Anisotropic coal permeability estimation by determining cleat compressibility using mercury intrusion porosimetry and stress–strain measurements

This paper presents a novel method to calculate the anisotropic and stress-dependent coal permeability by determining cleat compressibility using the Mercury Intrusion Porosimetry (MIP) and stress–strain measurements. Cleat compressibility is often assumed isotropic and constant in the literature and is usually obtained by numerical fitting to a matchstick permeability model e.g. the model by Seidle et al. (1992) despite the gross simplification of this representation of the coal pore network. This paper provides a method to calculate anisotropic cleat compressibility using only MIP and stress–strain measurements which are easy to conduct, and without permeability information, which is harder to come by, requiring laboratory experiments on the core or through fitting field data. We report the measured stress–strain behaviour of a coal sample, with hydrodynamic loading/unloading over the range 0.5–4.0 MPa, and the permeability in face cleat (k _F ) and butt cleat (k _B ) directions using the Triaxial Stress Permeameter (TSR). The stress–strain measurement is used to calculate the anisotropic modulus of elasticity (E _F , E _B and E _V ) in face cleat, butt cleat and bedding plane directions, and cleat compressibility in the face cleat (C _fF ) and butt cleat (C _fB ) directions using the fractal dimension analysis with MIP measurement. Finally, the cleat compressibilities are used to calculate the anisotropic coal permeability by Seidle et al. (1992) permeability model and compared with the measured permeability of the coal sample.