Ion Transport in Fractured Shale
Résumé
Recent modeling and experimental studies elucidate the controls on ion transport in fractured shale. Modeling based on the code CrunchClay is presented for a fracture-clay matrix system that includes electrostatic effects on transport. The electrostatic effects include those associated with the development of a diffusion potential as captured by the Nernst-Planck equation, and the formation of a diffuse layer (EDL) bordering negatively charged clay particles within which partial anion exclusion occurs. A dual continuum formulation accounts for diffuse layer and bulk water pore space, with the diffuse layer model obtained by volume averaging ion concentrations in the Poisson-Boltzmann equation. The simulation results demonstrate the lack of retardation for anions (e.g., 36Cl‑) of the contaminant plume within the fracture flow system because they are largely excluded from the charged clay rock, while the migration of cations that accumulate in the EDL (e.g., 90Sr++) are strongly attenuated. This behavior has been validated experimentally in transport experiments conducted in the Wolfcamp Shale. Samples without fractures showing the typical behavior in shale, with strong anion retardation due to their exclusion in the EDL. In contrast, where fractures are present, bromide (the representative anion) breaks through earlier than uncharged solutes (D2O) and cations due to the fact that they can migrate down the electrically neutral fracture with no retardation from matrix diffusion.