Particle Diffusion in Complex Nanoscale Pore Networks

We studied the diffusion of particles in the highly irregular pore networks of chalk, a very fine-grained rock, by combining three-dimensional X-ray imaging and dissipative particle dynamics (DPD) simulations. X-ray imaging data were collected at 25 nm voxel dimension for two chalk samples with very different porosities (4% and 26%). The three-dimensional pore systems derived from the tomograms were imported into DPD simulations and filled with spherical particles of variable diameter and with an optional attractive interaction to the pore surfaces. We found that diffusion significantly decreased to as much as 60% when particle size increased from 1% to 35% of the average pore diameter. When particles were attracted to the pore surfaces, even very small particles, diffusion was drastically inhibited, by as much as a factor of 100. Thus, the size of particles and their interaction with the pore surface strongly influence particle mobility and must be taken into account for predicting permeability in nanoporous rocks from primary petrophysical parameters such as surface area, porosity, and tortuosity