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

Abstract

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.

Microphysics of Inelastic Deformation in Reservoir Sandstones from the Seismogenic Center of the Groningen Gas Field

“…d denotes the assumed grain contact flaw spacing in 2-D, chosen to be equivalent to the characteristic interspacing of grain contact surface irregularities seen in micrographs; µ denotes the assumed friction coefficient of the intergranular illite; B is a constant describing the consolidation behavior of illite; E q is the Young's modulus of quartz grains; Y is the fracture energy of quartz and φ cr is the intragranular porosity fraction assumed to prevail after multi-edge cracking Pijnenburg et al (2019a, b) shows that these graincontact clay films are thicker in the more porous sandstones. Roughly estimating these thicknesses from the micrographs (Pijnenburg et al 2019a) or split-cylinder (Pijnenburg et al 2019b) samples deformed up to stages 1, 3d or 3c (refer Fig. 1).…”

“…8.1.3). A simple analysis shows that compaction by intergranular slip plus clay consolidation is virtually time or rate insensitive at effective stresses pertaining to the current state of depletion of the Groningen reservoir (σ 1 = 57 MPa; σ 3 = 27 MPa), so that reservoir compaction by these mechanisms is expected to be similarly time or rate insensitive (Pijnenburg et al 2019b). In accordance with this, long-term compaction experiments performed on the Slochteren sandstone over weeks (Pijnenburg et al 2018) to months (Hol et al 2015) at similar stresses, temperature (100 °C) and chemical conditions (4 M salt brine) to those seen in the Groningen field showed that the bulk of stage 2 inelastic compaction is instantaneous, while decelerating creep deformation contributed a modest 10-20% to the inelastic strain accumulated during active loading.…”

“…A recent microstructural study by Pijnenburg et al (2019b) showed that stage 2, inelastic deformation of Slochteren sandstone is accompanied by consolidation of and slip on µm-thick, (intergranular) clay films present on the surfaces of and within the contacts between the sandstone grains, plus minor intragranular cracking. A rough estimate of the inelastic strain magnitude associated with clay film deformation (ε ≈ 0.3%), at the experimentally explored stress changes (σ 1 from 41 to 91 MPa, σ 3 = 41 MPa, P p = 1 MPa), showed similar results to the values measured during testing (0.4%).…”

“…This series of simplified models represents a first exploratory attempt to test whether the microphysical mechanisms described by Pijnenburg et al (2019b) can (semi-)quantitatively account for the inelastic behavior of the Slochteren sandstone seen at the sample scale in our triaxial deformation experiments. Our hope is that, if so, the underlying physical processes will be explored in further detail in the future, using more rigorous approaches for linking grainscale to aggregate-scale behavior, i.e., by specialists in homogenization treatments (Bardet and Vardoulakis 2001;Fortin et al 2003), granular mechanics modeling (Einav 2007a, b;Tengattini et al 2014) and the discrete element method (Kawamoto et al 2016;Van den Ende et al 2018).…”

Drill core from seismically active sandstone gas reservoir yields clues to internal deformation mechanisms

“…Because the Groningen reservoir lies close to its maximum burial depth (Gaupp and Okkerman, 2011), and because long-term clay consolidation depends mainly on effective normal stress (Brown et al, 2017), intergranular clay films compacted during burial would almost certainly deform further as the effective vertical stress increased from ∼30 to 57 MPa during depletion of the Groningen reservoir. As demonstrated by Pijnenburg et al (2019b), such stress changes are sufficient to inelastically compact individual 2-5-µm-thick intergranular clay films in the Slochteren sandstone by ∼4%, i.e., by 0.1-0.2 µm. Coupled with the intergranular sliding (clay film shearing) required by the in situ condition of 1-D vertical strain, this implies reservoir compaction strains of 0.1-0.2% for typical detrital grain sizes of 100-200 µm (Pijnenburg et al, 2019b).…”