Regional Permeability Estimates from Fracture Apertures

Regional Permeability Estimates from Fracture Apertures in Outcrop, Edwards Aquifer, Central Texas

ZAHM, C.K.*, and R. MARRETT, Department of Geological Sciences, The University of Texas at Austin, Austin, TX, 78712, USA {marrett@mail.utexas.edu)
(*current address:} Department of Geology, Colorado School of Mines, Golden, CO, 80401, USA, {czahm@mines.edu)

ACCEPTED FOR PRESENTATION: SEPM - Fluid Flow in Carbonates Research Conference, Sept. 1998

The use of fracture aperture data from an outcrop with an area as small as 100 m² to make estimates of regional permeability for an aquifer that is greater than 7,700 km² may seem ambitious. However, that is the premise of this paper. This investigation utilizes innovative techniques that use fracture aperture data measured in outcrop to estimate regional-scale permeability in the Barton Springs Segment of the Edwards aquifer of central Texas. Fracture apertures were collected along transect lines oriented perpendicular to the predominant fracture strike in four outcrop locations. Each outcrop is a large pavement of carbonate bedding-plane exposure located in the streambed of Barton Creek. The sampled apertures were used to generate frequency-size distribution plots where the y-axis represents the ranked number of the associated fracture aperture and the x-axis represents aperture. The cumulative number, N, representing the number of fractures larger than the associated fracture aperture, was plotted against the fracture aperture on log-log scale. Segments of the data that could be fitted with a straight line and that were not affected by censoring and truncation biases were considered to be self affine (i.e. fractal) and were described using power law equations. Fractal modeling was used to estimate the permeability contributions for the truncated data. Permeability estimates were based on data for the range of apertures that were effectively sampled, and only relied up on fractal modeling for the ineffectively sampled aperture range. \par Permeability was estimated to be approximately 10^-9 m² for three of the four outcrops studied. The estimated permeabilities are similar to permeabilities calculated from specific capacity tests conducted on water wells in the Edwards aquifer. In addition, the estimated permeabilities concur with regional permeability measurements reported for the Edwards aquifer by other studies. In contrast to the other three outcrops, permeability for the fourth outcrop was estimated to be 10^-6 m² . Dissolution along the fractures was observed in this outcrop and fracture aperture was enhanced causing the estimate of permeability to be higher. The calculated permeability for this outcrop is consistent with values obtained in other studies where o pen conduits such as caves and enhanced dissolution features are present. Textural differences in carbonate lithology do not appear to have significant bearing on the ability to estimate permeability from outcrop using this method. Estimates of permeability only considered fracture aperture and did not include the effects of connectivity, fracture roughness and/or channeling.

Return to Physical Hydro Home Page

Last modified 07/23/01