关键词: 页岩气, 多尺度孔隙, 粗糙度, 连通性, 各向异性, 可变管径, 三维分形, 传质机理

Abstract: Shale exhibits inherent anisotropy and transverse isotropy as a result of its mineral composition and preferred orientation, leading to pronounced disparities in mass transfer characteristics along the directions parallel and perpendicular to the bedding planes. Multi-scale interconnected pores serve as effective conduits for shale gas transport, while the topographical heterogeneity of mineral grains at the microscopic scale induces an observable roughness on the pore walls. Establishing a comprehensive mass transfer model that integrates spatial pore distribution with mass transfer characteristics under multiple flow regimes, alongside determining the evolution laws of shale permeability, remains a critical challenge. In this study, anisotropic parameters, including TOC, pore size, surface diffusion coefficients, and stress sensitivity coefficients, were experimentally determined following oriented coring along both parallel and perpendicular directions relative to the bedding planes of the core samples. Multi-scale pore distribution and connectivity were characterized through integrated high-pressure mercury intrusion and nuclear magnetic resonance experiments, while the nanometer-scale roughness morphology was characterized using atomic force microscopy (AFM) in conjunction with a physical model of conical roughness elements. Based on the three-dimensional fractal theory, an anisotropic mass transfer model for multi-scale micro-nano pores in shale was established. This model integrates factors including multi-scale pore size distribution, connectivity, roughness, water film, stress sensitivity, and real gas effects, while coupling transport mechanisms such as viscous flow, Knudsen diffusion, surface diffusion, and desorption. The research results indicate that:(1) TOC, pore size, connectivity, roughness, surface diffusion coefficient, and stress sensitivity coefficient exhibit significant anisotropic characteristics; (2) The developed model comprehensively incorporates the anisotropic gas mass transfer characteristics of shale, with an average relative error between predictions and experimental results ranging from 3.24 % to 5.12 %. The ratio of parallel to vertical permeability reaches 17, and factors such as connectivity, roughness, pore size distribution, and tortuosity fractal characteristics exert significant influences on the mass transfer outcomes. (3) Matrix permeability is positively correlated with TOC, pore size, connectivity, and surface diffusion coefficient, with organic matter exhibiting greater sensitivity to pore size and connectivity compared to inorganic matter. Conversely, permeability is negatively correlated with roughness and the ratio of the porosity-permeability correction coefficient; increased capillary roughness further restricts the Knudsen diffusion of gas molecules.

Key words: shale gas, multi-scale pore, roughness, connectivity, anisotropy, variable pipe diameter, three-dimensional fractal, mass transfer mechanism