Laboratory study of hydrothermal deformation in unconsolidated St Peter quartz sand
We report on hydrostatic and triaxial hydrothermal deformation experiments of water-saturated St Peter quartz sand at temperatures between 24 and 225C. At all conditions, quartz sand deforms by a combination of elastic and inelastic mechanisms. During triaxial loading, all samples initially show quasi-elastic deformation followed by a substantial change of volumetric strain rate at yield. Samples dilate with low acoustic emission (AE) rates and minimal grain crushing when deformed at low effective pressures, and compact with high AE rates and substantial grain cracking/crushing at greater pressures. Yieldstrength data for a given temperature define an elliptical envelope consistent with critical state and CAP models from soil mechanics. Yield strength at low effective pressure is temperature-insensitive whereas yield strength at high effective pressure is lowered at elevated temperature. Deformation is consistent with the Arrhenius behavior expected for thermally-activatedsubcritical crack growth, as are activation energies estimated using the high pressure, temperature-sensitive yield data. The results indicate that increased stresses and temperatures associated with subsurface burial to the depths associated with currently productive hydrocarbon reservoirs are sufficient to significantly alter the yield strength of deforming granular media in systematic and predictable ways. Hence, models for reservoir deformation should take thermally-activated damage mechanisms into account. 2008, ARMA, American Rock Mechanics Association.