Mapping of Stress Sensitivity Affected by Water Variation to Microscopic Pore Distributions in Medium- and High-Rank Coals

The stress sensitivity and compressibility of the pore system in coal greatly influence the coal permeability and porosity, which are important to coalbed methane development, while water transformation under stress and drying process is essential to obtain insights into water interference on pore structure. In this study, microscopic pore distributions obtained by different test methods are compared, and the stress sensitivity and compressibility of micropore-transition pores and mesopore-macropores, and the water migration under different saturations are investigated by nuclear magnetic resonance of cores of four different rank coals. According to the combined results of low-pressure N-2 adsorption and mercury intrusion porosimetry, the distribution of T-2 spectra was in good agreement with the pore size distribution, which was dominated by micropores and transition pores (MI-T pores). The stress sensitivity of mesopores and macropores (ME-MA pores) in the completely water-saturated middle-rank coal samples was higher than that of the high-rank coal samples. ME-MA pores, featuring more complex heterogeneity than the whole pore system, provided more compression space but the compressibility varied significantly among different ranks of coal samples due to the low proportion of ME-MA pores. Besides, the pore space after compression could not be restored to its original state. Different ranks of coal cores varied in the relationships among stress sensitivity, compressibility, and heterogeneity. During the drying process, coal water tended be first evaporated from coal evaporation surface, smooth micro-fractures, open macropores and large particle voids.

Automated summary

The stress sensitivity and compressibility of the pore system in coal greatly affect coal permeability and porosity, which are crucial for coalbed methane development. Understanding water transformation under stress and during drying is important for analyzing water's impact on pore structure.