Coal cleat permeability for gas movement under triaxial, non-zero lateral strain condition: A theoretical and experimental study

As the permeability of coal seams is mainly determined by the network of natural fractures known as the cleat system, estimation of cleat permeability is of utmost importance for the carbon dioxide sequestration process in deep coal seams. The main objective of this study is to develop a new mathematical model for predicting cleat permeability under non-zero lateral strain conditions such as the conditions encountered in laboratory triaxial experiments. By applying the theory of elasticity to the constitutive behaviour of fractured rocks, a theoretical relationship between permeability and gas injecting pressure, confining pressure, axial load and gas adsorption in triaxial tests is developed. The new model was then verified using experimentally-determined permeability data of two coal samples. Results indicate that the new model can fairly accurately predict the combined effects of effective stress and coal matrix swelling on cleat permeability for both CO2 and N2 injections at various injection pressures. The model also provides quite accurate prediction of the effect of confining pressure on cleat permeability for both CO2 and N2 injections. The model includes parameters for fractured rock properties, namely Poisson's ratio and Young's modulus. The model can be applied to predict cleat permeability, regardless of cleat size. When the accuracy of the new model is compared with the existing Gilman and Beckie [5] model, with increasing injecting pressure both models show similar increments of N2 permeability and different reductions for CO2 permeability. This is due to the zero lateral strain assumption of the existing model, which is not applicable to the swelling process under triaxial test condition. The new model is more accurate for the prediction of CO2 cleat permeability under triaxial test condition.