An experimental exploration of the transport and capture of abiotic colloids in a single fracture

The effect of microscale surface roughness in fracture walls is shown to accelerate colloid transport relative to a tracer and to increase colloid capture. These results are from experiments on duplicate laboratory experiments using synthetic, neutrally buoyant, abiotic, colloidal particles and two dense rhyolitic tuff cores. One core has a single, rough-walled fracture with an average aperture of 450 μm. We have used a profilometer to measure the fracture topography and calculate aperture distributions and surface areas in the reconstructed core. The other core has a smooth, channelized slit that is a parallel-plate analogue to the fractured core. Identical tracer and particle pulses were injected into each core. Particle capture by electrostatic attachment in the parallel-plate core gives a retention capacity of 55×109 particles m−2. In the rough-walled fracture core, average particle transport rates are a full fracture volume faster than average tracer transport rates. Reynolds flow simulation indicates the presence of highly channelized fluid flow that likely contributes to this accelerated particle transport. Capture in the fractured core was 90×109 particles m−2; retention capacity is unknown because particle concentrations in the effluent achieved a steady state value of only 85% of the injected concentration.