Comprehensive evaluation and interpretation of mercury intrusion porosimetry data of coals based on fractal theory, Tait equation and matrix compressibility

Accurate characterization of coal pore structure is vital for understanding the mechanism of gas storage and migration. As an effective method for pore evaluation, mercury intrusion porosimetry (MIP) remains questionable without addressing the effects of interparticle voids filling and matrix compression properly. In this study, three intact coal samples with different metamorphic grades, one tectonic coal sample and two activated carbon (AC) samples were selected. Fractal theory was applied to identify the three stages of MIP test and to simulate the interparticle voids volume quantitatively using extrapolation method. Compared with the conventional matrix compressibility (MC) method for MIP data correction, we put forward the semi-empirical Tait equation (TE) method to compensate for the deficiencies of MC method. The results indicate that the simultaneous mercury filling of interparticle voids account for 1.9-3.4% of the total intrusion volumes for coals and 8.3% for AC. However, the proportions of matrix compression could reach 46.72-80.41% for coals and 14.30% for AC. In combination with low pressure N2 gas adsorption (LPN2GA) method, total pore volume (TPV) in the range of 33-320 nm from TE method is in closer proximity to the results using Barret-Joyner-Halenda (BJH) model. Whereas, MC method is superior in correcting MIP data of smaller pore sizes (7-33 nm). Overlapped pore size distributions (PSD) indicate that peak numbers, peak positions and incremental pore volumes of MIP corrected data show good consistency with LPN2GA measurement. For samples of low strength and with abundant inkbottle shaped pores, it is inappropriate to correct their MIP data.

Automated summary

Accurate characterization of coal pore structure is essential for understanding gas storage and migration mechanisms. This study used fractal theory and the new Tait equation method to analyze MIP data of coal, finding that simultaneous interparticle void filling affects 1.9-3.4% of total intrusion volumes, while matrix compression can account for 46.72-80.41%. When combined with low pressure N2 gas adsorption, the TE method provides TPV values closer to the BJH model for pore sizes in the range of 33-320 nm, while the MC method is more suitable for correcting smaller pore sizes in MIP data.