Stress sensitivity of medium- and high volatile bituminous coal: An experimental study based on nuclear magnetic resonance and permeability-porosity tests

Coalbed methane (CBM) development is in progress in northwestern China, where there are deposits of medium volatile bituminous coal (MVBC) and high volatile bituminous coal (HVBC). Stress sensitivity can lead to low gas-production during CBM development. In this study, three groups of HVBC samples and one group of MVBC samples are applied for low-field nuclear magnetic resonance (LF-NMR) and permeability-porosity tests (P-PT) under confining pressure. Each group contained two cores with similar physical properties, and one is used for LF-NMR and the other for P-PT under confining pressure. The key apertures dominating the reduction of permeability are revealed by comparative analysis of the results of LF-NMR and PP-T under confining pressure. Influences of multiscale pores and structure of macro-pores and fractures (MP-F) on stress sensitivity are discussed. In addition, this paper discusses the methods for determination of compressibility coefficients. The results shows that MP-F are the key apertures influencing stress sensitivity. Compressibility of macro-pore is the greatest, followed by meso-pores (MEP), and that of micro-pores and transition pores (MP-TP) is the lowest. Stress sensitive stages are influenced by structure of MP-F. Method A (based on P-PT) is suitable for calculating compressibility coefficient of coal with a high proportion of MP-F and method B (based on LF-NMR) has advantages for calculating compressibility coefficients of MP-TP, MEP, and MP-F, respectively. An improved model, considering matrix deformation of low rank coal and compressibility coefficient of key apertures, is proposed to calculate permeability under increased effective stress.