A SIMPLE MODEL OF FRACTURED AQUIFER PERMEABILITY

Fractured aquifer permeability is often characterized using the concept of a representative elementary volume, normally considered to be 100 cubic meters or more in fractured aquifers. However, a well-known issue in these aquifers is an apparent increase in aquifer permeability with scale or the “scale effect”. A simple model of fractured aquifer permeability was tested that uses distributed apertures instead of assuming equal apertures. The model was tested against core, well, and regional model permeability data from the San Antonio segment of the Edwards aquifer. Using outcrop aperture data, the model reproduced the well and regional scale permeability values that vary by ten orders of magnitude. The model also suggests that the “scale effect” can be fully explained as an effect of selective sampling of a fractured aquifer. The model indicates that a representative elementary volume for the Edwards aquifer may not exist or is much larger than 4 kilometers. The modeling also shows the truncations apparent in fracture aperture distribution data are apparent in well and regional model distributions. This suggests that while a “scale effect” may exist in permeability data, using fracture aperture distributions may be a more consistent tool for evaluating these aquifers.