Assessment of Anomalous Transport through Rough-walled Fractures
The University of Texas at Austin, Department of Geological Sciences, Austin, Texas, USA
Anomalous transport ubiquitously occurs across all scales within fractured geological media. Detailed characterization of anomalous transport through single fractures is thus critical for predicting the fate of solutes and other fluid-borne entities through fractured media. Our direct numerical simulations of solute transport through two-dimensional rough-walled fractures showed early arrival and heavy tailing in breakthrough curves (BTCs), which are salient characteristics of anomalous transport. Analyses for dispersion coefficients (DADE) using the standard advection-dispersion equation (ADE) led to errors which increased linearly with fracture roughness. Estimated Taylor dispersion coefficients deviated from estimated DADE even at higher Peclet numbers. Alternatively, we thus used continuous time random walk (CTRW) model with truncated power law transition rate probability to characterize the anomalous transport. CTRW modeling markedly and consistently improved fits to the BTCs relative to those fitted with ADE solutions. The degree of deviation of transport from Fickian to anomalous is captured by the parameter β of the truncated power law. We found that β is proportional to fracture roughness. We also found that the CTRW transport velocity can be quantified based on the mean flow velocity. Along with the ability to predict β, this is a major step towards prediction of transport through CTRW using measurable physical properties.
AAPG Search and Discovery Article #90183©2013 AAPG Foundation 2013 Grants-in-Aid Projects