Petroleum Reservoir Evaluation and Development >

2020 , Vol. 10 >Issue 4: 17 - 24

DOI: https://doi.org/10.13809/j.cnki.cn32-1825/te.2020.04.003

CBM Geology

Differences in methane adsorption and desorption characteristics of low, medium and high rank coal reservoirs at different temperatures

  • Dongmin MA ,
  • Zheng GAO ,
  • Yue CHEN ,
  • Hui ZHANG ,
  • Kai SHAO ,
  • Zhicang ZHANG ,
  • Xun WU ,
  • Fu YANG
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  • 1. College of Geology and Environment, Xi’an University of Science and Technology, Xi’an, Shaanxi 710054, China
    2. Key Laboratory of Coal Resources Exploration and Comprehensive Utilization, Ministry of Natural Resources, Xi’an, Shaanxi 710021, China
    3. Geological Research Institute for Coal Green Mining, Xi’an University of Science and Technology, Xi’an, Shaanxi 710054, China
    4. Shaanxi Coalbed Methane Development and Utilization Co., Ltd., Xi’an, Shaanxi 710119, China

Received date: 2019-12-31

  Online published: 2020-08-07

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Abstract

In order to study the difference in adsorption/desorption characteristics and the desorption hysteresis of CH4 in different coal ranks, low, medium, and high coal rank samples are collected for the experiments of micro-component determination, liquid nitrogen adsorption, and isothermal adsorption/desorption to systematically analyze the composition, pore structure, differences in adsorption/desorption characteristics and desorption hysteresis effects of different coal rank sample materials. Combined with the calculation results of methane adsorption heat, the mechanism of coalbed methane desorption hysteresis from the energy perspective is discussed. The results show that: ①The reflectance Ro,max of the vitrinite group of coal samples are 0.43 %, 1.26 %, and 3.27 %, respectively. Low-rank coal samples have low vitrinite group content, high inert matter group content, high volatile content and fixed carbon. Medium-rank and high-rank samples are reversed. The degree of coal metamorphism increases. Porosity, BET specific surface area, BJH total pore volume, and fractal dimension D2 change in a “V” shape, and D1 changes in an inverted “V” shape. ②Under isothermal condition, the residual adsorption amount becomes larger as the coal rank rises, and the desorption becomes more difficult. As the temperature rises, the residual adsorption capacity increases first and then decreases. With 40 ℃ as the inflection point, the temperature influences both the degree of activation of gas molecules and the pore structure of coal. ③When the pressure is the same, the higher the coal rank, the faster the methane adsorption rate at the low-pressure stage（p<4 MPa）, the adsorption capacity increases rapidly at the high-pressure stage（p>4 MPa）, and the adsorption capacity does not increase significantly. ④For the three coal samples, DFS4 #, SGZ11 #, and SH3 #, in the desorption process, their isosteric heat of adsorption are all greater than that in the adsorption process, indicating that the desorption process needs to continuously absorb heat from outside. Therefore, the methane in the adsorption state needs to absorb energy from the external environment, and the energy difference between the adsorption and desorption processes will cause desorption hysteresis. The methane in the free state enters into the micropores due to high pressure, resulting in the expansion and deformation of coal matrix, changes in pore structure, limit of methane desorption, and finally a desorption hysteresis effect.

Key words： metamorphic degree; isothermal adsorption; isobaric adsorption; fractal dimension; adsorption heat; desorption hysteresis

Cite this article

Dongmin MA , Zheng GAO , Yue CHEN , Hui ZHANG , Kai SHAO , Zhicang ZHANG , Xun WU , Fu YANG . Differences in methane adsorption and desorption characteristics of low, medium and high rank coal reservoirs at different temperatures[J]. Petroleum Reservoir Evaluation and Development, 2020 , 10(4) : 17 -24 . DOI: 10.13809/j.cnki.cn32-1825/te.2020.04.003

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