HR: 1340h
AN: T33C-1501    [Abstracts]
TI: Discrete Compaction Bands in Porous Sandstone: Stress Scaling and Energetics of Propagation
AU: * Baud, P
EM: pbaud@eost.u-strasbg.fr
AF: Institut de Physique du Globe (CNRS/ULP), 5 rue Rene Descartes, Strasbourg Cedex, 67084, France
AU: Tembe, S
EM: stembe@ic.sunysb.edu
AF: SUNY Stony Brook, Department of Geosciences, Stony Brook, NY 11794-2100, United States
AU: Wong, T
EM: teng-fong.wong@sunysb.edu
AF: SUNY Stony Brook, Department of Geosciences, Stony Brook, NY 11794-2100, United States
AB: Compaction bands are a kinematic end-member of strain localization in porous rock, forming thin tabular structures normal to the maximum compressive stress with negligible shear offset. They are associated with significant porosity loss and permeability reduction. In the field where they were first identified, such localized features typically have thickness on the order of 1-10 mm and trace lengths on the order of 1-10 m. Recent investigations in the laboratory have detailed the development of comparable discrete compaction bands in several sandstones with porosities ranging from 23% to 25% at stress states in the transitional regime from brittle faulting to cataclastic. In many respects the geometric attributes of discrete compaction bands as observed in the laboratory are qualitatively similar to compaction bands that have been documented in the field. Nevertheless, there are at least two discrepancies between the laboratory and field structures. First, the stress level inferred for compaction band development in the field are lower than that required in the laboratory. Second, the dimensions as well as the damage intensity measured in the field are also appreciably lower than those in the laboratory deformed samples. If the laboratory measurements can be realistically extrapolated to the geologic setting, the mechanical basis for the growth of the band, stress state and damage intensity should first be established. In this study we address these questions by compiling field observations and laboratory data on the stresses associated with the compaction band failure mode and the geometric attributes of the bands. To complement existing laboratory data on discrete compaction bands in the Bentheim sandstone, we conducted a suite of experiments on the Diemelstadt and Bleurswiller arkosic sandstones. The propagation and geometric attributes of the bands produced in the deformed samples were characterized using acoustic emission data, X- ray CT images and conventional microscopy techniques. Synthesis of field and laboratory data for five sandstones over length scales of 10^-3-10 m shows that the thickness and length of compaction bands seem to obey a quadratic scaling relation where the thickness scales approximately with the square root of the band length. In a recent analysis Rudnicki (2007) formulated several models for compaction band propagation, and in particular he proposed a combined anti-crack/dislocation model which would indeed result in the quadratic scaling that we identified in the field and laboratory data. On the basis of this fracture mechanics model, we explore the mechanical interpretation for the broad range of stresses associated with compaction band propagation on the field and laboratory settings and obtained a scaling relation in which the stress level is inversely proportional to compaction band thickness. Thus for the laboratory bands, higher propagation stresses are predicted, as is the potential for significant damage, while for compaction bands in the field, lower stresses and less damage are expected. Together the laboratory and field data constrain the critical strain energy release rate in the model to be on the order of 2-40 kJ/m², comparable with lab measurements of the compaction energy.
DE: 5114 Permeability and porosity
DE: 8010 Fractures and faults
DE: 8020 Mechanics, theory, and modeling
DE: 8030 Microstructures
DE: 8168 Stresses: general
SC: Tectonophysics [T]
MN: 2007 Fall Meeting