The Temperature Dependence of Time-Dependent Deformation in Sandstone

With increasing depth in the crust, the deformation mechanics of porous sandstones transition from dilatant, brittle deformation to compactant, ductile deformation. Both deformation modes exhibit a time dependency due to chemical interactions between the pore fluid and the rock matrix, which allows deformation to occur at stresses below the conventional 'critical level'. To date, the majority of experimental work on sandstone deformation has been carried out at room temperature. However, at the crustal depths required for ductile behaviour, the temperature is likely around 75-200 ^oC and it is not known what effect this increased temperature has on the mechanical behaviour of sandstone.

Here, the effect of elevated temperatures (up to 150^oC) on sandstone deformation was explored, with a focus on the brittle-ductile transition and the fully ductile regimes. This was executed through triaxial deformation experiments, performed at a range of effective pressures on fluid saturated cores, of either Bleurville or Locharbriggs sandstone. Samples were deformed at room temperature or at elevated temperatures under either constant strain rate (10^-5 s^-1) or constant stress (creep) conditions.

Constant strain rate tests in the ductile regime last for a few hours and show that at 150^oC the differential stress required for the onset of compaction is reduced by 10-20 MPa, with the exact amount being a function of sandstone composition and the effective pressure. In addition, the pressure of the brittle-ductile transition is also reduced by the temperature increase.

During constant stress tests, which run for a few days to a few weeks, samples are initially loaded at a constant strain rate, before being held at a set stress value. The strain rate at which the sandstone continues to deform decreases with increasing time and can reach rates as low as 10^-8 s^-1. At 150^oC the sandstone underwent compactant creep at similar strain rates to its room temperature counterpart, but with a differential stress reduction of 20-30 MPa. By carrying out experiments at a range of temperatures and effective pressures, a prelimary empirical relationship has been established, stating that at a given amount of compactive strain, the strain rate is proportional to an exponential function of the applied stress, multiplied by a temperature dependent prefactor.