Study on full-scale pores characterization and heterogeneity of coal based on low-temperature nitrogen adsorption and low-field nuclear magnetic resonance experiments

Abstract

Abstract The characteristics and heterogeneity of pores in coal are of great significance for understanding the production mechanism of coalbed methane. The paper used low-temperature nitrogen adsorption and low-field nuclear magnetic resonance experiments to study the characteristics of full-scale pores development with different metamorphic coals (0.58%≤R0,max≤3.44%), and the heterogeneity of pores was analyzed by fractal theory. The results showed that pores with different sizes in low-rank coal were well developed, and the connectivity of these pores was favorable. The proportion of micropores in middle-rank coal was 73.56%, and the connectivity between transitional pores, mesopores, and macropores was poor. And the proportion of micropores in high-rank coal was 92.74%, and a mount of these pores were closed or semi-closed, the connectivity between micropores and transitional pores was terrible. As the metamorphism degree increased, the total porosity grew gradually, while the effective porosity and permeability decreased. The heterogeneity of pores was controlled by coal metamorphic, characteristics of pore structure, pore size distribution, and connectivity. Affected by coalification, the DL1 (characterizes the roughness of adsorption pores surface, which ranges from 2.13 to 2.45) and DL2 (characterizes the complexity of adsorption pores structure, which ranges from 2.56 to 2.77) fell firstly and then rose, while the DN (characterizes the heterogeneity of seepage pores, which ranges from 2.92 to 2.95) improved constantly. The adsorption pores surface roughness and structure complexity gained with the raised of specific surface area and pore volume, so the DL1 and DL2 showed the power-law growth. The uniformity of pore structure enhanced with the increase of seepage pores proportion, and the DN decreased linearly. When the volume of seepage pores was determined, the complexity of the pore structure was weakened by pores connectivity, thus the DN reduced gradually. The results can provide theoretical support for predicting coalbed methane recoverability and improving coalbed methane production efficiency.