Inelastic Deformation of the Slochteren Sandstone : Stress-Strain Relations and Implications for Induced Seismicity in the Groningen Gas Field

Pore pressure reduction in sandstone reservoirs generally leads to small elastic plus inelastic strains. These small strains (0.1%–1.0% in total) may lead to surface subsidence and induced seismicity. In current geomechanical models, the inelastic component is usually neglected, though its contribution to stress-strain behavior is poorly constrained. To help bridge this gap, we performed deviatoric and hydrostatic stress cycling experiments on Slochteren sandstone samples from the seismogenic Groningen gas field in the Netherlands. We explored in situ conditions of temperature (T = 100 °C) and pore fluid chemistry, porosities of 13% to 26% and effective confining pressures (≤320 MPa) and differential stresses (≤135 MPa) covering and exceeding those relevant to producing fields. We show that at all stages of deformation, including those relevant to producing reservoirs, 30%–50% of the total strain measured is inelastic. Microstructural observations suggest that inelastic deformation is largely accommodated by intergranular displacements at small strains of 0.5%–1.0%, with intragranular cracking becoming increasingly important toward higher strains. The small inelastic strains relevant for reservoir compaction can be described by an isotropic, Cam-clay plasticity model. Applying this model to the depleting Groningen gas field, we show that the in situ horizontal stress evolution is better represented by taking into account combined elastic and inelastic deformation than it is by representing the total deformation behavior using poroelasticity (up to 40% difference). Therefore, inclusion of the inelastic contribution to reservoir compaction has a key role to play in future geomechanical modelling of induced subsidence and seismicity

Intergranular Clay Films Control Inelastic Deformation in the Groningen Gas Reservoir: Evidence From Split-Cylinder Deformation Tests

Production of oil and gas from sandstone reservoirs leads to small elastic and inelastic strains in the reservoir, which may induce surface subsidence and seismicity. While the elastic component is easily described, the inelastic component, and any rate-sensitivity thereof remain poorly understood in the relevant small strain range (≤1.0%). To address this, we performed a sequence of five stress/strain-cycling plus strain-marker-imaging experiments on a single split-cylinder sample (porosity 20.4%) of Slochteren sandstone from the seismogenic Groningen gas field. The tests were performed under in situ conditions of effective confining pressure (40 MPa) and temperature (100 °C), exploring increasingly large differential stresses (up to 75 MPa) and/or axial strains (up to 4.8%) in consecutive runs. At the small strains relevant to producing reservoirs (≤1.0%), inelastic deformation was largely accommodated by deformation of clay-filled grain contacts. High axial strains (>1.4%) led to pervasive intragranular cracking plus intergranular slip within localized, conjugate bands. Using a simplified sandstone model, we show that the magnitude of inelastic deformation produced in our experiments at small strains (≤1.0%) and stresses relevant to the Groningen reservoir can indeed be roughly accounted for by clay film deformation. Thus, inelastic compaction of the Groningen reservoir is expected to be largely governed by clay film deformation. Compaction by this mechanism is shown to be rate insensitive on production timescales and is anticipated to halt when gas production stops. However, creep by other processes cannot be eliminated. Similar, clay-bearing sandstone reservoirs occur widespread globally, implying a wide relevance of our results

Deformation behavior of sandstones from the seismogenic Groningen gas field : Role of inelastic versus elastic mechanisms

Reduction of pore fluid pressure in sandstone oil, gas, or geothermal reservoirs causes elastic and possibly inelastic compaction of the reservoir, which may lead to surface subsidence and induced seismicity. While elastic compaction is well described using poroelasticity, inelastic and especially time‐dependent compactions are poorly constrained, and the underlying microphysical mechanisms are insufficiently understood. To help bridge this gap, we performed conventional triaxial compression experiments on samples recovered from the Slochteren sandstone reservoir in the seismogenic Groningen gas field in the Netherlands. Successive stages of active loading and stress relaxation were employed to study the partitioning between elastic versus time‐independent and time‐dependent inelastic deformations upon simulated pore pressure depletion. The results showed that inelastic strain developed from the onset of compression in all samples tested, revealing a nonlinear strain hardening trend to total axial strains of 0.4 to 1.3%, of which 0.1 to 0.8% were inelastic. Inelastic strains increased with increasing initial porosity (12–25%) and decreasing strain rate (10−5 s−1 to 10−9 s−1). Our results imply a porosity and rate‐dependent yield envelope that expands with increasing inelastic strain from the onset of compression. Microstructural evidence indicates that inelastic compaction was controlled by a combination of intergranular cracking, intergranular slip, and intragranular/transgranular cracking with intragranular/transgranular cracking increasing in importance with increasing porosity. The results imply that during pore pressure reduction in the Groningen field, the assumption of a poroelastic reservoir response leads to underestimation of the change in the effective horizontal stress and overestimation of the energy available for seismicity

