Strong field evidence of directional permeability scale effect in fractured rock

Permeability is a key parameter that controls mass and energy transport in geologic media. Its spatial variation is generally agreed within the research community, while the scale effect in permeability is still under considerable debate. This debate is mainly due to the lack of consistent measurement and interpretation of site data. Here, we show a strong field evidence of directional permeability scale effect from cross-hole pneumatic injection tests conducted in unsaturated fractured tuff at a field site in central Arizona, USA. The analysis reveals that permeability increases with the radial distance measured between the centroids of injection and monitoring locations of the test, which we consider to be the measurement scale. These results are compared to previously analyzed single-hole tests conducted at multiple measurement scales. We hypothesize that the permeability scale effect is controlled by the connectivity of fluid conducting fractures, which also increases with scale. This observation and hypothesis is consistent with existing scaling theories in fracture connectivity, although subsurface fracture connectivity cannot be measured directly with present technology. These results suggest the need to obtain permeability estimates that are commensurate with the scale of intended use.