Modeling Induced Seismicity: Inter-Seismic Quasi-Static Triggering in a Discretely Fractured Poroelastic Medium

Jin, Lei (Stanford University) | Zoback, Mark (Stanford University)

OnePetro 

Fluid injection into the subsurface perturbs the pore pressure and alters the effective stress quasi-statically, inducing seismicity on fractures of certain orientations (we hereinafter do not distinguish between a fracture and a fault in this study). This process is traditionally considered as a decoupled hydroshear process: the effective normal stress on a fracture simply decreases by the amount of fluid overpressure, whereas the shear stress remains unchanged (e.g., Byerlee, 1978; Scuderi & Collettini, 2016; Mukuhira et al, 2016), resulting in a direct increase in the Coulomb stress, which, when driven from negative to zero, signifies the occurrence of seismicity. Such a decoupled mechanism remains as the basis of some prevalent statistical models of induced seismicity in a permeable porous medium (e.g., Shapiro et al., 2005; Rothert & Shapiro, 2007). In this class of models, a statistically random critical pore pressure is used as a proxy of the frictional strength of a preexisting fracture and the pore pressure evolution is governed by simple linear fluid diffusion; the modeled spatialtemporal distribution of seismicity, however, is often inconsistent with observations. As a remedy, some nonlinear diffusion models have been developed by adding a pressure-dependent diffusivity (Hummel & Shapiro, 2012; Johann et al., 2016; Carcione et al., 2018). The diffusion-based seismicity models can be further extended by incorporating, e.g., random stress heterogeneity (Goertz-Allmann & Wiemer, 2012), fractures following distributions derived from field observations (Verdon et al., 2015), and even empirical seismic emission criteria for generating synthetic seismograms (Carcione et al., 2015). This decoupled mechanism also underlies some studies that invert for distributions of permeability (Tarrahi & Jafarpour, 2012) and pore pressure (Terakawa et al., 2012; Terakawa, 2014) from induced seismicity data. However, the decoupled mechanism inherently cannot explain the remoting triggering of seismicity in areas not subjected to pressure perturbation (Stark & Davis, 1996; Megies & Wassermann, 2014; Yeck et al., 2016); it also directly contradicts the commonly observed depletioninduced faulting (Zoback & Zinke, 2002).

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