Comments on Calculation of Diffusion Coefficients from Hydrocarbon Concentration Profiles in Rocks: GEOLOGIC NOTE

Abstract

Diffusion of hydrocarbons is an important transport process in the geologic environment; it has applications in the primary migration of hydrocarbons and in the sealing properties of shales. Previous work in the literature reports effective diffusivities calculated from hydrocarbon concentration profiles in rocks. Those effective diffusivities may be applied to transport within rocks similar to the sampled rocks, but the effective diffusivities comprise several factors that must be considered when one applied them to different rocks. For example, with increasing carbon number, the reported effective diffusivities show a large decrease that results not from a trend in the intrinsic diffusivity but from a trend in the organic-carbon/water partitioning of the hydrocarbons. As such, the effective diffusivities do not apply to rocks with little or no organic carbon. This note presents a model whereby the previously published effective diffusion coefficients are interpreted with respect to diffusion within the water-saturated pore space and partitioning between the aqueous and organic-carbon phases of the rock. This model allows a more accurate extrapolation of diffusion measurements to new systems. However, the model does not conflict with the previous qualitative conclusion that aqueous diffusion is an important transport mechanism for light hydrocarbons only.