Tracing causes for the stress sensitivity of elastic wave velocities in dry Castlegate sandstone

The stress sensitivity of elastic wave velocities in dry rock is a resultant of two types of processes-elastic and non-elastic processes. Both processes are activated under stress and both are associated with stress-induced changes in the rock structure. Although they are of the same origin, their s...

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Veröffentlicht in:Geophysical journal international 2013, Vol.192 (1), p.137-147
Hauptverfasser: Stroisz, Anna Magdalena, Fjær, Erling
Format: Artikel
Sprache:eng
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Zusammenfassung:The stress sensitivity of elastic wave velocities in dry rock is a resultant of two types of processes-elastic and non-elastic processes. Both processes are activated under stress and both are associated with stress-induced changes in the rock structure. Although they are of the same origin, their stress-dependency may differ. In this study, a set of tests that separate the elastic and non-elastic processes is used to evaluate the impact of each process on the stress sensitivity of the elastic wave velocities. The tests rely on comparing the stress sensitivity of wave velocities during uniform loading/unloading over a long stress interval (involving both elastic and non-elastic processes) with the stress sensitivity seen in low-amplitude stress oscillations (assumed to be affected mainly by elastic processes). Our study on dry, weak sandstone shows that the impact of elastic and non-elastic processes on the stress sensitivity of the elastic wave velocities is significantly different. This implies that the processes ought to be described separately in order to provide a better foundation for predictive rock physics models. Observations on artificially fractured samples indicate that large, horizontal fractures reduce the axial wave velocities, whereas they have no notable impact on the stress sensitivity of the velocities. This suggests that the closed macro-fractures contain damaged areas with reduced stiffness which are apparently insensitive to stress changes. A few basic processes-elastic opening and closure of cracks, friction-controlled shear sliding of closed cracks associated with opening or closure of wing cracks, and crushing of asperities in fractures or grain contacts-may be used to intuitively explain the observations.
ISSN:0956-540X
1365-246X
DOI:10.1093/gji/ggs029