Discrete Element Method modelling of Groningen reservoir compaction using a new contact model describing elastic and inelastic grain-scale interactions

Predicting reservoir compaction resulting from fluid depletion is important to assess potential hazards and risks associated with fluid production, such as surface subsidence and induced seismicity. Globally, many producing oil and gas fields are experiencing these phenomena. The giant Dutch Groningen gas field, the Netherlands, is currently measuring up to 35 cm of surface subsidence and experiencing widespread induced seismicity. To accurately predict reservoir compaction, reservoir-scale models incorporating realistic grain-scale microphysical processes are needed. As a first step towards that aim, Discrete Element Method (DEM) modeling can be used to predict the compaction behavior of granular materials at the cm/dm-scale, under a wide range of conditions representing realistic in-situ stress and pressure conditions.

Laboratory experiments on the reservoir of the Groningen gas field, the Slochteren sandstone, have shown elastic deformation, inelastic deformation due to clay film consolidation, and inelastic deformation due to grain sliding and grain failure. Since the available contact models for DEM modeling do not yet incorporate all of these grain-scale processes, a new contact model, the Slochteren sandstone contact model (SSCM), was developed to explicitly take these mechanisms into account and integrate them into Particle Flow Code (PFC), which is a powerful DEM approach.

In SSCM the blunt conical contact with an apex angle close to 180˚ is assumed to properly model the elastic behavior, as well as the grain failure mechanism. Compacting an assembly of particles with this type of contact model, results in a range of contact shapes, from point to long contacts, which is compatible with microstructural observations of Slochteren sandstone.  The deformation of thin intergranular clay coatings is implemented following the microphysical model proposed by Pijnenburg et al. (2019a).

The model allows for the systematic investigation of porosity, grain size distribution and intergranular clay film content on compaction behavior. The model was calibrated against a limited number of hydrostatic and deviatoric stress experiments (Pijnenburg et al. 2019b) and verified against an independent set of uniaxial compressive experiments (Hol et al. 2018) with a range of porosities, grain size distributions and clay content. The calibrated model was also used to make predictions of the compaction behavior of Slochteren sandstone. These predictions were compared to field measurements of in-situ compaction and showed an acceptable match if the uncertainties of field measurements are considered in calculations.

References:

Pijnenburg, R.P.J., Verberne, B.A., Hangx, S.J.T. and Spiers, C.J., 2019. Intergranular clay films control inelastic deformation in the Groningen gas reservoir: Evidence from split‐cylinder deformation tests. Journal of Geophysical Research: Solid Earth.

Pijnenburg, R.P.J., Verberne, B.A., Hangx, S.J.T. and Spiers, C.J., 2019. Inelastic deformation of the Slochteren sandstone: Stress‐strain relations and implications for induced seismicity in the Groningen gas field. Journal of Geophysical Research: Solid Earth.

Hol, S., van der Linden, A., Bierman, S., Marcelis, F. and Makurat, A., 2018. Rock physical controls on production-induced compaction in the Groningen Field. Scientific reports, 8(1), p.7156.