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Transport of polydisperse colloids in a saturated fracture with spatially variable aperture

Chrysikopoulos Constantinos, Scott C. James

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Summary

A particle tracking model is developed to simulate the transport of variablysized colloids in a fracture with a spatially variable aperture. The aperture of the fractureis treated as a lognormally distributed random variable. The spatial fluctuations of theaperture are described by an exponential autocovariance function. It is assumed thatcolloids can sorb onto the fracture walls but may not penetrate the rock matrix. Particleadvection is governed by the local fracture velocity and diffusion by the Stokes-Einsteinequation. Model simulations for various realizations of aperture fluctuations indicate thatlognormal colloid size distributions exhibit greater spreading than monodispersesuspensions. Both sorption and spreading of the polydisperse colloids increase withincreasing variance in the particle diameter. It is shown that the largest particles arepreferentially transported through the fracture leading to early breakthrough while thesmallest particles are preferentially sorbed. Increasing the variance of the aperturefluctuations leads to increased tailing for both monodisperse and variably sized colloidsuspensions, while increasing the correlation length of the aperture fluctuations leads toincreased spreading.

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