A comparative study of the nanopore structure characteristics of coals and Longmaxi shales in China

Abstract

Both of the coalbed methane (CBM) and shale gas reservoirs are dominated by nanometer‐scale pores with their nanopore structures controlling the occurrence, enrichment, and accumulation of natural gas. Low‐pressure nitrogen gas adsorption (LP‐N2GA), low‐pressure carbon dioxide gas adsorption (LP‐CO2GA), high‐pressure methane adsorption (HPMA), and field emission scanning electron microscope (FE‐SEM) experiments were conducted on 14 different‐rank coal samples and nine Longmaxi shale samples collected from various basins in China to compare their nanopore characteristics. The FE‐SEM results indicate that the pore structures of both the coal and shale samples consist of nanometer‐sized pores that primarily developed in the organic matter. The types of their isothermal adsorption curves are similar. However, the coal and shale samples possess various hysteresis loops, which suggest that the nanopores in shale are open‐plated, whereas those in coal are semi‐open. Furthermore, the specific surface area (SSA) and pore volume (PV) of the micropores in coal are much larger than those of the mesopores, with the micropore SSAs accounting for 99% of the total SSA in the coal samples. However, the micropore SSAs in the shale samples only account 42.24% of the total SSA. These different nanopore structures reflect their different methane adsorption mechanisms. The methane adsorption of coal is primarily controlled by the micropore SSA, whereas that of shale is primarily controlled by the mesopore SSA. If we use mesopore SSA to analyze its impact on methane adsorption capacity of coal and shale, it will be mismatched. However, no mismatching relationship exists between the total SSAs and adsorption capacities of coal and shale. This study highlights the controlling effect of total SSA on methane adsorption capacity.

Gas sorption-induced pore radius change and their impact on gas apparent permeability

“…(b) pore structure plays a crucial role in pore radius reduction as different nanopore structures reflect the gas sorption mechanisms. Zhou et al [47] showed that the mesopores are more controlled by shale than micropores and providing more than 50% total SSA than micro-SSA's and same results was found in this study. For example, Sample Y-105-1 and Y-105-15 have contributed micro-SSA's about 42.96% and 42.72% whereas meso-SSA's about 57.04% and 57.28%, respectively, and (c) organic matter and mineral components also affect pore radius as different shale sample containing different SSA in mineral components [21] and [12].…”

Predicting adsorbed gas capacity of deep shales under high temperature and pressure: Experiments and modeling

“…The larger the SSA, the greater the adsorption capacity (Chalmers et al, 2012;Yu et al, 2016a;Xiang et al, 2019). Due to the large SSA of shale, about 10∼30 m 2 /g (Zhou et al, 2019), it has a certain adsorption capacity; (4) affected by the adsorption force, under a certain pressure, methane molecules are preferentially adsorbed in the micropores (< 2 nm), and the micropores are the main storage space for the adsorbed gas (Zhu et al, 2020).…”

Clarifying the Effect of Clay Minerals on Methane Adsorption Capacity of Marine Shales in Sichuan Basin, China

“…The LTNA curves of the representative shale samples are shown in Figure 4, which belong to Type II adsorption isotherms [47,48]. The hysteresis loop between the adsorption and desorption isotherms is similar to a Type H3, which reflects the open parallel-plate pores with good connectivity, and this type of pore structure is favorable for gas migration [49].…”