic emission monitoring and spatially distributed pore fluid pressure measurements, allowing us to establish how permeability heterogeneity develops during progressive compaction. Three sandstones were tested in the compactant regime: Locharbriggs sandstone, which is microstructurally heterogeneous with beds of higher and lower initial permeability; a low porosity (21%) Bleurville sandstone, which is microstructurally homogeneous and produces localised compaction bands; and a high porosity (24%) Bleurville sandstone, which is also homogeneous but produces compaction in a more diffuse pattern. At regular intervals during compactive deformation, a constant pore pressure difference was imposed at the upper and lower boundaries of the cylindrical samples, and steady-state flow allowed to become established. Following this, local pore pressure measurements were made at four locations, allowing us to derive estimates of the local permeability. In all samples, progressive compaction produced overall reductions in permeability. In addition, localised compaction also produced internal reorganisation of the permeability structure. Localised compaction bands caused local decreases in permeability, while more diffuse compaction produced a more homogeneous overall reduction in permeability.