Loading rate dependence of permeability evolution in porous aeolian sandstones

Mechanical properties of rocks are characterized by their notable dependence on the applied deformation rate. However, little is known about the strain rate dependence of fluid flow properties since most laboratory tests are conducted using a single, high strain rate. We have investigated the effect of loading rate on the permeability of porous sandstones by carrying out triaxial compression tests at four different temperatures and strain rates with continuous monitoring of permeability, acoustic emission (AE), and pore fluid chemistry. All tests are characterized by an initial permeability decrease due to inferred compaction of favorably oriented cracks. The amount of initial permeability reduction increases with decreasing strain rate, thus implying a more efficient initial compaction at slower strain rates. At a later stage of loading, permeability correlates with stress, ion concentration, or AE damage depending on the strain rate used. High strain rate tests are characterized by a positive power law or logarithmic correlation between permeability and AE damage. At slow strain rates, permeabilities decrease exponentially with mean effective stress and axial strain for the Locharbriggs sandstone. The Clashach sandstone exhibits a linear correlation between permeability and exit pore fluid concentrations (Si, Mg, Fe, Al) if a slow strain rate is used. These observations have important implications for the applicability of room temperature, high strain rate laboratory data to the conditions that prevail in the Earth's crust

Scaling and universality in rock fracture

We present a detailed statistical analysis of acoustic emission time series from laboratory rock fracture obtained from different experiments on different materials including acoustic emission controlled triaxial fracture and punch-through tests. In all considered cases, the waiting time distribution can be described by a unique scaling function indicating its universality. This scaling function is even indistinguishable from that for earthquakes suggesting its general validity for fracture processes independent of time, space, and magnitude scales