Time-dependent Brittle Creep in Rock: The Influence of Confining Pressure and Temperature
Abstract
The characterization of time-dependent brittle creep deformation is fundamental to understanding the long-term evolution and dynamics of the Earth’s crust. The presence of water promotes environment-dependent stress corrosion cracking that allows rock to deform at a constant stress below its short-term failure stress over extended periods of time. Here we report illustrative results from an experimental study of the influence of an elevated temperature on time-dependent brittle creep in water-saturated samples of Bentheim sandstone (initial porosity, Φ = 23%), Darley Dale sandstone (Φ = 13%) and Crab Orchard sandstones (Φ = 4%). We present data obtained from both conventional, constant stress creep experiments and from stress-stepping creep experiments performed under effective confining pressures in the range 10 MPa to 50 MPa and at temperatures from 20° to 75°C. Deformation was monitored throughout each experiment by measuring simultaneously three proxies for evolving crack damage: (1) axial strain, (2) porosity change and (3) the output of acoustic emission (AE) energy, all as functions of time. Results from conventional creep experiments demonstrate that the primary control on creep strain rate and time-to-failure is the applied differential stress. They also suggest the existence of a critical level of crack damage beyond which deformation accelerates and ultimately leads to sample failure on a localized fault. The influence of effective confining pressure was investigated in stress-stepping experiments. In addition to the expected mechanical influence of elevated effective pressure, our results also demonstrate that stress corrosion cracking is inhibited at higher effective confining pressures, with creep strain rates reduced by about 3 orders of magnitude as effective confining pressure is increased from 10 to 50MPa. We have used the same technique to investigate the influence of an elevated temperature. Our results show that, for the same applied differential stress and confining pressure, creep strain rates are increased by between 2 and 3 orders of magnitude, as sample temperature is increased from 20° to 75°C. Extrapolating the opposing influences of increasing temperature and pressure suggests that temperature is the dominant influence on the evolution of creep deformation. Therefore creep strain rates are likely to increase with depth in the shallow crust.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.T41A1997M
- Keywords:
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- 5102 PHYSICAL PROPERTIES OF ROCKS / Acoustic properties;
- 5104 PHYSICAL PROPERTIES OF ROCKS / Fracture and flow;
- 8045 STRUCTURAL GEOLOGY / Role of fluids;
- 8118 TECTONOPHYSICS / Dynamics and mechanics of faulting