Pore-scale characterization and fractal analysis for gas migration mechanisms in shale gas reservoirs

Abstract The microstructures and the gas migration mechanisms of unconventional gas reservoirs are important to gas exploitation. In this chapter, the microstructures of Longmaxi shale are characterized by their nitrogen adsorption-desorption isotherms, scanning electron microscopy data, and several fractal parameters. The gas migration models in a single micro-nano capillary and fractal porous media are established and verified. The mesopores are abundant in shale samples according to the nanopore size distribution. The adsorption capacity and nanopore distribution of shale are likely to be controlled by the total organic carbon content to some extent. The surface fractal dimensions among the shale samples are close, which indicates that the irregularity of the pore surface is similar for the shales from the Longmaxi Formation. The pore fractal dimensions of shales display great differences, which implies that the pore distribution of shale is strongly heterogeneous. The self-similarity of the pore distribution of shale is confirmed by the analytical fractal equation. Furthermore, the heterogeneity of pore distribution can be quantitatively represented by the normalized lacunarity. The succolarity of shale is too small to build a meaningful correlation with permeability but can partly explain the low-permeable capacity of shale. The gas migration in a single capillary and the fractal shale matrix is mainly controlled by viscous flow, transition diffusion, and surface diffusion of adsorbed gas. The transition diffusion dominates the gas migration when pressure is relatively low. While under high pressure, viscous flow dominates the gas transport. Moreover, the surface diffusion of adsorbed gas also has some contributions to the gas transport process under low pressure.