The effect of grain size and porosity on the nature of compaction localisation in high-porosity sandstone

Abstract

Compaction bands are a type of deformation band with negligible shear in porous rocks such as sandstones and are important because they can impede subsurface fluid flow. However, the microstructural properties which favour their growth are not fully understood. We experimentally investigate the effect of porosity and grain size on compaction localisation using synthetic sandstones that permit control of the starting porosity (27%, 32%, 37%) and mean grain size (ranging from 314 to 987 μm). Each sample is subjected to the same triaxial test conditions and shortened by 5% axial strain at an effective confining pressure of 85% of their grain crushing pressure (P*). Discrete compaction bands oriented normal to the axial loading direction are found only in the sample with the lowest starting porosity (27%) and smallest grain size (314 μm), while diffuse bands are observed for the same porosity at a larger grain size of 411 μm. No compaction bands are observed for any grain size in either the 32% or 37% starting porosity samples. Porosity analysis indicates grain size reduction does not necessarily correspond to porosity reduction indicating that compaction by grain rearrangement is as effective as localisation through comminution for these high-porosity synthetic sandstones.