Time‐dependent compaction band formation in sandstone

Michael J Heap,Patrick Baud,Philip G Meredith,Nicolas Brantut

doi:10.1002/2015jb012022

Abstract

AbstractCompaction 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 localization 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 = 0.24) under constant stress (creep) conditions in the compactant regime. Our experiments show that inelastic strain accumulates at a constant stress in the compactant regime, manifest as compaction bands. While creep in the dilatant regime is characterized by an increase in porosity and, ultimately, an acceleration in axial strain rate to shear failure, compaction creep is characterized 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 and 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 compactant 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.

Highlights

Inelastic compaction in porous sandstone formations can be manifest as discrete bands that form orthogonal to the maximum principal stress, called compaction bands [Hill, 1989]
The onset of dilatational microcracking, determined using a combination of the onset of acoustic emissions (AEs) activity and a change in the rate of porosity reduction, occurred at a differential stress of about 40 MPa, and macroscopic sample failure occurred following the peak stress; the peak stress occurred at a differential stress of 72 MPa and an axial strain of about 1%
We find that the inelastic strain associated with the growth of a compaction band is about the same regardless of whether the band grew under a constant stress or a constant strain rate (Figure 13a)

Summary

Introduction

Inelastic compaction in porous sandstone formations can be manifest as discrete bands that form orthogonal to the maximum principal stress, called compaction bands [Hill, 1989]. Triaxial deformation experiments on porous sandstone samples, conducted under a constant strain rate and using a conventional triaxial stress path, have shown that compaction bands can develop over a wide range of effective pressures [Wong et al, 1997; Holcomb and Olsson, 2003; Baud et al, 2004; Fortin et al, 2006; Tembe et al, 2008; Charalampidou et al, 2011] These laboratory studies demonstrated that inelastic compaction (i.e., permanent porosity reduction) leads to a reduction in permeability [Wong et al, 1997; Zhu and Wong, 1997; David et al, 2001]. After more than a decade studying compaction bands, field observations [Schultz et al, 2010; Eichhubl et al, 2010], laboratory

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Discussion

Conclusion