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Itasca, United States

Han Y.,Itasca Consulting Group Inc. | Damjanac B.,Itasca Consulting Group Inc. | Nagel N.,Itasca Houston Inc.
46th US Rock Mechanics / Geomechanics Symposium 2012 | Year: 2012

In this paper, we present a microscopic numerical system for simulating the interaction between the natural fractures and hydraulic fracturing. In this system, the intact rock mass is represented by bonded particle model in Particle Flow Code (PFC); the pre-existing natural fractures are simulated by smooth-joint contact model; the fluid flow in the porous media and fracture and the buildup and dissipation of pore pressure are modeled by the pipe flow over the network connecting all the pores; and, the hydraulic fracturing is treated as dynamic mechanical and hydraulic pressure boundary or interior conditions along the hydraulic fractures. In our model, the hydro-mechanical response of the porous matrix, the fluid flow in the pore channels, the coupling of the matrix volumetric deformation and the pore fluid dissipation, and the reactivation and further development of natural fractures, are modeled naturally and realistically in a physically correct manner. After each component of the system is described in great details, an example is provided to illustrate the complete procedure of applying the developed system in solving practical problems. Copyright 2012 ARMA, American Rock Mechanics Association. Source

Li B.,Wenzhou University | Li B.,Guilin University of Technology | Guo L.,Wenzhou University | Zhang F.-S.,Itasca Houston Inc.
Journal of Central South University | Year: 2014

A three-dimensional numerical torsion shear test is presented on hollow cylinder specimen which is performed on a spherical assemblage with fixed principal stress axes using the discrete element code PFC3D. Stack wall technique boundary conditions are employed and optimized to reasonably capture the microstructure evolution. Parametric studies are conducted in terms of the ratio κ, normal and shear stiffness of particles, wall stiffness and friction coefficients. Afterwards, in comparison with physical test, numerical results for a fixed principal stress angle (α=45°) are presented. The results show that the numerical test could capture the macro-micro mechanical behavior of the spherical particle assembly. The evolution of the coordination number demonstrates that particles in shear banding undergo remarkable decrease. The effects of localization on specimens illustrate that global stress and strain recorded from a hollow cylinder apparatus could not represent the localized response. The shearing band initiation and evolution from porosity and shear rate are visualized by contour lines in different shear strains. © 2014 Central South University Press and Springer-Verlag Berlin Heidelberg. Source

Kallu R.R.,University of Nevada, Reno | Keffeler E.R.,RESPEC Consulting and Services | Watters R.J.,University of Nevada, Reno | Agharazi A.,Itasca Houston Inc.
International Journal of Mining Science and Technology | Year: 2015

Estimating weak rock mass modulus has historically proven difficult although this mechanical property is an important input to many types of geotechnical analyses. An empirical database of weak rock mass modulus with associated detailed geotechnical parameters was assembled from plate loading tests performed at underground mines in Nevada, the Bakhtiary Dam project, and Portugues Dam project. The database was used to assess the accuracy of published single-variate models and to develop a multivariate model for predicting in-situ weak rock mass modulus when limited geotechnical data are available. Only two of the published models were adequate for predicting modulus of weak rock masses over limited ranges of alteration intensities, and none of the models provided good estimates of modulus over a range of geotechnical properties. In light of this shortcoming, a multivariate model was developed from the weak rock mass modulus dataset, and the new model is exponential in form and has the following independent variables: (1) average block size or joint spacing, (2) field estimated rock strength, (3) discontinuity roughness, and (4) discontinuity infilling hardness. The multivariate model provided better estimates of modulus for both hard-blocky rock masses and intensely-altered rock masses. © 2015. Source

Li B.,Wenzhou University | Zhang F.,Itasca Houston Inc. | Gutierrez M.,Khalifa University | Gutierrez M.,Colorado School of Mines
Acta Geotechnica | Year: 2015

This paper presents results of three-dimensional simulations of the hollow cylindrical torsional shear test using the discrete element method. Three typical stress states that can be applied in the hollow cylindrical apparatus (HCA), i.e. triaxial, torsional compression and pure torsional, are examined in terms of the distributions of stresses and strains in the HCA sample. The initiation and propagation of the shear bands in the sample were characterized by porosity and shear strain rate distributions in the sample. The results show that the shear strain rate contour is a better indicator for shear band development than the porosity contours. It is demonstrated that the stresses and strains measured in the shear zone are significantly different from the boundary measurements and the average values used in HCA testing. Initially, the peak strength measured from the boundary forces was found to be slightly lower than that measured in the shear band. Subsequently, due to the formation of shear band, the stress ratio from boundary forces decreased significantly especially when the major principal stress is oriented 30° and 45° from the vertical. The evolutions of porosity, coordination number and particle rotation at different locations in the sample were also monitored. Finally, the appropriateness of the HCA is evaluated in comparison with previously published data. © 2014, Springer-Verlag Berlin Heidelberg. Source

Nagel N.B.,Itasca Houston Inc. | Sanchez-Nagel M.,Itasca Houston Inc.
Proceedings - SPE Annual Technical Conference and Exhibition | Year: 2011

In horizontal well shale completions, multiple stages, each often with multiple clusters, are used to provide sufficient stimulated area to make an economic well. Each created hydraulic fracture alters the stress field around it. When hydraulic fractures are placed close enough together, the well-known stress shadow effect occurs in which subsequent fractures are affected by the stress field from the previous fractures. The effects include higher net pressures, smaller fracture widths and changes in the associated complexity of the stimulation. The level of microseismicity is also altered by stress shadow effects. For example, it is commonly seen that the number of microseismic events is significantly reduced from the toe to the heal of the well, where the first frac stage is conducted at the toe of the well. In this paper, we present the results of a numerical evaluation of the effect of multiple hydraulic fractures on stress shadowing as a function of fracture spacing, shale rock mechanical properties, and the in-situ stress ratio. In addition, utilizing the inherent ability of discrete element models to evaluate shear and tensile failure along fracture surfaces, shear failure, as a proxy for microseismicity, is evaluated as a function of fracture-induced stress and stress shadowing. The results of the study provide a means to optimize shale completions by understanding the effect of stress ratio, rock mechanical parameters, and hydraulic fracture spacing on the stress shadow effect and the potential for changing fracture complexity. Copyright 2011, Society of Petroleum Engineers. Source

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