Research on influence of pore structure connectivity of low gas permeability on coal seam permeability

Abstract

In order to study the pore structure characteristics and pore connectivity of low-permeability coal, low-field nuclear magnetic resonance technology was used to analyze the raw coal sample. The T2 curve of the coal sample under saturated water and residual water conditions was used to obtain the free fluid value, irreducible fluid value, effective porosity, pore size distribution, and permeability etc. A quantitative calculation model of pore connectivity was established on the basis of these works. The results showed that the porosity of the experimental coal sample was changing from 6.30% to 11.02%, and the pore size was distributed in three stages, the pore size was mainly concentrated in the 0 nm to 100 nm segment. The fissures and the pores which including macropore and medium ones were not developed. Adsorption pore was inversely proportional to porosity. Percolation pore was proportional to porosity. The pore connectivity of experimental coal sample was varying between 31.50% and 62.05%. The pore connectivity was affected by the pore size, and the pore connectivity rate of the experimental coal sample with a pore size below 100 nm was up to 58.45%. The connectivity rate of pores with pore size over 100 nm was above 94%. Pore connectivity has a good positive correlation with porosity, effective porosity, and permeability.

Keywords

• low gas permeability coal
• porous structure
• pore connectivity
• permeability
• nuclear magnetic resonance