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

We present a quasi-static and hydro-mechanical fully coupled approach to the modeling of inter-seismic triggering of seismicity in an arbitrary fractured and fluid-saturated poroelastic solid. The discrete fracture network (DFN) we consider is hybrid and at a dual-scale consisting of large-scale deterministic fractures and small-scale stochastic fractures following certain distributions. The fracture-poro-elasticity fully coupled modeling is carried out using our Jin & Zoback (2017) nonlinear computational model by explicitly resolving the deterministic fractures only. This provides inputs for the subsequent seismicity modeling that accounts for the entire hybrid dual-scale DFN. A new stress updating algorithm is developed accounting for the competing poroelastic stress compensation and seismicity-induced stress loss on fractures. Our model can therefore naturally produce multiple cycles of seismicity triggering. As an example, we perform a numerical experiment and generate a synthetic catalog of induced seismic events, and then analyze (1) seismicity distribution in relation to the fluid pressure, poroelastic stress and fracture distribution, (2) event type, (3) spatial-temporal characteristics, (4) stress history, (5) seismic source parameters and their characteristics and (6) activated DFN and permeability changes. Our modeling provides explanations to some curious observations in real data and can serve as a physics-based tool for predicting induced seismicity in complex geological media. 1998; Altmann et al., 2014; Segall & Lu, 2015; Jin & Zoback, 2015a) and numerical solutions (Murphy et al., 2013; Chang & Segall, 2016a; Chang & Segall, 2016b; Chang & Segall, 2017; Fan et al., 2016; Deng et al., 2016; Zbinden et al., 2017) have been proposed, providing poromechanics-based models of induced seismicity. At a smaller scale, numerical simulation of fluid-induced microseismicity, typically motivated by applications to stimulation of hydrocarbon and geothermal reservoirs, have been reported (e.g., Maillot et al., 1999; Baisch et al., 2010; Wassing et al., 2014; Zhao & Young, 2011; Yoon et al., 2014; Raziperchikolaee et al., 2014; Riffault et al., 2016). Irrespective of the scale of interest, these studies substantiate the critical role of poroelastic coupling in triggering seismicity. Despite these overwhelmingly strong evidences, some debates persist, mainly from those who advocate the simple diffusion-only models (Johann et al., 2016). They claim that their diffusion models approximate poroelastic models if the Biot coefficient α is small; they also argue that when α is less than 0.3, it is the pore pressure rather than the poroelastic stress that dominates the hydroshear on fractures; they even further question the Segall (2015, 2016a) poroelastic models in which α is greater than 0.3, and simply hypothesize that for nearly impermeable rocks, the α should also be negligible. However, one must realize that α is a measurement of the rock solid’s susceptibility to the influence of the fluid and vice versa; it is not a property directly related to the permeability of the rock itself. As a matter of fact, laboratory experiments show that α of unconventional reservoir rocks is indeed primarily between 0.3 and 0.9 (e.g., Ma & Zoback, 2017). Fluid-solid coupling is undoubtedly important in triggering seismicity. However, the aforementioned poroelastic models only include fractures limited in both number and distribution, therefore, the role of fractures cannot be sufficiently explored; the fractures are also explicitly represented as entities following the same fluid and solid rheologies as in the rest of the domain, therefore, the medium is effectively ‘porous’ only. Such simplifications may come with certain consequences. There have been studies suggesting that accounting for both poroelastic coupling and an arbitrary discrete fracture network (DFN) permitted to have a separate set of fluid and solid constitutive behaviors can significantly impact the outcome of modeled induced seismicity (Jin & Zoback, 2016a; Jin & Zoback, 2016b; Jin & Zoback, 2018c). Pertaining to this issue, some studies resolve very regularly distributed fractures (e.g., Safari & Ghassemi, 2016); others attempt to include an arbitrary DFN, among which, e.g., some focus on the fluid pressure and solid deformation only within fractures but not the matrix (Farmahini-Farahani & Ghassemi, 2016), some consider coupling only upon the occurrence of seismicity (Bruel, 2007). None of these models produces repeating events frequently detected in fluid-induced seismicity catalogs (e.g., Baisch & Harjes, 2003; Moriya et al., 2003; Deichmann et al., 2014; Duverger et al, 2015). To date, a model of induced seismicity considers fluidsolid full coupling in an arbitrarily fractured medium and at the same time capable of producing seismic source parameters and repeating seismic events is generally lacking. In this study, within the framework of singlephase poromechanics of arbitrarily fractured porous media developed by Jin & Zoback (2017), we first propose a coupled fracture-poro-elasticity model of induced seismicity in a fluid-saturated and discretely fractured porous solid. The model focuses on the interseismic triggering of seismicity and do not explicitly model the co-seismic rupture process. Seismicity-induced stress redistribution on a fracture is accomplished via a novel fracture stress updating algorithm. Further, a stochastic perturbation is introduced to the shear stress drop on a fracture induced to slip to allow for realistic seismic source characteristics. The model is also naturally capable of producing repeating seismic events. The details are described below.