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

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/N adsorption (LPCO/NGA), 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.