3-D mechanical analysis of complex reservoirs: a novel mesh-free approach

Abstract

SUMMARY Building geomechanical models for induced seismicity in complex reservoirs poses a major challenge, in particular if many faults need to be included. We developed a novel way of calculating induced stress changes and associated seismic moment response for structurally complex reservoirs with tens to hundreds of faults. Our specific target was to improve the predictive capability of stress evolution along multiple faults, and to use the calculations to enhance physics-based understanding of the reservoir seismicity. Our methodology deploys a mesh-free numerical and analytical approach for both the stress calculation and the seismic moment calculation. We introduce a high-performance computational method for high-resolution induced Coulomb stress changes along faults, based on a Green's function for the stress response to a nucleus of strain. One key ingredient is the deployment of an octree representation and calculation scheme for the nuclei of strain, based on the topology and spatial variability of the mesh of the reservoir flow model. Once the induced stress changes are evaluated along multiple faults, we calculate potential seismic moment release in a fault system supposing an initial stress field. The capability of the approach, dubbed as MACRIS (Mechanical Analysis of Complex Reservoirs for Induced Seismicity) is proven through comparisons with finite element models. Computational performance and suitability for probabilistic assessment of seismic hazards are demonstrated though the use of the complex, heavily faulted Gullfaks field.

Numerical and experimental simulation of fault reactivation and earthquake rupture applied to induced seismicity in the Groningen gas field

“…As shown in a number of previous studies (e.g. Buijze, van den Bogert et al, 2019;Buijze et al, 2017;Haug et al, 2018;Mulders, 2003;van Wees et al, 2019;Zbinden et al, 2017), depletion of offset reservoir compartments results in normal and shear stress concentrations developing at the top of the hanging wall and at the base of the footwall (Figure 3-9a and b), while shear stress minima develop at the top of the footwall and base of the hanging wall (Figure 3-9b). The net result of these stress changes is that maxima in SCU (Equation 3-22) appear at the top of the hanging wall and at the base of the footwall, resulting in fault reactivation at these two locations after 7 MPa of depletion (Figure 3-9d).…”

“…Many modeling studies have assumed linear elastic material properties to model the stresses induced by depletion of the Groningen field as well as the response of other fields to a variety of subsurface activities (Buijze, van den Bogert et al, 2019;Haug et al, 2018;Mulders, 2003;Nagelhout & Roest, 1997;Roest & Kuilman, 1994;Segall, 1989;Segall et al, 1994;van Wees et al, 2019, Zbinden et al, 2017. The stressstrain response resulting from the present elastoplasic MCC model was near-linear in the stress range of interest.…”

“…The effect of the decrease in stress path parameter on seismicity may thus be masked by ongoing compaction. It would be an interesting avenue for future studies to evaluate the effect of stress path changes on fault reactivation in a field-scale model with multiple faults -such as the geomechanical reservoir simulator MACRIS (Candela, Thibault et al, 2019;van Wees et al, 2019).…”

“…Sensitivity to the nucleation process could be investigated by comparing against the model realizations in this study. The analysis could even be extended to 3D, by using recent 3D fast models for the computation of stress changes in faulted, geometrically complex reservoirs (van Wees et al, 2019), and/or analytical solutions for elongated, along-strike rupture propagation for ruptures confined along the fault dip (Weng & Ampuero, 2019). The current study forms a first step in understanding the alongdip behavior of production-induced rupture, considering stresses and friction that are heterogeneous along the fault dip.…”

“…Geomechanical modeling indicates that these events are likely induced by (a combination of) production-related poroelastic stress changes, stress changes caused by gradients in compaction of the sandstone reservoir where the depleting reservoir compartments are offset by faults (i.e. differential compaction), stress changes due to salt caprock creep (where salt caprock is present), and by delayed pore pressure diffusion from faults into the depleting reservoir (Buijze, van den Bogert et al, 2019;Haug et al, 2018;Mulders, 2003;Nagelhout & Roest, 1997;Roest & Kuilman, 1994;Segall, P., 1989;Segall, P. et al, 1994;van Wees et al, 2019;Zbinden et al, 2017). The relationship between pressure decrease, reservoir compaction, fault stressing and thus-forced fault reactivation is one of the key ingredients for understanding the spatiotemporal behavior of seismicity and assessing the potential size of the induced events.…”