Tracer tests and contaminant transport rates in dual-porosity formations with application to the WIPP

Abstract

Transport of contaminants was analyzed for dual-porosity formations in which advection occurs only through fractures, but contaminants can diffuse into the rock matrix porosity. Transport was shown to occur in three distinct regimes: fracture, dual-porosity and total porosity transport. In the fracture transport regime, contaminant transport is controlled by fracture porosity alone. This was shown to end too quickly to have any significance for nuclear waste repositories like the Waste Isolation Pilot Plant (WIPP). Dual-porosity transport, in which contaminants are predominantly inside the matrix porosity and in the process of diffusing deeper into the rock, was shown to be controlled by a single formation parameter, called here the transport length, which can be estimated from tracer tests. Although conventional two-well tracer tests yield ambiguous values for fracture width and porosity, they provide useable estimates of the transport length. The effect on tracer tests of random heterogeneity and anisotropy in the formation was analyzed; the arithmetic mean of inferred values from different tests is the correct average for the transport length and the harmonic mean is the correct average for the kinematic porosity. Data were analyzed from nine tracer tests in a dual-porosity dolomite in the Culebra formation above the WIPP repository, including one test repeated at a different flow rate. The effect of dual-porosity on the mean transport rate of contaminants potentially leaking from the WIPP repository was shown to be negligible. Thus, for off-site transport at the WIPP, the formation is adequately modeled as a single-porosity, matrix-only medium.