Volume-Based Modelling of Fault Reactivation in Porous Media Using a Visco-Elastic Proxy Model

Abstract

The injection of fluids into the subsurface takes place in the context of a variety of engineering applications such as geothermal power generation, disposal of wastewater, CO 2 storage and enhanced oil recovery. These technologies involve not only the underground emplacement of fluids in a geologic formation but also affect the stress state of these rocks. If the rock's strength is surpassed, these stress changes can even lead to failure. In this context, we present a conceptual approach to model fault reactivation in porous media. As a starting point for developing and implementing this approach, the already existing combined hydroand geomechanical model within the open-source simulator DuMu x was chosen. For the evaluation of shear slip on the fault plane, the classical Mohr-Coulomb failure criterion is used. Based on the energy balance from Kanamori (Earthquake thermodynamics and phase transformations in the earth's interior, international geophysics, vol 76. Academic Press, London, pp [293][294][295][296][297][298][299][300][301][302][303][304][305] 2001), where a slip event on fault is described as a transformation of elastic energy into seismic waves, heat and an amount of energy required to cause fracture, we interpret failure as a dissipation of elastic energy. Furthermore, seismic data allow to infer a constant stress drop over a wide range of scales (Abercrombie and Leary in Geophys Res Lett 20 (14): [1511][1512][1513][1514] 1993). These findings are incorporated into our model by altering the material properties during the slip event. In detail, the linear elastic material law is replaced by a visco-elastic behaviour, which reproduces the characteristics mentioned above. This, in turn, leads to additional displacements, which are interpreted as the slip on the fault plane. Our results indicate that this pragmatic approach is capable of modelling fault reactivation without resolving the fault as a discrete surface but as a elements representing a fault zone instead.

Editorial

“…The developed preconditioner exhibits good scalability properties for challenging problems regardless of dominant physics. Beck et al (2016) present a conceptual approach to model fault reactivation in porous media. They interpret failure as a dissipation of elastic energy and thus replace the originally linear elastic material law by a visco-elastic law.…”

Modelling fault reactivation with characteristic stress-drop terms

“…The energies, into which the elastic energy is transformed, are not considered since they are either dissipated or negligible for the stress redistribution. There are other recently introduced approaches (Berge et al, 2017;Ucar et al, 2018;Gómez Castro et al, 2017) that calculate the stress drop required for reaching the equilibrium, which is in fact the excess shear stress. Our approach uses a constant value, and therefore ends up either beyond the equilibrium or does not yet reach it, the latter case would trigger immediately another failure event.…”

Numerical Modeling of Injection‐Induced Earthquakes Using Laboratory‐Derived Friction Laws

“…Poroelastic coupling plays a major role in the presence of faults and fractures (Rutqvist et al, ; Rutqvist & Stephansson, ; Segura & Carol, ). Faults are typically represented either as 3‐D failure zones with slip‐weakening rheology (Beck et al, ; Borja, ; Rutqvist et al, ), or as frictional contact surfaces (Ferronato et al, ; Ghassemi & Tao, ; Jha & Juanes, ; Morris, Hao, et al, ). The latter approach, adopted in this study, is consistent with the observed structure of mature faults, which suggests that shearing during earthquake slip events localizes within a thin granular core (Rice, ).…”

“…Measures of tendency to slip, mobilized friction angle, Coulomb failure analysis (Reasenberg & Simpson, ), and estimates of coseismic fault slip based on simplified fault rheologies have proven very useful to understand the geomechanical challenges posed by subsurface energy technologies. Thermohydromechanical models of induced seismicity have focused on fault reactivation through estimates of tendency to slip (De Simone et al, ; Jacquey et al, ; Kim & Hosseini, ; Li & Laloui, ; Morris, Detwiler, et al, ; Rutqvist et al, ; Vidal‐Gilbert et al, ; Vilarrasa et al, ; Vilarrasa et al, ), estimates of coseismic slip from slip‐weakening friction laws (Beck et al, ; Cappa & Rutqvist, , ; Jeanne et al, ; Jha & Juanes, ; Morris, Hao, et al, ; Rinaldi et al, ; Rohmer, ; Taron & Elsworth, ), or connecting poroelastic stressing to models of earthquake production rate (Chang & Segall, , ; Dieterich, ; Segall & Lu, ). Over the past decade, there have been numerous contributions to hydromechanical modeling of induced seismicity, often combining two separate codes to sequentially solve for fluid flow and rock mechanics (Cappa & Rutqvist, , ; Jha & Juanes, ; Rutqvist et al, ).…”