ABSTRACT: The brittle-ductile transition (BDT) in Berea sandstone is characterized by: (1) stress drops smaller than brittle tests, (2) a change from dilational to compactive behavior, (3) multiple, narrow cross-cutting cataclastic shear zones at about 35° to s1. Shear localization via narrow cataclastic zones in BDT exhibit: (4) pervasive matrix microfracturing in BDT as opposed to microfracture 'halo' around brittle shear fractures, and (5) strong microfracture orientation parallel to s1 in BDT away from narrow cataclastic zones instead of more random orientation associated with localized brittle failure. A decrease in the strain rate from 7.5 x 10-5sec.-1 by an order of magnitude changes the brittle-transitional boundary from about 80 MPa to about 50 MPa. The lower strain rate also shifts the effective confining pressure range for BDT towards lower confining pressures. These results suggest that the genesis of small faults or deformation bands in porous rocks may be related to BDT deformation.

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