4.6 Article

The Impact of Kerogen Tortuosity on Shale Permeability

Journal

SPE JOURNAL
Volume 26, Issue 2, Pages 765-779

Publisher

SOC PETROLEUM ENG
DOI: 10.2118/204469-PA

Keywords

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Funding

  1. College of Petroleum Engineering and Geosciences at King Fahd University of Petroleum and Minerals

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This study investigated the diffusion mechanism of natural gas in kerogens, finding a direct link between diffusion coefficient and kerogen porosity. Furthermore, the relationship between tortuosity and diffusivity in kerogens was explored, confirming the significant impact of diffusion processes on shale permeability at the reservoir scale.
Shale-matrix-associated transport phenomena exhibit multiple mechanisms including advective-, diffusive-, and adsorptive-driven transport modes, depending on the pore type. Diffusive processes are governed by the shale organic constituents known as kerogens. Kerogens, composed of fine-scale organic microstructures, vary with respect to their petrophysical properties, depending on their origin and maturity level. The extent to which kerogens contribute to the overall transport is governed by their ability to diffuse hydrocarbons contained within. The diffusion coefficient is a crucial parameter used to quantify diffusivity based on the interactions between the host material and the diffusing molecules. Kerogen as a hosting medium allows for diffusion of natural gas at various rates based on several factors. One of these factors, kerogen porosity, is conjectured to significantly influence diffusive transport phenomena. In this paper, taking advantage of the predictive power of molecular dynamics (MD) simulation, we investigate the impact of kerogen porosity on the diffusivity coefficient of natural gas. Starting from a single type II kerogen macromolecule, several kerogen structures for a realistic range of porosity values were created and, subsequently, used for diffusivity calculations of methane molecules. Simulation results suggest a direct link between diffusion and kerogen porosity, allowing for delineation of the diffusion tortuosity factor. Furthermore, the microscale tortuosity-diffusivity relationship in kerogens was investigated at the reservoir scale by means of a shale permeability model. The results substantiate the critical impact of the diffusion process on the shale permeability.

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