Microphysics of Time-Dependent Deformation of Clay-Bearing Sandstone: Grain Shape Matters

Reliable assessment of the long-term effects of hydrocarbon production from sandstone reservoirs, on induced subsidence and seismicity, requires an understanding of the processes that lead to associated reservoir compaction. The compaction is typically small (strain < 1%) and often considered to be purely elastic, but in reality is partly permanent and possibly even time-dependent. This means compaction may continue even after production stops. Empirical models are often used to evaluate future reservoir compaction, however the reliability of such extrapolation in time is questionable without underpinning in terms of the governing processes. We develop a simplified microphysical model with a variable microstructure (i.e., unit cells with varying grain packings), aimed at capturing the deformation mechanisms observed in triaxial compression experiments on clay-bearing Bleurswiller sandstone (porosity 21%). The mechanisms include rate-independent consolidation of intergranular clay films, rate-dependent intergranular slip along the clay films and stress corrosion cracking of quartz/clastic grains. Our model predicts mechanical behaviour consistent with the main features and trends observed in the experimental data. Stresses and strains are localized in unit cells with relatively low and high grain contact inclination, respectively, over the full range of deviatoric loading. Sensitivity analysis showed that quartz/clastic grain shape (aspect ratio) is an important factor determining the modelled mechanical behaviour. The present model provides mechanistic underpinning for existing geomechanical models accounting for rate-dependent deformation relevant to hydrocarbon production-induced subsidence and seismicity, at field conditions and timescales.