Investigation of coal pore and fracture distributions and their contributions to coal reservoir permeability in the Chan
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ORIGINAL PAPER
Investigation of coal pore and fracture distributions and their contributions to coal reservoir permeability in the Changzhi block, middle-southern Qinshui Basin, North China Zhigang Du 1,2,3 & Xiaodong Zhang 1,4 & Qiang Huang 2 & Shuo Zhang 1 & Chenlin Wang 1 Received: 1 December 2017 / Accepted: 22 July 2019 # Saudi Society for Geosciences 2019
Abstract Pores and fractures in coal have an important influence on gas desorption, diffusion, and seepage during coalbed methane (CBM) recovery. The occurrence and spatial distribution of pores and fractures in the Changzhi CBM block coals of the middle-southern Qinshui Basin, North China, were investigated by means of scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and low-temperature liquid nitrogen isotherm adsorption/desorption (LT-N2AD). In addition, pore and fracture contributions to the permeability of the coal reservoir were evaluated. The results show that there are many pores and fractures with different widths and lengths in the coals, most of which are isolated. The microfractures serve as a bridge for gas migration between pores, cleats, and macrofractures. Based on the relationship between cross-cutting microfractures and cleats, the multiple populations of microfractures in the coals can be classified as 1st class and 2nd class microfractures. Statistical analyses reveal that the aperture distributions of these fractures are heterogeneous and that the expected fracture aperture distributions can be described by a log-normal Gaussian function. Based on the statistically expected fracture aperture and its actions on gas flow, three levels of potential flow paths and three permeability domains are concluded to occur in coal reservoirs, including cleats, 1st class microfractures, and 2nd class microfractures. For this study area, due to the occurrence of mineralized cleats, the CBM reservoir permeability is significantly reduced. The current coal reservoir permeability contribution is mostly from the 1st class and 2nd class microfractures. Lastly, based on the pore and fracture spatial distributions of the coal and the three statistically expected permeability domains, the coupled pore-fracture network configuration models of coal reservoirs were constructed, which are useful for evaluating coal reservoir permeability. Keywords Qinshui Basin . Changzhi block . Pore and fracture . Reservoir permeability
Introduction Editorial handling: Barry Katz * Xiaodong Zhang [email protected] 1
School of Energy Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, Henan, China
2
School of Civil Engineering, Luoyang Institute of Science and Technology, Luoyang 471003, Henan, China
3
Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of Education (China University of Mining and Technology), Xuzhou 221008, Jiangsu, China
4
Collaborative Innovation Center of Coalbed Methane (Shale Gas) in Central Plains Economic Zone, Jiaozuo 454000, Henan, China
Coalbed methane (CBM) is now recog
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