HR: 0800h
AN: T21B-0485    [Abstracts]
TI: Using Borehole Breakouts in Quartz-Rich St. Meinrad Sandstone as Indicators of In-Situ Stress Magnitudes, and the Effect of Drilling Variables
AU: * Haimson, B C
EM: bhaimson@wisc.edu
AF: University of Wisconsin, 1509 University Avenue, Madison, WI 53706-1595
AB: Laboratory drilling experiments were conducted to determine the effects of several drilling variables on the development of breakouts in the quartz-rich, 28% porosity, fine-grained, St. Meinrad sandstone. Resulting breakouts were all fracture-like, i.e. tabular and very long and narrow throughout. Tests showed that breakout depth (or length) was directly proportional to the horizontal principal far-field stress differential, suggesting a potential use of this breakout dimension to estimate in situ stress magnitudes. However, drilling larger borehole diameters produced increasingly longer breakouts, suggesting that lab-based results need be scaled to fit field conditions. In addition, we infer that fracture-like breakouts could reach lengths of several meters in typical oil-field wellbores, and be potentially a source of sand production. Increasing the drilling fluid (water) flow-rate tended to create deeper breakouts under the same far-field stress conditions, while raising the drilling-bit penetration rate had the opposite effect. On the other hand, varying the type of drilling bits used had no discernable effect on breakout dimensions or shape. One important characteristic of the fracture-like breakouts in St. Meinrad sandstone is their near-constant width regardless of testing conditions, indicating no correlation with far-field stress, and suggesting that they result from the emptying of induced compaction bands. We conclude that breakouts in St. Meinrad sandstone could potentially be used as stress magnitude indicators if their depth could be measured by logging tools, if drilling variables were kept constant during field operations and were accurately simulated in the laboratory, and if scale effects were properly taken in consideration.
DE: 5104 Fracture and flow
DE: 8164 Stresses: crust and lithosphere
DE: 8166 Stresses: deep-seated
DE: 8168 Stresses: general
SC: Tectonophysics [T]
MN: Fall Meeting 2005