(1) University of Wisconsin, Madison, WI
ABSTRACT: Deformation and Failure in Two Classes of Porous Sandstones
We are investigating deformational and failure mechanics in porous sandstones by drilling vertical boreholes in rock blocks subjected to far-field true triaxial stresses (> > ). The high stress concentration along the springline emanating from the borehole wall brings about compressive failure under otherwise stable far-field stress conditions. We identify two major classes among the sandstones we have tested. Class I sandstones contain 50-70% quartz grains and considerable amounts of feldspar, are generally well cemented, have poor grain sphericity, and porosities that vary between 15 and 30%. They fail by developing dilatant intra- and trans-granular microcracks parallel to and borehole wall. The cracks create thin separated layers of split grains that are ejected in succession into the borehole, forming ‘dog ear’ shaped breakouts. No localized deformation ahead of the breakout is observed. Class II sandstones (porosity: 15-30%) contain mainly quartz (90-99%), and have rounded grains bonded primarily through suturing. Borehole failure in class II rocks culminates in long and narrow tabular breakouts that resemble fractures, which extend normal to direction. Microscale observations suggest that the failure process begins by localized deformation in the direction along the springline, facilitated by grain roundness and weak bonding, and resulting in grain compaction within a narrow band. Grains are mostly intact within this band, but some secondary grain splitting and crushing is also visible. This zone compares well with field-observed compaction bands. Borehole breakouts in class II sandstones appear to be created by the emptying of the loosened grains within the compaction bands with the aid of the circulating drilling fluid.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.