Diffusion coefficients and multi-scale diffusion models of shale oil and gas

ZHANG Mingdi; ZHANG Guangdong; ZHU Langtao; MENG Sijin

doi:10.12090/j.issn.1006-6616.2025140

Volume 32 Issue 4

Aug. 2026

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Article Navigation > Journal of Geomechanics > 2026 > 32(4): 877-888

ZHANG M D，ZHANG G D，ZHU L T，et al.，2026. Diffusion coefficients and multi-scale diffusion models of shale oil and gas[J]. Journal of Geomechanics，32（4）：877−888 doi: 10.12090/j.issn.1006-6616.2025140

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Diffusion coefficients and multi-scale diffusion models of shale oil and gas

doi: 10.12090/j.issn.1006-6616.2025140

1.
Southwest Oil and Gas Branch of Sinopec Corporation, Chengdu 610095, Sichuan , China
2.
School of Petroleum Engineering, Southwest Petroleum University, Chengdu 610500, Sichuan, China

Funds: This research was financially supported by the Natural Science Foundation of China (Grant No.51504202).

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Received: 2025-05-16
Revised: 2025-12-26
Accepted: 2026-01-30

Available Online: 2026-05-08

Published: 2026-08-28

Abstract

Abstract

Objective 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. Methods 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. Results 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. Conclusions 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. Significance 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.
- shale oil and gas,
- diffusion coefficient,
- multiscale diffusion model,
- fractal theory,
- bedding fractures

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