Rate-dependent inelastic deformation of Slochteren sandstone: implication for reservoir compaction in the Groningen gas field

Abstract

Hydrocarbon production from sandstone reservoirs causes elastic and inelastic reservoir compaction, potentially leading to surface subsidence and even seismicity, such as observed in the Groningen gas field, Netherlands. Inelastic compaction can partly be instantaneous, though rate-/time-dependent processes may play a role on the longer term. Therefore, compaction may continue even if production is stopped. To reliably evaluate the impact of post-abandonment behaviour, mechanism-based rate-/time-dependent compaction laws are needed. We performed triaxial compression experiments on Slochteren sandstone (reservoir of the Groningen field) samples, with porosity ϕ = 14.6-18.9%, to investigate the effect of strain rate (rates of 10 - 6 - 10 - 8 s - 1 ) under conventional triaxial and uniaxial strain (i.e. zero-lateral strain) boundary conditions. Under triaxial conditions, lowering of stress-strain curves was observed with decreasing strain rate at all differential stresses, the effect being enhanced at higher temperature and pore fluid pH. By contrast, strain rate had limited effect on axial stress vs. strain behaviour under uniaxial strain conditions, though decreasing strain rate, as well as increasing fluid pH, resulted in a smaller increase in confining pressure required to maintain a zero-displacement lateral boundary condition. The mechanical data, complemented by microstructural analysis, suggest that subcritical cracking, coupled with grain rearrangement, was the dominant mechanism causing inelastic deformation under triaxial and uniaxial strain conditions. Our results suggest that the amount of reservoir compaction will be limited after production stops. However, time-dependent deformation will lead to changes in the in-situ state of stress, which should be included in models assessing reservoir compaction and induced seismicity in the Groningen field.