Structural and Fractal Characterizations of Nanopores in Middle-Rank Tectonically Deformed Coals – Case Study in Panguan Syncline

Abstract

The reservoir properties of tectonically deformed coals (TDCs) differ significantly compared with their neighboring primary coals which are also known as unaltered or underformed coals. However, the heterogeneity of nanopores in TDCs under the syncline control has been seldom reported, and also the middle-rank level was minimally investigated to date. Thus, in this paper, the structures and multiscale fractal characteristics of nanopores in middle-rank TDCs under the controlling effect from Panguan Syncline were investigated via high-pressure mercury injection (HPMI), low-pressure CO 2 /N 2 adsorption (LPCO 2 /N 2 GA), and fractal theory. The results show that both the pore volume (PV) and specific surface area (SSA) of macropores increase significantly in the stage of cataclastic–schistose coal. For ductile deformed coals, the PV increases, while the SSA remains stable. The SSA of mesopores increases slightly in the brittle deformation stage, but significantly in the ductile deformation stage. For micropores, both the PV and SSA for TDCs are significantly higher than primary coals. Moreover, the ductile deformation has a more significant promotion effect for the microporous PV and SSA than the brittle deformation. The fractal dimension of the adsorption pore (induced from the Sierpinski model) increases; however, that of seepage pores (Sierpinski model) decreases with the enhancement of tectonic deformation. The fractal dimension for mesoporous (induced from the FHH model, Frenkel–Halsey–Hill) at 2–6 nm keeps stable in the stage of cataclastic–schistose coal but significantly increases in the ductile deformation stage. For mesopores of 6–100 nm, their heterogeneities were also enhanced in the ductile deformation stage. The fractal dimension of 0.3–0.6 nm micropores is close to 3 and changes slightly with the enhancement of tectonic deformation, indicating that the heterogeneity of smaller micropores is stronger than that of larger micropores. The results are of broad interest for CBM exploration and gas outburst prediction.

Pore Size Distribution and Fractal Characteristics of Deep Coal in the Daning–Jixian Block on the Eastern Margin of the Ordos Basin

“…The desorption branch separated from the adsorption branch as the relative pressure decreased. When ink bottle–shaped pores were present in coal, the relative pressure due to complete evaporation of the liquid at the bottleneck was much lower than the required coagulation pressure for the radius of the bottle, resulting in the sudden release of liquid in the bottle and a sharp decrease in the return line ( P / P 0 = 0.5). , An inconspicuous inflection point occurred at this point, indicating that the mesopore morphology of pores with smaller diameters (<4 nm) exhibited tiny bottlenecks and slit/wedge–shaped pores that were almost closed on one side.…”

“…The adsorption/desorption, diffusion, seepage, and output of CBM are closely related to the developmental characteristics and connectivity of the pores in coal, as its main storage site. , Research on coal reservoir pores has evolved in recent years from qualitative to quantitative investigations. Numerous scholars have conducted studies utilizing microcomputed tomography, nuclear magnetic resonance spectroscopy, , and low-pressure CO 2 adsorption (LP-CO 2 GA), low-pressure N 2 adsorption (LP-N 2 GA), − and high-pressure mercury intrusion (HPMI) − experiments to explore the pore morphology, specific surface area (SSA), pore volume (PV), pore size distribution (PSD), and other microscale and nanoscale pore characteristics of coal.…”

Methane Adsorption Behavior and Energy Variations of Brittle Tectonically Deformed Coal under High Temperature and High Pressure

“…All the tested samples were collected from the newly exposed structural positions and mining faces of the Xinjing, Er, and Wu mines, following the method of sampling of coal seams (GB/T482-2008) and the method of sampling of coal petrology (GB/T19222-2003). According to micro- and macrodeformation observations (Figure ), nine coal samples with typical deformation features were selected and classified into four deformation types: primary structure, cataclastic, mortar, and granulitic coal (with the increase in brittle deformation intensity). ,,, Table lists the results of the industrial analysis [the moisture, ash, volatile matter, and fixed carbon contents and the maximum vitrinite reflectance ( R o,max )], which was conducted by following the standards of GB/T212-2008 and GB/T6948-2008. Noticeably, with the increase in coal deformation, the ash yield of TDCs generally increases.…”