@conference {Nagel2012a,
	title = {{Hydraulic fracturing optimization for unconventional reservoirs - The critical role of the mechanical properties of the natural fracture network}},
	booktitle = {Society of Petroleum Engineers - SPE Canadian Unconventional Resources Conference 2012, CURC 2012},
	volume = {1},
	year = {2012},
	pages = {227{\textendash}237},
	abstract = {The success of many shale plays depends on the optimal stimulation of natural fractures, and the characterization of the natural fracture systems is a key issue often leading to the construction of a discrete fracture network (DFN). The DFN chiefly consists of fracture spacing, fracture dip, and dip direction from numerous sources, and, in some cases, is matched to well test data allowing for the determination of hydraulic properties. However, a commonly missing component of natural fracture characterization, and the component most important to evaluating the coupled hydro-mechanical behavior of the fractures during a hydraulic fracture stimulation, is the evaluation of the mechanical behavior of the fractures. The important mechanical parameters of the fractures include: a) elastic properties such as shear and normal stiffness, which relate changes in pore pressure to changes in aperture; b) strength parameters such as cohesion and friction angle, which define when a fracture may shear and open; c) dilational properties, which relate fracture opening to shear slippage; d) fracture toughness, which determines the pressure required to extend a fracture; and e) initial aperture. In this paper, using a new, state-of-the-art, fully-coupled, 3D distinct element hydraulic fracturing simulator, mechanical fracture parameters were evaluated in a parametric study in order to determine their impact on the effectiveness of hydraulic fracture stimulations. Copyright 2012, Society of Petroleum Engineers.},
	isbn = {9781622764884},
	url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-84875645702\&partnerID=tZOtx3y1},
	author = {Nagel, Neal B. and Garcia, Xavier and Lee, Byungtark and Sanchez-Nagel, Marisela}
}
@conference {Nagel2011,
	title = {{Discrete element hydraulic fracture modeling - Evaluating changes in natural fracture aperture and transmissivity}},
	booktitle = {Society of Petroleum Engineers - Canadian Unconventional Resources Conference 2011, CURC 2011},
	volume = {2},
	year = {2011},
	pages = {1350{\textendash}1363},
	abstract = {Previous works have presented the results of successful simulations of fluid injection into naturally fractured shale using a Discrete Element Model (DEM). The simulations included coupled fluid flow-deformation analysis, failure type and extent calculations, as well as a series of parametric analyses. The parameters investigated included: 1) injection rate and its effect on the overall fracturing results, and 2) fluid viscosity, which had a significant influence on the ratio of tensile (mode 1) failure versus shear failure. With the huge growth in the stimulation of naturally fractured formations such as fractured shales, it is clear that the industry needs new hydraulic fracturing simulation tools beyond the limits imposed by pseudo3D fracturing models. DEMs, in which both matrix block behavior and fracture behavior are explicitly modeled, offer one option for the specific modeling of hydraulic fracture creation and growth in a naturally fractured formation without, for example, the assumption of bi-planar fracture growth. In this paper, we extend the previous works to quantify, for fractured shale gas plays, the effect of stress orientation, fluid viscosity, and rock mechanical properties in terms of changes in fracture aperture and transmissivity. Changes in fracture transmissivity directly correlate with improvements in well productivity - the primary goal of the stimulation. The results of the study provide a means to improve shale completions by understanding the effects of the DFN orientation relative to the stress field, fluid viscosity, and rock mechanical properties on changes in fracture aperture, fracture transmissivity, and formation effective permeability, which directly relate to well productivity. Copyright 2011, Society of Petroleum Engineers.},
	isbn = {9781618394217},
	url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-84860594665\&partnerID=tZOtx3y1},
	author = {Nagel, Neal and Damjanac, Branko and Garcia, Xavier and Sanchez-Nagel, Marisela}
}
