Permeability characteristics of layered composite coal-rock under true triaxial stress conditions

Abstract It is important to understand the gas flow properties in coal and sandstone under different stress conditions for underground carbon dioxide storage and coal bed gas drainage in underground engineering. In an actual project, the rock layers are complex and varied, and the in situ stress presents a significant three-dimensional anisotropy ( σ 1 > σ 2 ≠ σ 3 ). Although a large amount of data on the permeability of coal have been reported previously, studies on the permeability of layered composite coal rock under true triaxial stress conditions are lacking. In this study, the permeability of layered composite coal rock under true triaxial stress conditions was measured by using a true triaxial apparatus. Under experimental stress conditions, the permeability always decreased as the principal stress increased. The thickness changes in coal seams and sandstone formations have major influences on the evolution of composite coal rock permeability. Furthermore, the normalized permeability increases with the thickness ratio of sandstone to coal. We analyzed the stress state of layered composite coal rock by considering the interaction of coal and sandstone under true triaxial stress conditions. Based on the anisotropy consideration of coal rock and the stress difference of composite coal rock masses, a permeability model for layered composite coal rock under true triaxial stress condition was proposed (TCP model: layered composite coal rock permeability model). It was found that this model can well reflect the permeability characteristics of composite coal rock under true triaxial stress conditions while simultaneously considering the effect of the three principal stresses and the mutual strain restraint effects of coal and sandstone.