Investigation of fluid diffusion kinetics in nanochannels using micro-Raman spectrometry

Abstract

Fluid diffusion kinetics in nanopores is crucial for shale oil exploitation but influenced by complex pore structure and fluid-wall interactions. Core-scale experiments are difficult to decouple diffusion in nanopores and micron-pores, molecular simulations are time-consuming when handling pores with diameters larger than 10 nm, and nanofluidic experiments via conventional optical methods are challenging to measure fluid concentrations. Currently, there is no reliable method to investigate fluid diffusion in typical shale nanopores (diameters = 10~100 nm). Here, we report a nanofluidic method combining microscopic Raman spectroscopy to investigate diffusion in nanochannels imitating shale nanopores. A “channel-channel-cell” chip design enables real-time detection of fluid concentrations in microcells and measurement of diffusion coefficients in nanochannels, and a self-made temperature control module enables precise adjustment of fluid temperature. By this method, interdiffusion experiments of n-octane-1-octene mixture and n-octane-cyclooctane mixture in nanochannels (depths = 21~173 nm) are conducted. We report that the diffusion in nanochannels still conforms to Fick’s diffusion law, and the diffusion coefficients in channels with a minimum depth of 21 nm and at different temperatures (22~110℃) exhibit no obvious deviation from the bulk phase, suggesting that fluid-wall interactions have no significant effect on diffusion kinetics in our experiments. The consistency of the experimental results and classical predictions also validates the reliability of our method, which fills the gap in researches on fluid diffusion in nanopores and has promising application prospects. Diffusion in nanochannels with more types of fluids, more complex channel structures and smaller depth of channel can be furthered investigated by this method.