Open AccessStudy on the Applicability of Reservoir Fractal Characterization in Middle–High Rank Coals with NMR: Implications for Pore-Fracture Structure Evolution within the Coalification Process
Author(s) -
Haihai Hou,
Qiuhong Qin,
Longyi Shao,
Guodong Liang,
Yue Tang,
Huajie Zhang,
Qiangqiang Li,
Shujun Liu
Publication year - 2021
Publication title -
acs omega
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.779
H-Index - 40
ISSN - 2470-1343
DOI - 10.1021/acsomega.1c03904
Subject(s) - fractal dimension , maceral , porosity , coal , mineralogy , fractal , coalbed methane , materials science , vitrinite , geology , chemistry , composite material , coal mining , mathematics , petrography , organic chemistry , mathematical analysis
In order to evaluate the applicability of the pore-fracture structure fractal characterizations in coal reservoirs and confirm the internal relationships between the porosity, permeability, coal metamorphic grade, and pore-fracture structure, the pore-fracture features of 21 middle-high rank coal samples from Anhe, Jiaozuo, and Huaibei coalfields in northern China were investigated using a low-field nuclear magnetic resonance (NMR). All the coal samples are characterized by low moisture content ( M ad ), low and medium ash yield ( A ad ), and high vitrinite (V) in coal maceral. The adsorption space fractal dimension ( D A ) is positively correlated with the Langmuir volume ( V L ) under the three-peak transverse relaxation time ( T 2 ) spectrum. The fractal dimension of all effective T 2 points under saturated water ( D NMR ) is positively correlated with V L and the adsorption pore volume, but negatively correlated with the volume ratio of seepage pores and fractures. The free flow space fractal dimension ( D M ) is negatively correlated with the porosity of full saturated water (Φ F ) and the porosity of movable water (Φ M ). There is a negative correlation between Φ F and the seepage space fractal dimension ( D S ) in the coal samples with one-peak and two-peak T 2 spectra, but a positive correlation can be found with the three-peak T 2 spectrum. Therefore, it is necessary to consider the types of T 2 spectral peak as a prerequisite to analyze the correlations between pore-fracture parameters and NMR fractal dimensions. With the increase of coal rank, the adsorption pore content, Φ F , and bulk volume immovable (BVI) fraction first increase and then decrease, whereas the seepage pore content, fracture development, bulk volume movable (BVM) fraction, and BVM/BVI first decrease and then increase. The inflection points of these changes correspond to the maximum vitrinite reflectance ( R o,max ) at 2.6-2.8%, which would be attributed to the third coalification jump. Generally, D A is the fractal dimension representing the coal pore surface, and D S and D M are closely related to the pore structure. Furthermore, D NMR not only represents the roughness of the pore surface but also the complexity of the pore structure.