HR: 0830h
AN: T41D-0249    [PDF]
TI: Influence of Intermediate Stress on Yielding of Berea Sandstone
AU: * Jourine, S
EM: s0j0185@spindletop.tamu.edu
AF: Texas A&M University, Department of Petroleum Engineering, College Station, TX 77843 United States
AU: Karner, S L
EM: karner@geo.tamu.edu
AF: Texas A&M University, Center for Tectonophysics Geology and Geophysics Department, College Station, TX 77843 United States
AU: Kronenberg, A K
EM: kronenberg@geo.tamu.edu
AF: Texas A&M University, Center for Tectonophysics Geology and Geophysics Department, College Station, TX 77843 United States
AU: Chester, F M
EM: chesterf@geo.tamu.edu
AF: Texas A&M University, Center for Tectonophysics Geology and Geophysics Department, College Station, TX 77843 United States
AB: The onset of brittle failure of Berea sandstone has been investigated under varying stress states by subjecting solid and hollow cylindrical samples to confining pressure P$_{c}$ (to 120 MPa) and axial stress $\sigma$$_{z}$ (to 260 MPa) in a conventional triaxial deformation apparatus. Inelastic yielding of solid and hollow samples is marked by cascading acoustic emissions and axial displacements that depart from the initial linear elastic response. For hollow samples, uniaxial ($\sigma$$_{z}$ $>$ $\sigma$$_{\Theta}$ = $\sigma$$_{r}$ = 0) and biaxial ($\sigma$$_{z}$ $>$ $\sigma$$_{\Theta}$ $>$ $\sigma$$_{r}$ = 0 or $\sigma$$_{\Theta}$ $>$ $\sigma$$_{z}$ $>$ $\sigma$$_{r}$ = 0) stress states are obtained with deviators that (1) are maximum at the inner wall, (2) decay in a predictable manner with increasing radius prior to yielding, and (3) are insensitive to frictional boundary conditions at piston-sample contacts throughout much of the sample. X-ray CT scans of deformed samples reveal different modes of failure associated with the stress states imposed. Zones of anomalous X-ray density within hollow samples that mark deformed sandstone are localized near the inner wall, nearly midway between sample ends, and inner walls have lost their cylindrical geometry. Axial cavities of hollow samples become elliptical when axial stress is the intermediate principal stress ($\sigma$$_{z}$ = $\sigma$$_{2}$), with geometries resembling wellbore break-outs used to infer in-situ horizontal stresses in scientific drilling studies, while toroidal spalls are developed when the tangential stress is the intermediate stress ($\sigma$$_{\Theta}$ = $\sigma$$_{2}$). Our results for solid specimens under conventional triaxial conditions ($\sigma$$_{z}$ $>$ $\sigma$$_{r}$ = $\sigma$$_{\Theta}$ $>$ 0) are in agreement with results reported for Berea sandstone. However, the results for hollow specimens require a yield criterion that includes intermediate stress $\sigma$$_{2}$. Yield criteria that fit our combined data may be expressed in terms of first and second invariants of stress I$_{1}$ and J$_{2}$; for example, using the modified Wiebols and Cook (1968) criterion, J$_{2}$$^{1/2}$ = A + B I$_{1}$ + C I$_{1}$$^{2}$, data for Berea sandstone are well matched by parameters A = 16.5 MPa, B = 1.05, and C = -0.0013 MPa$^{-1}$.
DE: 5104 Fracture and flow
DE: 8010 Fractures and faults
DE: 8020 Mechanics
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2003 Fall Meeting