Estimation of fracture porosity in an unsaturated fractured welded tuff using gas tracer testing

Abstract

Kinematic fracture porosity is an important hydrologic transport parameter for predicting the potential of rapid contaminant migration through fractured rock. The transport velocity of a solute moving within a fracture network is inversely related to the fracture porosity. Since fracture porosity is often one or two orders of magnitude smaller than matrix porosity, and fracture permeability is often orders of magnitude greater than matrix permeability, solutes may travel significantly faster in the fracture network than in the surrounding matrix. This dissertation introduces a new methodology for conducting gas tracer tests using a field portable mass spectrometer along with analytical tools for estimating fracture porosity using the measured tracer concentration breakthrough curves.Field experiments were conducted at Yucca Mountain, Nevada, consisting of airpermeability transient testing and gas-tracer-transport tests. The experiments were conducted from boreholes drilled within an underground tunnel as part of an investigation of rock mass hydrological behavior. Air-permeability pressure transients, recorded during constant mass flux injections, have been analyzed using a numerical inversion procedure 1 to identify fracture permeability and porosity. Dipole gas tracer tests have also been conducted from the same boreholes used for air-permeability testing. Mass breakthrough data has been analyzed using a random walk particle-tracking model, with a dispersivity that is a function of the advective velocity. The estimated fracture porosity using the tracer test and air-injection test data ranges from .001 to .015. These values are an order of magnitude greater than the values estimated by others using hydraulically estimated fracture apertures. The estimates of porosity made using air-permeability test data are shown to be highly sensitive to formation heterogeneity. Uncertainty analyses performed on the gas tracer test results show high confidence in the parameter estimates made.

Transport of radon gas into a tunnel at Yucca Mountain—estimating large-scale fractured tuff hydraulic properties and implications for the operation of the ventilation system

“…This can be attributed to the scale difference between the radon and the air-injection data, as well as to heterogeneity in the properties of the TSw fracture continuum. The estimated porosity of the TSw fracture continuum is an order of magnitude smaller than values measured by Freifeld [2001] using gas-tracer tests in boreholes. This difference is not unreasonable, given that the radon data were fairly insensitive to the ffacture porosity over this mnge.…”

“…This yields a lower net permeability and higher porosity. In general, the estimated permeability of the TSw fracture continuum obtained from the radon data is one to four orders of magnitude larger than values obtained from borehole air-injection tests conducted by Lecain [1997] and Freifeld [2001]. This can be attributed to the scale difference between the radon and the air-injection data, as well as to heterogeneity in the properties of the TSw fracture continuum.…”

Experimental and numerical simulation of dissolution and precipitation: implications for fracture sealing at Yucca Mountain, Nevada

“…Fracture sealing could occur in small aperture fractures and in localized areas where fluid flux rates are much higher, due to focussed flow and reflux of condensate. Sonnenthal and Spycher, 2001 Fracture fluid flux for the Yucca Mountain system was calculated using the following assumptions and values: a long-term infiltration rate of 10 mm/y, a fracture porosity of 0.1 to 1.0% (Freifeld, 2001), and that all of the infiltrating water flows through the fracture network. …”

Phase I Contaminant Transport Parameters for the Groundwater Flow and Contaminant Transport Model of Corrective Action Unit 97: Yucca Flat/Climax Mine, Nevada Test Site, Nye County, Nevada, Revision 0

“…Limited lateral porosity data have been collected at Yucca Mountain from surface outcrops (e.g., Flint et al [1996]) and from tunnels (e.g., Freifeld [2001]); however, these data are from the unsaturated zone and do not represent the full suite of HGUs at Yucca Flat. To circumvent the problem of limited data for estimation of a spatial statistic (e.g., the correlation length) in the horizontal plane, it is common to assume a ratio of anisotropy in the principal directions and then scale the statistic relative to the known orientation.…”