Time-dependent compaction band formation in sandstone

Michael Heap (1) , Nicolas Brantut (2) , Patrick Baud (1) , Philip Meredith (2)

1 IPGS-GE - Géophysique expérimentale (IPGS)
2 UCL - University College of London [London]

Compaction bands in sandstone are laterally-extensive planar deformation features that are characterized by lower porosity and permeability than the surrounding host rock. As a result, this form of localisation has important implications for both strain partitioning and fluid flow in the Earth’s upper crust. To better understand the time-dependency of compaction band growth, we performed triaxial deformation experiments on water-saturated Bleurswiller sandstone (initial porosity = 24%) under constant stress (creep) conditions in the compactive regime. Our experiments show that inelastic strain accumulates at a constant stress in the compactive regime, manifest as compaction bands. While creep in the dilatant regime is characterised by an increase in porosity and, ultimately, an acceleration in axial strain rate to shear failure, compaction creep is characterised by a reduction in porosity and a gradual deceleration in axial strain rate. The global decrease in the rates of axial strain, acoustic emission energy, and porosity change during creep compaction is punctuated at intervals by higher rate excursions, interpreted as the formation of compaction bands. The growth rate of compaction bands formed during creep is lower as the applied differential stress, and hence background creep strain rate, is decreased. However, the inelastic strain associated with the growth of a compaction band remains constant over strain rates spanning several orders of magnitude (from 10–8 to 10–5 s–1). We find that, despite the large differences in strain rate and growth rate (from both creep and constant strain rate experiments), the characteristics (geometry, thickness) of the compaction bands remain essentially the same. Several lines of evidence, notably the similarity between the differential stress dependence of creep strain rate in the dilatant and compactive regimes, suggest that, as for dilatant creep, subcritical stress corrosion cracking is the mechanism responsible for compactant creep in our experiments. Our study highlights that stress corrosion is an important mechanism in the time-dependent porosity loss, subsidence, and permeability reduction of sandstone reservoirs.

Domaines: Géophysique [physics.geo-ph]

Michael Heap, Nicolas Brantut, Patrick Baud, Philip Meredith. Time-dependent compaction band formation in sandstone. Journal of Geophysical Research, 2015, 120, pp.4808-4830. ⟨10.1002/2015JB012022⟩. ⟨hal-01180533⟩