Fluid extraction from sandstone reservoirs leads to reservoir compaction, potentially inducing surface subsidence and seismicity, as observed in the Groningen Gas Field, Netherlands. Such compaction is partly elastic, but can additionally be caused by instantaneous plastic and rate/time-dependent processes, such as subcritical crack growth, meaning that compaction may continue even if production is stopped. Despite the need to evaluate the impact of post-abandonment reservoir behavior (>10–100 years), few mechanism-based, rate/time-dependent compaction laws exist. Compaction due to grain breakage, either via critical or subcritical crack growth, is driven by tensile stresses acting on surface and volume flaws. We performed high-resolution 3D linear elastic finite element method simulations on simplified grain assemblies to investigate the effect of stress–strain boundary conditions, porosity and mineralogical variations on grain-scale stress fields. Our simulations showed tensile stress concentrations at grain contact edges and on pore walls, which increased in magnitude with increasing aggregate porosity and local porosity variation. The fraction of surface area with tensile stresses sufficient to extend flaws with a size up to 30μm showed a clear correlation with compactive yield envelopes for the Groningen reservoir sandstone. This suggests that compactive failure is related to the probability of pre-existing surface flaws, falling in a pore surface region where the Griffith criterion is satisfied. A preliminary, time-independent failure probability model, using the observed tensile stress distribution, qualitatively predicts a non-linear increase in grain cracking during deviatoric loading, and suggests a new route to predict sandstone compaction through brittle grain failure.

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 10-6-10-8s-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.

Shale host rock and containment potential are largely determined by the connected pore network in the rock, and the connection between the pore network and the naturally present or mechanically induced fracture network together determines the total bulk permeability. Pore connectivity in shales is poorly understood because most of the porosity is present in sub-micrometer-sized pores that are connected through nanometer-sized pore throats. We have used a number of different techniques to investigate the microstructure and permeability of Early Jurassic shales from the UK (Whitby Mudstone), under intact and fractured conditions. Whitby Mudstone is a clay matrix-rich rock (50-70%), with different mineralogical layers on the sub-millimeter scale and very low natural permeability (10-19 to 10-22 m2), representative of many gas shales and caprocks present in Europe. Artificial fracturing of this shale increases its permeability by 2-5 orders of magnitude at low confining pressure (5 MPa). At high confining pressures (30 MPa), permeability changes were more sensitive to the measuring direction with respect to the bedding orientation. Given the distinct lack of well-defined damage zones, most of the permeability increase is controlled by fracture permeability, which is sensitive to the coupled hydro-chemo-mechanical response of the fractures to fluids.