Compactive Deformation of Sandstone Under Crustal Pressure and Temperature Conditions

Mark Jefferd,Philip G Meredith,Nicolas Brantut,Oliver Plümper,Thomas M Mitchell

doi:10.1029/2020jb020202

Mark Jefferd, Philip G Meredith + Show 3 more

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https://doi.org/10.1029/2020jb020202

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Abstract

AbstractThe transition from macroscopically brittle to macroscopically ductile deformation in porous sandstones is known to be pressure dependent, with compactive, ductile behavior occurring only once significant effective pressures have been reached. Within the crust, such effective pressures are associated with burial depths in the range 0.5–6 km, where the temperature is likely 35°C–200°C. To test the importance of such elevated temperature on the strength and deformability of sandstone, a series of constant strain rate, triaxial deformation experiments were performed on three different water saturated sandstones at either ambient temperature or 150°C. For each sandstone, an effective pressure range was used which spanned both the brittle and ductile deformation regimes, up to a maximum of 120 MPa. In the brittle regime, we observed a temperature‐dependent lowering of the yield stress of between 8% and 17%. Within the ductile regime, we observed an even greater reduction in the yield stress of between 9% and 37%. A further notable observation is that the transition from dilatant, brittle behavior to compactive, ductile behavior tends to occur at a lower effective pressure at elevated temperature. The weakening observed at elevated temperature can be explained by a reduction in fracture toughness, which is shown mathematically to cause greater weakening in the ductile regime than in the brittle regime. The apparent reduction in fracture toughness at elevated temperature is potentially driven by a combination of a reduction in surface energy and, to a minor extent, an increase in subcritical crack growth rate.

Highlights

The inelastic deformation of porous sandstone under applied stress depends on the local physical and chemical environment (e.g., Baud et al, 2000)
Joints and faults are representative of localized dilatancy that occurs in the brittle regime (Aydin & Johnson, 1983), while compaction bands are an example of localized compaction structures that can result from grain crushing in the ductile regime (Mollema & Antonellini, 1996)
We demonstrated through triaxial deformation experiments performed on water saturated samples of Bleursville, Locharbriggs, and Boisze sandstones, that increasing the temperature from room temperature to 150°C weakens sandstone by an non negligible amount in both the brittle and ductile regime

Summary

Introduction

The inelastic deformation of porous sandstone under applied stress depends on the local physical and chemical environment (e.g., Baud et al, 2000). The underlying deformation mechanisms can typically be grouped into those corresponding either to the macroscopically brittle regime or the macroscopically ductile regime (Bemabe & Brace, 1990; Bésuelle et al, 2003; Wong et al, 1997; Wong & Baud, 2012). Joints and faults are representative of localized dilatancy that occurs in the brittle regime (Aydin & Johnson, 1983), while compaction bands are an example of localized compaction structures that can result from grain crushing in the ductile regime (Mollema & Antonellini, 1996). Similar structures can be formed in laboratory experiments, and it has been demonstrated that the transition from macroscopically brittle behavior to macroscopically ductile behavior is highly dependent on the effective confining pressure (Handin et al, 1963; Wong & Baud, 2012; Wong et al, 1997)

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