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

Abstract

Reservoir compaction, surface subsidence, and induced seismicity are often associated with prolonged hydrocarbon production. Recent experiments conducted on the Groningen gas field's Slochteren sandstone reservoir rock, at in-situ conditions, have shown that compaction involves both poroelastic strain and time independent, permanent strain, caused by consolidation and shear of clay films coating the sandstone grains, with grain failure occurring at higher stresses. To model compaction of the reservoir in space and time, numerical approaches, such as the Discrete Element Method (DEM), populated with realistic grain-scale mechanisms are needed. We developed a new particle-interaction law (contact model) for classic DEM to explicitly account for the experimentally observed mechanisms of nonlinear elasticity, intergranular clay film deformation, and grain breakage. It was calibrated against both hydrostatic and conventional triaxial compression experiments and validated against an independent set of pore pressure depletion experiments conducted under uniaxial strain conditions, using a range of sample porosities, grain size distributions, and clay contents. The model obtained was used to predict compaction of the Groningen reservoir. These results were compared with field measurements of in-situ compaction and matched favorably, within field measurement uncertainties. The new model allows systematic investigation of the effects of mineralogy, microstructure, boundary conditions, and loading path on compaction behavior of the reservoir. It also offers a means of generating a data bank suitable for developing generalized constitutive models and for predicting reservoir response to different scenarios of gas extraction, or of fluid injection for stabilization or storage purposes.