Impact of Coal Ranks and Confining Pressures on Coal Strength, Permeability, and Acoustic Emission

Abstract

Hydraulic fracturing is extensively applied as an effective stimulation technology to enhance coalbed methane recovery. Coal strength and permeability play critical roles in the performance of hydraulic fracturing, and acoustic emission (AE) provides useful information to evaluate this. Coal strength, permeability, and AE are all governed by coal rank and stress condition. However, the influence of coal rank and stress condition remains unclear. In this work, the coal strength, permeability, and AE are investigated under different confining pressures for different coal ranks in the laboratory. The results show these properties are mainly controlled by coal structure, particularly the development of pores and cracks. The compressive strength of coal presents a U-shaped trend with the vitrinite reflectance. However, as the confining pressure increases, the compressive strength raises, but its increase rate declines. In the process of loading stress, the permeability, and AE activities can be divided into three phases: compaction and elastic deformation phase, nonelastic deformation phase, and macroscopic failure phase. The initial permeability of low and medium-rank coals is higher than those of high-rank coals. In addition, as the confining pressure increases, AE activities and permeability weaken. The occurrence of abundant AE activities do not always suggest the generation of new cracks, but it can be regarded as an indicator, where the coalbed reservoir exhibits high permeability. Finally, based on these results, a new damaged model and a constitutive relationship between AE counts and stress are proposed to describe the coal-damaged evolution and predict its stress–strain behavior.