Issen, Kathleen A.1, Vennela Challa1
(1) Clarkson University, Potsdam, NY
ABSTRACT: Theoretical Modeling of Localized Deformation Structures in Porous Sandstone
Porous rocks in both field and laboratory settings are frequently observed to fail by localized deformation. Shear bands, in which shear deformation is often accompanied by either dilatant or compactant deformation, are the structures most extensively observed and described. However, two other types of localized deformation structures were identified in recent field and laboratory examinations of high porosity sandstone. Compaction bands, consisting of pure compressional deformation, form perpendicular to maximum compression, while dilation bands, consisting of pure dilational deformation, form perpendicular to minimum compression/maximum extension. These bands are of significant interest since they affect the porosity and permeability of the original rock, consequently affecting fluid flow.
Rudnicki and Rice model of bifurcation of strain from homogeneous deformation to localized deformation can be used to analyze band formation in rock at scales varying from laboratory specimens to field structures. This approach reveals that strain types from pure compaction to compacting shear to dilating shear to pure dilation are theoretically possible. A two yield surface model combined with the bifurcation approach, in which the first yield surface corresponds to a dilatant, frictional mechanism, while the cap corresponds to a compactant mechanism, successfully describes strain localization in high porosity sandstone under axisymmetric compression. The predicted band angles agree with experimentally observed compaction bands and shear bands. Conditions for dilation band formation under axisymmetric extension are similarly determined using the two yield surface model suggesting that dilation band formation could be a common deformation mode in high porosity sandstone.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.