Elasto-Plastic Behavior of Reservoir Rock: Depletion Experiments and Numerical Modeling

Abstract

Depletion-induced seismic events, such as those in the Groningen gas field, are closely linked to reservoir compaction. Although the stress-strain path of a compacting reservoir might be described using only elastic equations, inelastic deformation has been shown to significantly contribute, in both earlier compaction experiments and field-scale studies. The current and future stress states in the subsurface, which influence fault instability and control the elastic energy available for elastic rupture (earthquakes), can only be constrained when considering inelastic deformation. To quantify the contribution of inelastic strain in a compaction reservoir, pore pressure depletion experiments with pseudo-uniaxial strain, and supporting hydrostatic cycling tests were performed on Bleurswiller sandstone. Both hydrostatic cycling experiments and pressure depletion experiments were carried out under realistic in-situ reservoir pressures. Depletion experiments were conducted at a range of durations, to determine whether there is time-dependency of the inelastic strain component. The experiments show that the inelastic contribution is substantial, particularly under reservoir-like stress conditions (>57% of the total volumetric strain). However, high uncertainty limited the detection of any time-dependent strain. Hydrostatic test data were used to calibrate a Modified Cam-Clay plasticity model, which produced encouraging results under reservoir boundary conditions, despite being based on results from relatively simple experiments. Future improvements to the experimental design, including longer test durations and improved stress control, along with better constraints on the Poisson's ratio, will help to enhance the accuracy and applicability of such models in reservoir settings.