Jefferd, Mark;
(2020)
Sandstone Deformation under Crustal Hydrothermal Conditions.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
With increasing depth in the crust, the increase in effective pressure causes a transition in the deformation mechanism of sandstones from dilatant cracking and fracturing, to compactive grain crushing and a reduction in porosity. These two deformation regimes are referred to as the brittle and ductile deformation regimes of porous rocks, and while the pressure dependence on the brittle to ductile transition has been experimentally well constrained, there is a sparsity of experimental data exploring the effect that elevated temperature has on the brittle to ductile transition and on the ductile regime as a whole. In this study, a series of constant strain rate triaxial deformation experiments were performed on three water saturated sandstones at either room temperature or 150◦C. Increasing the deformation temperature to 150◦C caused a reduction of 8 to 18% in the differential yield stress in the brittle regime and a reduction of 9 to 37% in the ductile regime. The weakening measured at elevated temperature is attributed primarily to a reduction in fracture toughness but also with increased rates of subcritical cracking. Additional ductile creep experiments were performed on Bleursville sandstone at room temperature, 75◦C and 150◦C to probe the role of subcritical cracking in more detail. At a given creep stress, the creep strain rates were generally at least an of magnitude higher at 150◦C than at room temperature. In addition, the stress sensitivity of strain rate to a change in differential stress was reduced at higher temperature, and in concordance with prior experimental and theoretical work, it is established that increased rates of stress corrosion likely explain the reduction in stress sensitivity. It is therefore concluded that the weakening effect of elevated temperature is greater at lower strain rates.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | Sandstone Deformation under Crustal Hydrothermal Conditions |
| Event: | UCL (University College London) |
| Open access status: | An open access version is available from UCL Discovery |
| Language: | English |
| Additional information: | Copyright © The Author [year]. Original content in this thesis is licensed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) Licence (https://creativecommons.org/licenses/by/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request. |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10098472 |
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