Nanocrystalline slip zones in calcite fault gouge show intense crystallographic preferred orientation: Crystal plasticity at sub-seismic slip rates at 18–150 °C

A central aim in fault mechanics is to understand the microphysical mechanisms controlling aseismic-seismic transitions in fault gouges, and to identify microstructural indicators for such transitions. We present new data on the slip stability of calcite fault gouges, and on microstructural development down to the nanometer scale. Our experiments consisted of direct shear tests performed dry at slip rates of 0.1–10 μm/s, at a constant normal stress of 50 MPa, at 18–150 °C. The results show a transition from stable to (potentially) unstable slip above ~80 °C. All samples recovered showed an optical microstructure characterized by narrow, 30–40-μm-wide, Riedel and boundary shear bands marked by extreme grain comminution, and a crystallographic preferred orientation (CPO). Boundary shear bands, sectioned using FIB-SEM (focused ion beam scanning electron microscopy), revealed angular grain fragments decreasing from 10 to 20 μm at the outer margins to ~0.3 μm in the shear band core, where dense aggregates of nanograins also occurred. Transmission electron microscopy, applied to foils extracted from boundary shears using FIB-SEM, combined with the optical CPO, showed that these aggregates consist of calcite nanocrystals, 5–20 nm in size, with the (104)[201] dislocation glide system oriented parallel to the shear plane and direction. Our results suggest that the mechanisms controlling slip include cataclasis and localized crystal plasticity. Because crystal plasticity is strongly thermally activated, we infer that the transition to velocity-weakening slip is likely due to enhanced crystal plasticity at >80 °C. This implies that tectonically active limestone terrains will tend to be particularly prone to shallow-focus seismicity

Inelastic Deformation of the Slochteren Sandstone: Stress-Strain Relations and Implications for Induced Seismicity in the Groningen Gas Field

Pore pressure reduction in sandstone reservoirs generally leads to small elastic plus inelastic strains. These small strains (0.1%–1.0% in total) may lead to surface subsidence and induced seismicity. In current geomechanical models, the inelastic component is usually neglected, though its contribution to stress-strain behavior is poorly constrained. To help bridge this gap, we performed deviatoric and hydrostatic stress cycling experiments on Slochteren sandstone samples from the seismogenic Groningen gas field in the Netherlands. We explored in situ conditions of temperature (T = 100 °C) and pore fluid chemistry, porosities of 13% to 26% and effective confining pressures (≤320 MPa) and differential stresses (≤135 MPa) covering and exceeding those relevant to producing fields. We show that at all stages of deformation, including those relevant to producing reservoirs, 30%–50% of the total strain measured is inelastic. Microstructural observations suggest that inelastic deformation is largely accommodated by intergranular displacements at small strains of 0.5%–1.0%, with intragranular cracking becoming increasingly important toward higher strains. The small inelastic strains relevant for reservoir compaction can be described by an isotropic, Cam-clay plasticity model. Applying this model to the depleting Groningen gas field, we show that the in situ horizontal stress evolution is better represented by taking into account combined elastic and inelastic deformation than it is by representing the total deformation behavior using poroelasticity (up to 40% difference). Therefore, inclusion of the inelastic contribution to reservoir compaction has a key role to play in future geomechanical modelling of induced subsidence and seismicity

Intergranular Clay Films Control Inelastic Deformation in the Groningen Gas Reservoir: Evidence From Split-Cylinder Deformation Tests

