HR: 0800h
AN: T41B-0586    [Abstracts]
TI: True Triaxial Strength and Brittle Fracture of the Granodiorite at the SAFOD Drillhole Wall, and the Potential for Estimating the Maximum Horizontal Principal Stress
AU: Lee, H
EM: Hikweon.Lee@bakeratlas.com
AF: Baker Atlas, 17015 Aldine Westfield Road, Houston, TX 77073-5101, United States
AU: * Haimson, B
EM: bhaimson@wisc.edu
AF: University of Wisconsin, 1509 University Avenue, Madison, WI 53706-1595, United States
AB: Salinian granodiorite core from the 1462-1470m segment of the SAFOD drillhole was used to derive its critical mechanical properties under true triaxial stress conditions, analyze shear localization and brittle fracture characteristics, and establish the strength criterion under dry conditions (Eos Trans. AGU, 87/52, Abstract T32C- 03). Here we report on a series of true triaxial tests on ‘unjacketed' specimens simulating stress conditions prevailing at the drillhole wall and responsible for borehole failure in the form of breakouts. Owing to numerous random cracks inherent in the core, only 11 rectangular prismatic specimens (19×19×38 mm3) were successfully tested, employing the University of Wisconsin polyaxial cell. The two larger principal stresses, σ₁ and σ₂, were transmitted through metal pistons, while σ₃ was applied by confining fluid pressure. Specimen sides facing σ₃ were left ‘unjacketed', i.e. in direct contact with the confining fluid, to simulate the condition of drilling-mud pressure applying the principal radial stress (σ₃) to the exposed borehole wall. The loading path called for first bringing σ₂ and σ₃ to preset levels and then increasing σ₁ at a constant strain rate (5x10^-6/sec) until brittle failure occurred. Invariably, failure occurred at σ₁ levels that were only about half as high as those in previously tested dry samples under the same σ₂ and σ₃ magnitudes. Instead of a shear fracture, or fault, steeply inclined in the direction of σ₃, as previously observed in the dry specimens, brittle failure took the form of a localized cluster of through-going extensile cracks parallel and adjacent to the faces subjected to σ₃. Since failure occurred at σ₁ values close to those at dilatancy onset in dry specimens, we infer that as soon as microcracks reopened, confining fluid rushed into those daylighting at the σ₃ faces and extended them along a path of least resistance, i.e. along a plane normal to σ₃. Thus brittle failure under drillhole wall conditions is drastically different from that conventionally expected, but is compatible with breakout formation mechanism in granite (Haimson, Int. J. Rock Mech., 2007). All the ‘unjacketed' true triaxial strength data can be fitted by a simple function in the octahedral shear stress versus octahedral normal stress domain, yielding a Nadai-type true triaxial strength criterion. The criterion can be used in conjunction with breakouts that have been located within the cored zone to yield the maximum horizontal in situ stress σ_H when the other two principal stress are known. Assuming that the state of stress at breakout-drillhole intersections (located for example by BHTV logging) is sufficient to bring about brittle failure (Vernik and Zoback, 1992), one can substitute the known principal stresses there (obtained from the Kirsch solution) for the corresponding values in the criterion. The in situ σ_v is given by the overburden density, σ_h is typically obtained from hydrofrac shut-in pressures, breakout width is extracted from BHTV logs, borehole fluid pressure is a function of its density, and the Poisson's ratio is obtained from mechanical lab testing. The only unknown, σ_H, is thus readily computed. An actual computation was not carried out because data on hydrofrac pressures and breakout dimensions were not available at the time of this submission.
DE: 5104 Fracture and flow
DE: 8045 Role of fluids
DE: 8164 Stresses: crust and lithosphere
DE: 8168 Stresses: general
SC: Tectonophysics [T]
MN: 2007 Fall Meeting