Deformation Behavior of Sandstones From the Seismogenic Groningen Gas Field: Role of Inelastic Versus Elastic Mechanisms

Abstract

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.

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

“…Summing up the elastic and inelastic contributions, we estimate a total vertical strain of 0.4% will have accumulated over the past 50 years of depletion. As can be seen, the inelastic component contributes approximately 50% to the total amount of deformation, which falls within the range previously reported in experimental studies on the strain partitioning in Slochteren sandstone during simulated pore pressure reduction (30-70%; Hol et al, 2015, Pijnenburg et al, 2018. Moreover, the modeled total strain of 0.4% corresponds suitably to the value of 0.3% estimated from in-situ compaction measurements in the Zeerijp-3 well (NAM, 2015;Cannon & Kole, 2017).…”

“…The external, effective vertical ( σ 1 eff ) and horizontal ( σ 3 eff ) stresses are transmitted onto the clay‐filled, grain‐to‐grain contacts, where these are markedly enhanced due to the smaller contact area. For a porosity of 20% typical for the center of the Groningen field (NAM, ), the enhancement of the contact normal stress ( ) and the contact shear stress ( ) typically amounts to a factor of 3 (Pijnenburg, , Chapter 5). For our model configuration, this gives: ≈ 3 ( σ 1 eff + σ 3 eff )/2 and ≈ 3 ( σ 1 eff ‐ σ 3 eff )/2.…”

“…Hysteretic stress-strain behavior is widely documented for sandstone (McKavanagh & Stacey, 1974;Shalev et al, 2014) and is thought to reflect semirecoverable slip along-and contraction/expansion of grain contacts (Tutuncu et al, 1998) or preexisting cracks (Walsh, 1965;David et al, 2012). Such behavior is generally perceived to persist up to a previously supported state of stress (Holt et al, 1994;Pijnenburg et al, 2018), beyond which newly induced, inelastic deformation may occur. For the Slochteren sandstone samples used here, the previously supported state of stress is given by that prevalent in the reservoir at the time of core extraction (Van Eijs, 2015), with the in situ P being 35 ± 2 MPa.…”

Multi‐scale simulation of rock compaction through breakage models with microstructure evolution

“…Standard plasticity models are usually unable to replicate plastic strains prior to the intersection of a yield surface and neglect any inelastic deformation caused by loadingunloading cycles taking place in the pre-yielding regime. However, ample evidence has proved that plastic strains start to accumulate much earlier than the classic yielding cap (van Thienen-Visser and Fokker, 2017; Pijnenburg et al, 2018;Hol et al, 2018). To circumevent this limitation, hypo-plastic modeling techniques can be incorporated (Einav, 2012).…”

Impact of Chemical Environment on Compaction Creep of Quartz Sand and Possible Geomechanical Applications

“…Upon production, the pore pressure ( P p ) in the reservoir decreases relative to the overburden stress ( σ v ), which increases the vertical effective stress ( σ v ,eff = σ v − P p ) acting on the load‐bearing structure of the reservoir rock. This leads to recoverable poroelastic and, in some cases, permanent, inelastic compaction of the reservoir rock (Bernabé et al, ; Pijnenburg et al, ; Shalev et al, ), potentially with a time‐ (creep) or rate‐dependent component (Doornhof et al, ; Nagel, ). Permanent compaction occurs when the effective stress acting on the rock becomes large enough to activate inelastic grain‐scale deformation processes, such as grain rearrangement (Menéndez et al, ), grain and grain contact failure by equilibrium or subcritical crack growth (Brantut et al, ; Brzesowsky, Hangx, et al, ; Brzesowsky, Spiers, et al, ), intergranular clay film deformation (Spiers et al, ), pressure solution (Dewers & Hajash, ; Gratier et al, ; Schutjens, ; Spiers et al, ), and intergranular frictional slip (Spiers et al, ).…”