HR: 0800h
AN: T41F-1307    [Abstracts]
TI: Borehole Breakouts and Compaction Bands in a Medium Porosity, Quartz-rich Sandstone
AU: Lee, H
EM: hilee@wisc.edu
AF: University of Wisconsin, 1509 University Avenue, Madison, WI 53706-1595 United States
AU: * Haimson, B C
EM: bhaimson@wisc.edu
AF: University of Wisconsin, 1509 University Avenue, Madison, WI 53706-1595 United States
AB: We conducted laboratory-drilling experiments in Coconino sandstone in which vertical boreholes were drilled into prismatic samples subjected to true-triaxial far-field stresses (\sigma$_{H}$ $>$ \sigma$_{v}$ $>$ \sigma$_{h}$) simulating field conditions. This early Permian aeolian sandstone comes from Arizona, and was selected because of its medium porosity (16-20%) and its rounded quartz grains (96%, with minor amounts of feldspar). Its fine-grains (0.1 mm) are well sorted and bonded through weak grain-contact suturing and some quartz overgrowth. Boreholes drilled under sufficiently high far-field stresses developed narrow and tabular fracture-like breakouts (constant width of 10 grain diameters) extending along the \sigma$_{h}$-springline and perpendicular to \sigma$_{H}$ direction. This behavior is consistent with other sandstones previously tested, which possessed similar composition and texture but had porosities in excess of 20%. However, breakout length in the Coconino was dependent on porosity. The higher porosity variety (20%) produced much longer breakouts than the 16% samples. SEM analysis of the damage zone ahead of the breakout tip revealed a narrow compaction band, having the same width as the breakout, localized along the \sigma$_{h}$-springline where compressive stress concentration is the highest. Within the band, grains only a few millimeters away from the breakout tip are already mostly intact, but debonded, with the sutured contact replaced by visible microcracks. The existence of a compaction zone is reflected by the reduction in the 2D porosity measured within the band. The initial failure mode leading to the localized compaction appears to be non-dilatant, since it involves no extensile microcracking. The zone of cracked and crushed grains immediately ahead of the breakout tip is a secondary failure zone within the compaction band as a result of excessive stress concentration there. Grain fragments and debonded grains are removed from the compaction band by the circulating drilling fluid, resulting in a long tabular fracture-like breakout. Our conclusion, based on testing a large number of sandstones, is that the failure mode leading to fracture-like breakouts is likely to occur in sandstones regardless of their porosity (at least within the tested 15-28%) and grain size (0.1-0.5 mm), as long as grains are rounded and overwhelmingly quartz, and bonding is through weak sutures. Such grain contacts are readily broken under sufficient normal stress owing to low fracture toughness along sutured surfaces. Debonded grains are then compacted through their relative movement into adjacent pores, creating a compaction band and facilitating fracture-like breakout formation.
DE: 8010 Fractures and faults
DE: 5104 Fracture and flow
DE: 5112 Microstructure
DE: 5114 Permeability and porosity
DE: 5194 Instruments and techniques
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting