Characterization of the coal pore system by resistivity and NMR methods

ABSTRACT

The development of the coal pore system is extremely significant for gas disaster prevention in coal mines and coalbed methane exploitation. To accurately, quantifiable, and non-destructively characterize the coal pore system, the relationship between total porosity, effective porosity, pore size distribution, and resistivity of coals with different degrees of metamorphic was studied by resistance test and low field NMR techniques. The dimensionless parameter ρ_P that characterize the coal pore system was defined by the variety of resistivity in dry and saturated conditions. The achieved results are presented as follows: (1) The greater the ρ_P (the influence of the coal pore system on the conductivity), the larger the total porosity; (2) The proportion of effective pores increased linearly with the addition of the ρ_P; (3) The proportion of micropores and small pores decreased linearly as the ρ_P increased, revealed that these pores had poor connectivity and complicated structure, and had a significant negative effect on the conductivity of saturated coal. (4) The proportion of mesopores and macropores gained linearly as the ρ_P increased, indicated these pores had well connectivity, and had a positive influence on the conductivity. The results showed that the variety of coal resistivity from dry to the saturated condition was mainly controlled by the pore system. Therefore, a new method to characterize the coal pore system was proposed, which was exceedingly effectively, conveniently, and non-destructively.

KEYWORDS:

• Resistivity
• nuclear magnetic resonance
• porosity
• effective porosity
• pore size distribution

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Funding

This work was supported by National Key Research and Development Program of China (2018YFC0808100), National Natural Science Foundation of China (51734007;51604101;51704099), Funded by China Postdoctoral Science Foundation Grant (2019M652536). This project was also supported by the Henan Postdoctoral Foundation (001801016), Key R&D and Promotion Special Projects of Henan Province (192102310511), State Key Laboratory Cultivation Base for Gas Geology and Gas Control (Henan Polytechnic University) (WS2017B06), Supported by Henan Postdoctoral Foundation, Funded by the Doctoral Fund of Henan Polytechnic University (No.B2018-59).