FRACTURE MODELING OF THE SAN ANTONIO SEGMENT OF THE EDWARDS AQUIFER, CENTRAL TEXAS

The San Antonio segment of the Edwards aquifer of central Texas is a prolific, well-studied, fractured karst aquifer. The average regional values of permeability of this aquifer are two orders of magnitude greater than the average permeability on the well scale and six orders of magnitude greater than the average permeability measured in core samples. This effect is referred to as the permeability “scale effect.” Previous work has attributed the permeability on the well scale to fractures. The purpose of this study was to determine whether outcrop data could predict the permeability of the aquifer on the regional scale. Well scale fracture density data from previous work were extrapolated to the regional scale to calculate how many connected fractures are encountered in a model cell. This number of fractures was then used to determine the estimate of regional permeability from the outcrop data. The outcrop permeability model yielded an appropriate mean value of regional permeability. This model suggests that the “scale effect” is only an artifact of sampling a larger portion of a fracture aperture distribution.