Production of oil and gas from sandstone reservoirs leads to small elastic and inelastic strains in the reservoir, which may induce surface subsidence and seismicity. While the elastic component is easily described, the inelastic component, and any rate-sensitivity thereof remain poorly understood in the relevant small strain range (≤1.0%). To address this, we performed a sequence of five stress/strain-cycling plus strain-marker-imaging experiments on a single split-cylinder sample (porosity 20.4%) of Slochteren sandstone from the seismogenic Groningen gas field. The tests were performed under in situ conditions of effective confining pressure (40 MPa) and temperature (100 °C), exploring increasingly large differential stresses (up to 75 MPa) and/or axial strains (up to 4.8%) in consecutive runs. At the small strains relevant to producing reservoirs (≤1.0%), inelastic deformation was largely accommodated by deformation of clay-filled grain contacts. High axial strains (>1.4%) led to pervasive intragranular cracking plus intergranular slip within localized, conjugate bands. Using a simplified sandstone model, we show that the magnitude of inelastic deformation produced in our experiments at small strains (≤1.0%) and stresses relevant to the Groningen reservoir can indeed be roughly accounted for by clay film deformation. Thus, inelastic compaction of the Groningen reservoir is expected to be largely governed by clay film deformation. Compaction by this mechanism is shown to be rate insensitive on production timescales and is anticipated to halt when gas production stops. However, creep by other processes cannot be eliminated. Similar, clay-bearing sandstone reservoirs occur widespread globally, implying a wide relevance of our results

Nanocrystalline slip zones in calcite fault gouge show intense crystallographic preferred orientation: Crystal plasticity at sub-seismic slip rates at 18–150 °C

A central aim in fault mechanics is to understand the microphysical mechanisms controlling aseismic-seismic transitions in fault gouges, and to identify microstructural indicators for such transitions. We present new data on the slip stability of calcite fault gouges, and on microstructural development down to the nanometer scale. Our experiments consisted of direct shear tests performed dry at slip rates of 0.1–10 μm/s, at a constant normal stress of 50 MPa, at 18–150 °C. The results show a transition from stable to (potentially) unstable slip above ~80 °C. All samples recovered showed an optical microstructure characterized by narrow, 30–40-μm-wide, Riedel and boundary shear bands marked by extreme grain comminution, and a crystallographic preferred orientation (CPO). Boundary shear bands, sectioned using FIB-SEM (focused ion beam scanning electron microscopy), revealed angular grain fragments decreasing from 10 to 20 μm at the outer margins to ~0.3 μm in the shear band core, where dense aggregates of nanograins also occurred. Transmission electron microscopy, applied to foils extracted from boundary shears using FIB-SEM, combined with the optical CPO, showed that these aggregates consist of calcite nanocrystals, 5–20 nm in size, with the (104)[201] dislocation glide system oriented parallel to the shear plane and direction. Our results suggest that the mechanisms controlling slip include cataclasis and localized crystal plasticity. Because crystal plasticity is strongly thermally activated, we infer that the transition to velocity-weakening slip is likely due to enhanced crystal plasticity at >80 °C. This implies that tectonically active limestone terrains will tend to be particularly prone to shallow-focus seismicity

Frictional properties and microstructure of calcite-rich fault gouges sheared at sub-seismic sliding velocities

We report an experimental and microstructural study of the frictional properties of simulated fault gouges prepared from natural limestone (96 % CaCO3) and pure calcite. Our experiments consisted of direct shear tests performed, under dry and wet conditions, at an effective normal stress of 50 MPa, at 18–150 °C and sliding velocities of 0.1–10 μm/s. Wet experiments used a pore water pressure of 10 MPa. Wet gouges typically showed a lower steady-state frictional strength (μ = 0.6) than dry gouges (μ = 0.7–0.8), particularly in the case of the pure calcite samples. All runs showed a transition from stable velocity strengthening to (potentially) unstable velocity weakening slip above 80–100 °C. All recovered samples showed patchy, mirror-like surfaces marking boundary shear planes. Optical study of sections cut normal to the shear plane and parallel to the shear direction showed both boundary and inclined shear bands, characterized by extreme grain comminution and a crystallographic preferred orientation. Cross-sections of boundary shears, cut normal to the shear direction using focused ion beam—SEM, from pure calcite gouges sheared at 18 and 150 °C, revealed dense arrays of rounded, ~0.3 μm-sized particles in the shear band core. Transmission electron microscopy showed that these particles consist of 5–20 nm sized calcite nanocrystals. All samples showed evidence for cataclasis and crystal plasticity. Comparing our results with previous models for gouge friction, we suggest that frictional behaviour was controlled by competition between crystal plastic and granular flow processes active in the shear bands, with water facilitating pressure solution, subcritical cracking and intergranular lubrication. Our data have important implications for the depth of the seismogenic zone in tectonically active limestone terrains. Contrary to recent claims, our data also demonstrate that nanocrystalline mirror-like slip surfaces in calcite(-rich) faults are not necessarily indicative of seismic slip rates