The effect of temperature, the nature of the pore fluid, and subyield differential stress on the permeability of phyllosilicate-rich fault gouge

Abstract

Elevated temperatures and subyield deviatoric stresses were applied to oriented natural samples of clay-bearing fault gouge under pressure in order to establish their effects on the water and argon permeability under conditions closer to those found at depth in fault zones. It was found that the application of subyield differential stress normal to the fault plane resulted in a permeability reduction by a factor of ∼2. Argon permeability showed a constant rate of decrease with increasing temperature up to 150°C and limited time-dependent compaction at temperature. More substantial time-dependent compaction at elevated temperature with water pore fluid reduced the permeability by a factor of 3 over ∼6 days. When water-wet, water permeability increased with temperature up to ∼80°C, and then reduced at higher temperatures. This behavior is interpreted to be due to a combination of thermal compaction and the destabilization of structured water films coating pore throats at elevated temperatures. Increasing the pore fluid pressure did not appear to play an important role in stabilizing water films in pore throats. Based on these results, the reduction of permeability due to blocking of pore throats by structured water films is unlikely to play a role in the deeper part of fault zones. However, in the shallow crust this permeability reduction mechanism may be responsible for a significant lowering of the permeability. This has important implications for fluids that may be traveling up large faults or for the modeling of fluid flow around waste repositories, particularly when these may create temperature perturbations by their presence.