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

Abstract

Physics-based assessment of the effects of hydrocarbon production from sandstone reservoirs on induced subsidence and seismicity hinges on understanding the processes governing compaction of the reservoir. Compaction strains are typically small (ε < 1%) and may be elastic (recoverable), or partly inelastic (permanent), as implied by recent experiments. To describe the inelastic contribution in the seismogenic Groningen gas field, a Cam-clay-type plasticity model was recently developed, based on the triaxial test data obtained for sandstones from the Groningen reservoir (strain rate ~ 10 −5 s −1). To underpin the applicability of this model at production-driven strain rates (10 −12 s −1), we develop a simplified microphysical model, based on the deformation mechanisms observed in triaxial experiments at in situ conditions and compaction strains (ε < 1%). These mechanisms include consolidation of and slip on µm-thick clay films within sandstone grain contacts, plus intragranular cracking. The mechanical behavior implied by this model agrees favourably with the experimental data and Cam-clay description of the sandstone behavior. At reservoir-relevant strains, the observed behavior is largely accounted for by consolidation of and slip on the intergranular clay films. A simple analysis shows that such clay film deformation is virtually time insensitive at current stresses in the Groningen reservoir, so that reservoir compaction by these mechanisms is also expected to be time insensitive. The Cam-clay model is accordingly anticipated to describe the main trends in compaction behavior at the decade time scales relevant to the field, although compaction strains and lateral stresses may be slightly underestimated due to other, smaller creep effects seen in experiments.

Compaction of the Groningen Gas Reservoir Sandstone: Discrete Element Modeling Using Microphysically Based Grain‐Scale Interaction Laws

“…In addition, increasing confinement leads to more fractures originating from the contacts that are perpendicular to the axial loading axis (i.e., parallel to lateral loading direction-see Figures 11d-11f). These results agree well with the experimental observation reported by Pijnenburg and Spiers (2020) for Slochteren sandstone.…”

“…Resulting fracture configuration include chipping/spalling, splitting and fragmentation, as observed, for example, by Karatza et al. (2019) using in‐situ X‐ray computed microtomography of a sand aggregate undergoing uniaxial compaction (see also Hangx et al., 2010; Hol et al., 2018; Pijnenburg & Spiers, 2020).…”

“…To date, several different approaches have been used to model post‐grain failure using PFC (Cil & Alshibli, 2014; Couroyer et al., 2000; Fu et al., 2017; Laufer, 2015; Marketos & Bolton, 2009; Sun et al., 2018; B. Wang et al., 2008). In our study, the particle removal approach (Couroyer et al., 2000) is chosen because microstructural observations (Pijnenburg & Spiers, 2020; Pijnenburg et al., 2019b) show extensive grain fragmentation upon failure, hence loss of local load supporting capacity provided subsequent strains are not too large, and because this approach is computationally efficient.…”

“…Here is a reference normal stress of 5 MPa (Brown et al., 2017) and γ is a consolidation constant, determined to be about 0.05 for illite compaction (Pijnenburg & Spiers, 2020).…”

Simulation of the inelastic deformation of porous reservoirs under cyclic loading relevant for underground hydrogen storage

“…The modified cam-clay model (MCC), as introduced by 52 , is the most widely used critical state model, which incorporates imposed loading sensitivity and hardening or softening hypothesis. Many researchers 53 , 54 have shown the applicability of modified cam clay plasticity model in the computation of inelastic deformation of sandstone rocks which is caused due to inter-granular cracking and slip of inter-granular clay films. Considering this hypothesis, MCC model is used in our work although the extent to which plasticity models can effectively describe inelastic deformation in the reservoir scale is still under research.…”