Pore Structure of Coals by Mercury Intrusion, N2 Adsorption and NMR: A Comparative Study

Li Li,Lu Lu,Yang Yang,Zhu Zhu,Cai Cai,Liu Liu,Zhang Zhang

doi:10.3390/app9081680

Abstract

Coalbed methane (CBM) mainly adsorb in massive pores of coal. The accurate characterization of pores benefits CBM resource evaluation, exploration and exploitation. In this paper, mercury intrusion porosimetry (MIP) and low temperature nitrogen adsorption (N2GA) combined with low field nuclear magnetic resonance (NMR) experiments were conducted to analyze the advantages and differences among different experimental techniques in pore characterization. The results show that the total porosity has a tendency to decrease first and then rise with the increase of coal rank, which is mainly caused by the compaction in early stage and the thermogenic gas produced in middle and late stages of coalification. The comparison between different techniques shows that NMR is superior to the conventional methods in terms of porosity and pore size distribution, which should be favorable for pore characterization. The N2GA pore size measurement, based on BJH model, is only accurate within 10‒100 nm in diameter. There is a peak misalignment between the NMR and MIP results in the pore size comparison. The reason for this phenomenon is that there is a centrifugal error in NMR experiment, which could cause a differential damage to the coal sample, resulting in partial loss of the nuclear magnetic signal.

Highlights

Coal generally has the characteristics of dual pore structure, low permeability, large pore surface area, and strong adsorption capability [1]
After the coal rank becomes semi-anthracite or anthracite, the internal fractures in the coal seam gradually increase and the total pore volume increases [10]. ∅N2GA results are significantly smaller for the reason that the proportion of pores with diameter between 1.7 and 200 nm of total pore volume is too small, but the trend along with the coal rank is similar to the results from other technologies
Three kinds of experimental technologies, supplemented by gas porosity and permeability experiments, are applied to evaluate the coal pore characteristics and compare the results of pore size distribution characterized by different experiments

Summary

Introduction

Coal generally has the characteristics of dual pore structure (matrix pore and fracture), low permeability, large pore surface area, and strong adsorption capability [1]. The International Union of Applied Chemistry (IUPAC) [2] proposed a pore size classification: micropores 50 nm in diameter. To better understand the effects of pore structure on both gas adsorption capacity and flow capability, the classification from Hodot [3] for coal pore size is proposed: super micropores (

Methods

Results

Discussion

Conclusion