Rice University

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Thesis Defense

Graduate and Postdoctoral Studies
Earth Science

Speaker: Tamunoisoala LongJohn
Masters Candidate

Microstructural Evolution of Stress and Porosity During the formation of Brittle Shear Fractures: A Discrete Element Model (DEM) Study

Thursday, May 4, 2017
1:30 PM  to 4:30 PM

327  Keith-Wiess Geological Laboratories


Brittle fracturing in rocks is a progressive process involving changes in stress, porosity, and strain. An understanding of how these properties change is essential for efficiency during drilling and producing of wells, and in hazardous waste disposal. However, it is often difficult and expensive to make observations of microstructural changes from laboratory and field measurements only. This study uses the discrete element method (DEM) to show that fractures correspond to zones of low particle abundance, high porosity, generally low stresses, and highly localized dilation and distortional strain. We probed the internal conditions of a numerical analog for sandstone during brittle deformation by conducting numerical biaxial experiments at different confining pressures. When biaxial compression begins, the bulk porosity is decreasing and differential stress is increasing. Internally, the stresses within the granular body are relatively uniform with anastomosing stress chains. At the yield stress, multiple dilational bands with a conjugate orientation start to open for relatively low confining pressures. These bands have a length of 7.5 mm and are oriented at an angle of ~35° from the maximum principal stress (?1). At the peak stress, high magnitude stress chains localize adjacent to the position of the developing through-going shear band and distortion is now evident. Finally, prior to stable sliding, the through-going shear fracture is fully developed. This fracture follows only a few individual dilational bands that have become dominant while the other dilational bands remain small and branch off the dominant fracture. Also, high stresses are transmitted across the fracture where porosity is low through a network/chain of particles in contact. With increasing confining pressure, distortion is favored over dilation as the mode of deformation, the number of stress chains across a shear fracture decreases while the thickness increases, and the steepness of the fracture increases.

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