To elucidate the diffusion mechanisms of shale oil and gas in nano- to microscale pore-fracture systems and overcome the limitations of conventional models in cross-scale prediction, this study investigates the diffusion characteristics and controlling factors of shale fluids across multiscale porous media.

High-temperature and high-pressure diffusion experiments were conducted on matrix and bedding-fractured core samples with varying petrophysical properties. A multiscale diffusion model was developed by coupling fractal theory, a tortuous capillary-bundle representation, and a confinement-corrected Peng–Robinson equation of state to account for pore-size distribution, tortuosity, phase saturation, and nanoscale fluid–wall interactions.

Nanoscale confinement leads to phase-diagram contraction and a leftward shift of the critical point, substantially weakening diffusion capacity in confined spaces. Porosity and permeability are the primary structural controls: when porosity increases from 5.68% to 10.53%, gas and liquid diffusion coefficients increase by approximately one order of magnitude; when bedding-fracture permeability increases from 1.69 ×10⁻¹⁵ m² to 404.88 ×10⁻¹⁵ m², diffusion coefficients rise by about three to four times. Increasing pressure (10~36 MPa) suppresses gas-phase diffusion but enhances liquid-phase diffusion, whereas higher temperatures (80~115 °C) promote diffusion overall.

The proposed fractal–confinement diffusion model accurately matches the experimental results and reveals key nanoscale mechanisms, including a ~10⁻⁵ reduction in effective diffusivity relative to bulk fluids and diffusion capacities in fractures that exceed those of the matrix by one to two orders of magnitude.

This model enables structural-sensitivity analysis and cross-scale prediction of diffusion processes, providing theoretical support for the quantitative characterization of shale-fluid migration and the optimization of reservoir development.