The relationships among stress, effective porosity and permeability of coal considering the distribution of natural fractures: theoretical and experimental analyses

Although the relationships among stress, effective porosity and permeability of coal are a fundamental research topic that has been studied for decades and are widely used in analyzing the mechanical behavior of coal seams and predicting coalbed methane production, most relevant studies are based on idealized models and do not consider the influence of natural fracture distributions. To obtain a comprehensive understanding of the interrelationships among stress, effective porosity and permeability of coal, a series of effective porosity and permeability determinations have been conducted under different overburden stresses using an automated permeameter–porosimeter considering the directional distribution of natural fractures in coal. The experimental results show that the directional distribution of natural fractures provides a substantial contribution to the anisotropy of the effective porosity and absolute permeability of coal, which exponentially decrease with increasing overburden stress. An existing permeability model was modified to reflect the influence of the natural fracture distribution on the power law relationship between effective porosity and permeability, i.e., the exponent is not constant, but a variable related to the natural fracture distribution. The anisotropic effective porosity sensitivity and stress sensitivity of coal are also discussed, and the coal mass is shown to have the highest effective porosity sensitivity and lowest stress sensitivity in the direction perpendicular to the bedding planes compared to those in other directions.