Three-dimensional pore-scale study of methane gas mass diffusion in shale with spatially heterogeneous and anisotropic features. (1st August 2020)

• Record Type: Journal Article
• Authors: Yin, Y.; Qu, Z.G.; Zhang, T.; Zhang, J.F.; Wang, Q.Q.
• Is Part Of: Fuel. Volume 273(2020)
• Publication Date: 2020-08-01
• Subjects: Shale gas -- Porous media -- Effective diffusivity -- Pore-scale -- Lattice Boltzmann method; Fuel -- Periodicals; Coal -- Periodicals; Coal; Fuel; Periodicals; 662.6
• DOI: 10.1016/j.fuel.2020.117750
• Languages: English
• ISSNs: 0016-2361

Abstract

Highlights: A series of 3D heterogeneous and anisotropic shale structures are reconstructed. 3D pore-scale lattice Boltzmann method is used to mimic gas diffusion process in shales. Comprehensive sensitivity analysis is performed to disclose gas diffusion properties in shales. An empirical formula is proposed to estimate the effective diffusivity of isotropic shales. Abstract: Understanding methane gas mass diffusion in shale is crucial for estimating gas production. However, shale is a typical ultra-tight porous medium that is characterized by its spatially heterogeneous and anisotropic features. The heterogeneous feature is derived from coexisting components including organic matter (OM), inorganic matter (IM), interparticle pores, microfractures, and bound water; the anisotropic feature is induced by the IM particles appearing in the form of flake-like structure. In this work, a three-dimensional (3D) pore-scale lattice Boltzmann model, which considers above heterogeneous and anisotropic features, is presented to study the methane gas diffusion in the shale. The elementary building block model and quartet structure generation set method are integrated to reconstruct 3D shale structures. Reconstructed shale structures are then characterized in terms of porosity, pore size distribution, and pore connectivity. The results show that the shale diffusivity increases with temperature, porosity, and OM volume fraction, while logarithmically decreasing with the increased pressure. Considering the anisotropic feature, the effective diffusivity parallel to the bedding plane is approximately 1.2–4.2 times higher than that perpendicular to the bedding plane. The microfracture orientation and aperture, compared to the microfracture shape and distribution pattern, play a more important role in determining the effective diffusivity. In contrast, the presence of bound water hampers gas diffusion. In addition, a general formula is proposed to conveniently estimate the diffusivity of isotropic shale, where the influences of temperature, pressure, porosity, OM volume fraction, and water saturation are fully considered.