Permeability and Porosity in Granular Rodpackings

Packings of granular rods have significantly higher porosity and permeability than those of ordinary, round granular materials while particle entanglement leads to a significantly greater structural integrity. Rods used as proppant to hold open cracks in bedrock from hydraulic fracture increase natural gas recovery rates by 10-13% and virtually eliminate the need for post-recovery processing to filter out unwanted sediment. We report on new experiments connecting the permeability and porosity of rod packings for a variety of particle aspect ratios and container sizes. We look for a previously observed deviation from linear, Darcy flow at low Reynold's numbers and test models connecting permeability with particle geometry and packing tortuosity. Separately, refractive index matched scanning tomography images the full 3d packing and allows us to calculate not only bulk porosity but the broad size distribution for both voids and throats, where sediment clogging is most likely. We identify the variety of possible paths across the pile for intruders of different sizes and use these to predict packing filtration properties, with implications for hydraulic dams and floodwater modulation.