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

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.

Coal Facies and Its Effects on Pore Characteristics of the Late Permian Longtan Coal, Western Guizhou, China

“…It is 9 Geofluids noteworthy that moist forest peat swamp facies samples from YV-1 well, which are medium-rank coal with R o of 0.95%-1.1%, have poorer porosity (2.66%-4.22%) and TPA (3.23-5.98 m 2 /g) and slightly larger MPD (8.1-8.2 nm) and (16.2-26.2 nm) than the same coal facies samples from PQ-1 well. This result is consistent with former researchers, and they indicated that micropores proportion reach peak during 0.75%-1.25% R o stage, while the peak of macropores occurs during 2.0%-2.5% R o stage (Figure 9) [33][34][35][36][37][38].…”

“…Especially in Guizhou Province, where the coal reservoirs are multilayer thin coal seams [7]. In addition, previous studies have proved that coal rank is another controlling factor to reservoir properties (Figures 9 and 12) [33][34][35][36][37], and coal rank reflects tectonic evolution of coal reservoir. Generally, medium rank coal has much higher 12 Geofluids proportion of micropores and lower proportion of macropores than low and high rank coals.…”

Adsorption Factors in Enhanced Coal Bed Methane Recovery: A Review

“…This technique expands the concept of the Langmuir adsorption isotherm by assuming perfect multilayer adsorption via the formation of monolayers [115]. The BET data are obtained by injecting N 2 gas at 77 K into a coal sample and provide information related to the type of pores in which adsorption occurs, the SSA of the material and the total pore volume [37,62,74,96,116,117]. The BET method is also used to calculate the SSA values of mesoporous solids in general [77,118].…”

Insights into the Nanoscale Microstructure Diversity of Different Rank Coals

“…12−14 For further understanding the heterogeneous pore structure of coal, many scholars have carried out a lot of research by using the above methods. The experimental results of Zhu et al 15 showed that due to the compaction in the early stage and the thermogenic gas produced in the middle and late stages of coalification, the total porosity tends to decrease first and then increase. Xu et al, 16 through a series of pore-detection experiments, showed that adsorption pores (<100 nm) are more well developed for disturbed coal than for primary coal.…”