Stress-Dependent Two-Phase Flow in Coal Cleats

Cleat characteristics, such as aperture and roughness, are critical for assessing coal seam gas well productivity. They affect both absolute permeability and relative permeability of coal. Significant challenges exist on the selection of coal relative permeability curve and understanding of its changes with cleat characteristics. This paper presents the effects of horizontal stress on cleat deformation and two-phase transport phenomena in coal cleat network. A discrete fracture network was first built to reproduce the realistic cleat network pattern. An improved experimental approach was developed to measure the cleat compressibility C_f directly using high precision non-contact video extensometer, which was then validated by comparing with the data available in the literature. Given a horizontal stress, the obtained C_f was adopted to analyse the aperture variation in face and butt cleats. The direct simulations based on the lattice Boltzmann method are conducted on cleat-system realisations with varying apertures, to evaluate the liquid distribution, relative permeability and capillary pressure at varied saturation.

In comparison with traditional indirect methods of C_f estimation via fitting permeability data, the directly measured compressibility offers good accurate and consistent results. The relationship between horizontal stress (and cleat aperture) and relative permeability was established, and the correlation was further fitted by the van Genuchten-based permeability model. The tortuosity index is found to be well linearly correlated to the cleat aperture. The developed framework facilitates the evaluation of cleat compressibility and builds the relationship between cleat aperture and coal relative permeability, which is useful in assessing the coal petrophysical properties at core scale.