HR: 0830h
AN: T41D-0257    [PDF]
TI: Effect of Confining Pressure on Compaction Localization in Notched Samples of Bentheim Sandstone: Experimental Observations and Finite Element Modeling
AU: * Vajdova, V
EM: vvajdova@ic.sunysb.edu
AF: SUNY Stony Brook, Department of Geosciences, SUNY SB, Stony Brook, NY 11794-2100 United States
AU: Wong, T
EM: Teng-fong.Wong@stonybrook.edu
AF: SUNY Stony Brook, Department of Geosciences, SUNY SB, Stony Brook, NY 11794-2100 United States
AU: Challa, V
EM: challav@clarkson.edu
AF: Clarkson University, Mechanical and Aeronautical Engineering, Potsdam, NY 13699-5725 United States
AU: Issen, K A
EM: issenka@clarkson.edu
AF: Clarkson University, Mechanical and Aeronautical Engineering, Potsdam, NY 13699-5725 United States
AB: In tectonic settings the coupled development of compaction and strain localization may significantly impact the stress field, strain partitioning and fluid flow, and therefore it is desirable to have a better understanding of how such localization develops at various burial depths. Field studies indicate that compaction localization may develop due to structural and stress heterogeneity. In a previous laboratory study to investigate these phenomena a stress concentration was introduced by a V-shaped circumferential notch in a cylindrical sample of Bentheim sandstone and conventional triaxial experiments were conducted at the confining pressure of 300 MPa. Our acoustic emission and microstructure data indicated that discrete compaction bands initiated from the notch tips and propagated by sequential increments as "anti-cracks". The transverse propagation of a compaction band was inferred to be faster than the axial displacement rate by 2 orders of magnitude. Energy dissipated for compaction band formation was estimated to be comparable to the shear fracture energy for shear band propagation. Guided by experimental observations, a finite element analysis was conducted to simulate the initiation and evolution of compaction localization. The ABAQUS model was developed using a Drucker-Prager with cap constitutive model, and the numerical simulations confirm that a stress concentration exists at the notch causing a stress state favoring an axially compacted zone to extend perpendicular to the maximum compressive stress. To clarify the pressure effect we conducted additional experiments at confining pressures of 250 and 350 MPa. Our mechanical data show that the critical stress for the initiation of a compaction band from a notch tip decreased with increasing confining pressure, similar to the yield stress for an unnotched sample that maps out a cap with negative slope in the stress space. Differential stress vs. axial strain plots from numerical simulations support the experimental observation that the yield stress shows negative pressure dependence. Preliminary microstructural observations indicate a similar failure mode for notched samples at the three different confining pressures. Synthesis of the experimental and numerical results can provide useful constraints on the stress singularity at the notch tip and how it influences development of compaction localization.
DE: 5104 Fracture and flow
DE: 5112 Microstructure
DE: 5114 Permeability and porosity
DE: 8020 Mechanics
DE: 8030 Microstructures
SC: Tectonophysics [T]
MN: 2003 Fall Meeting