Variable-temperature and variable-pressure adsorption characteristics of deep coalbed methane and their implications

Authors

• JIANG Wenping, CCTEG Xi’an Research Institute (Group) Co., Ltd., Xi’an 710077, ChinaFollow
• SUN Siqing, CCTEG Xi’an Research Institute (Group) Co., Ltd., Xi’an 710077, China
• JIN Xianglan, CCTEG Xi’an Research Institute (Group) Co., Ltd., Xi’an 710077, China
• TIAN Bofan, CCTEG Xi’an Research Institute (Group) Co., Ltd., Xi’an 710077, China
• TIAN Xinjuan, CCTEG Xi’an Research Institute (Group) Co., Ltd., Xi’an 710077, China

Abstract

Background Investigating the adsorption characteristics of coalbed methane (CBM) helps bridge key geological information concerning deep CBM, including occurrence state, free gas proportion, and gas-bearing properties, with the gas production potential and commercial exploitation of CBM wells. Such investigation represents a key component of the evaluation, exploration, and exploitation of CBM reservoirs. Methods Using deep coal samples collected from three representative regions, this study designed high-temperature and high-pressure methane isothermal adsorption experiments, together with variable-temperature and variable-pressure methane adsorption experiments, using nuclear magnetic resonance (NMR). The adsorption characteristics of the coal samples derived using the different experimental methods were subsequently compared. Results and Conclusions During high-temperature and high-pressure isothermal adsorption experiments conducted at 30 ℃, 50 ℃, 60 ℃, and 80 ℃, all coal samples exhibited similar bimodal NMR spectra. At a constant temperature, the total gas and adsorbed methane contents of the coal samples varied with pressure according to typical Langmuir curves, whereas the free gas content generally increased linearly with pressure. At the same pressure, the adsorbed, free, and total methane contents in the coal samples were consistently higher at low temperatures than at high temperatures. The experimental results also reveal that the adsorbed methane content was relatively sensitive to variations in pressure. In the variable-temperature and variable-pressure methane adsorption experiments conducted at 20 ℃ and 2.0 MPa, 30 ℃ and 6.0 MPa, 50 ℃ and 12.0 MPa, 60 ℃ and 15.0 MPa, 70 ℃ and 19.0 MPa, and 80 ℃ and 23.0 MPa), the NMR spectra were aligned with those derived from the isothermal adsorption experiments. Specifically, as pressure and temperature increased gradually, the total and adsorbed methane contents of the coal samples increased in accordance with the Langmuir curves, while the free gas content showed a roughly linear growth trend. In both types of experiments, the maximum proportion of free gas emerged at 80 ℃ and 23 MPa. Accordingly, the free gas proportion at a burial depth of approximately 4000 m was estimated to be not greater than 40%, suggesting that deep CBM remains predominantly adsorbed gas and that there are no minimal substantial differences in CBM storage mode between deep and medium-to-shallow coal seams. The total, adsorbed, and free methane contents derived from the variable-temperature and variable-pressure adsorption experiments were consistently higher than those obtained from the isothermal adsorption experiments. However, the results of the isothermal adsorption experiments at the final pressure were very close to those obtained from the variable-temperature and variable-pressure adsorption experiments under corresponding temperature and pressure conditions. Therefore, when conventional isothermal adsorption experiments are employed for a rough analysis of methane adsorption characteristics, the experimental temperature and final pressure are suggested to match the actual reservoir temperature and pressure. In contrast, variable-temperature and variable-pressure adsorption experiments should be used for precise analyses of methane adsorption characteristics and their differences. The results of the variable-temperature and variable-pressure adsorption experiments demonstrate that the methane adsorption rate decreased and even tended to stabilize at approximately 15 MPa, while an inflection point in the free gas content curve also occurred at this pressure. These findings indicate that CBM reservoirs at a burial depth of approximately 1500 m tend to undergo abrupt changes or exhibit unique characteristics. From the perspective of adsorption potential theory, this study elucidated the theoretical basis for identifying the burial depth threshold of deep CBM. Overall, the results of this study provide a significant reference for further understanding the reservoir characteristics, gas-bearing properties, and resource exploitation potential of deep CBM.

Keywords

deep coalbed methane (CBM), adsorbed gas, free gas, high-temperature and high-pressure isothermal adsorption experiment, variable-temperature and variable-pressure adsorption experiment

DOI

10.12363/issn.1001-1986.26.05.0309

Recommended Citation

JIANG Wenping, SUN Siqing, JIN Xianglan, et al. (2026) "Variable-temperature and variable-pressure adsorption characteristics of deep coalbed methane and their implications," Coal Geology & Exploration: Vol. 54: Iss. 9, Article 5.
DOI: 10.12363/issn.1001-1986.26.05.0309
Available at: https://cge.researchcommons.org/journal/vol54/iss9/5

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