52nd U.S. Rock Mechanics/Geomechanics Symposium,
2018. American Rock Mechanics Association
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ABSTRACT: The rapid growth of the North American shale gas industry has been made possible through technology advances in extended-reach horizontal drilling and multistage hydraulic fracture stimulations. However, the injection of large volumes of fluids during hydraulic fracturing have also raised concerns regarding related induced seismicity. Several recent empirical and numerical studies have investigated the effects of operational factors such as injection volume and rate on the magnitude distribution of induced seismicity events; studies on the influence of geological factors are more limited. A key geological factor is the influence of rock mass stiffness. Results are presented here investigating the effects of a stiffness contrast between adjacent formations on the magnitude distribution of induced seismicity events. A representative scenario based on the Montney play is modeled using a series of 3-D distinct-element simulations. Results show that triggered slip displacements across a fault that transects a stiffness contrast boundary are non-uniform, and that the larger slip displacements (and induced seismicity events) occur in the formation with the higher stiffness. This helps to explain observations of induced seismicity below the formation targeted by hydraulic fracturing.
In recent years, certain regions across Canada and the United States have experienced a significant increase in seismicity relative to historical baselines (Keranen et al. 2014; Ellsworth 2013; Farahbod, Kao, Walker, et al. 2015; Farahbod, Kao, Cassidy, et al. 2015). This increase has been linked to hydraulic fracturing and deep wastewater disposal wells associated with the development of new unconventional oil and gas resources (Horton 2012; BC Oil and Gas Commision 2012; BC Oil and Gas Commission 2014). The hydraulic fracturing process involves pumping fluids under high pressure into sections of a wellbore to generate fractures in order to increase the permeability and stimulated volume of the reservoir rock. At the same time, this injection of fluids into deep formations also serves to create localized increases in pore pressures, which in the presence of a critically stressed fault, can act to reduce the effective normal stresses acting on the fault, resulting in slip and induced seismicity. The generation of seismicity has raised public, industry, and regulator concerns in affected regions. For the most part, the events generated have very low magnitudes (< M3). However, there have been incidents of larger earthquakes (>M3) that in cases involving higher population densities and sensitive ground conditions have resulted in damage to infrastructure and property (Tagliabue 2013). To properly assess a targeted formation for induced seismicity hazard potential, it is important to study the factors that influence the triggering of fault slip and range of possible event magnitudes.
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