2024年8月26日17:00-18:30 にオンラインで第１１回 岩石ー流体セミナーを行いました。講演者は、ユトレヒト大学の篠原敬博さんです。砂岩質のガス貯留層の変形に関する実験、モデリングの研究についてです。１９名の参加者で活発なの議論が行われました。
Rate-dependent inelastic deformation behaviour of porous sandstones at reservoir conditions
Takahiro Shinohara1, Mark Jefferd1, Bart Verberne2, Chris Spiers1, Hans de Bresser1 and Suzanne Hangx1
1Utrecht University 2Shell Global Solutions International B.V.
Fluid extraction from sandstone oil, gas, or geothermal reservoirs causes elastic and inelastic compaction of the reservoir, which may lead to surface subsidence and induced seismicity, such as observed in the Groningen Gas Field, Netherlands. The inelastic compaction is partly caused by rate- or time-dependent processes, meaning that compaction may continue even if production is stopped. To reliably evaluate the impact of prolonged reservoir exploitation and post-abandonment behaviour (10-100 years), mechanism-based rate- or time-dependent compaction laws are needed.
We systematically investigated the effect of strain rate (rates of –
) in triaxial compression experiments performed on clay-bearing Bleurswiller sandstone (as an analogue of the Groningen reservoir sandstone) and almost clay-free Bentheimer sandstone, to explore the effect of mineralogy. Our results showed a systematic lowering of stress-strain curves with decreasing axial strain rate in Bleurswiller sandstone at differential stresses exceeding 40-50% of peak stress (i.e. comparable to typical reservoir stress conditions). By contrast, in Bentheimer sandstone, rate effects were only noticeable at differential stresses > 70% of peak differential stress. Further investigation of the deformation behaviour of Bleurswiller sandstone at varying confining pressure, temperature and pore fluid pH, complemented by microstructural analysis, suggested that the observed rate effects are likely controlled by rate-dependent intergranular frictional slip at lower differential stress, with an increased role of stress corrosion cracking at higher stress.
We then 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 our triaxial compression experiments on clay-bearing Bleurswiller sandstone. The mechanisms include rate-independent consolidation of intergranular clay films, rate-dependent intergranular frictional 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. 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.