Occurrence pattern of adsorbed and free gas in deep coal seams under the control of pore structure: A case study of the No. 8 coal seam in the Daning-Jixian block along the eastern margin of the Ordos Basin

Authors

• BIAN Liheng, Key Laboratory of Coalbed Methane Resources & Reservoir Formation Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, China; School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China; PetroChina Coalbed Methane Company Limited, Beijing 100028, ChinaFollow
• SHEN Jian, Key Laboratory of Coalbed Methane Resources & Reservoir Formation Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, China; School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, ChinaFollow
• SHI Rui, Key Laboratory of Coalbed Methane Resources & Reservoir Formation Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, China; School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China
• ZHANG Wei, PetroChina Coalbed Methane Company Limited, Beijing 100028, China
• MA Zhuang, PetroChina Coalbed Methane Company Limited, Beijing 100028, China
• LIANG Xuemin, PetroChina Coalbed Methane Company Limited, Beijing 100028, China
• DONG Jiying, Key Laboratory of Coalbed Methane Resources & Reservoir Formation Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, China; School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China
• ZHU Peipei, Key Laboratory of Coalbed Methane Resources & Reservoir Formation Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, China; School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China
• MA Tianyu, Key Laboratory of Coalbed Methane Resources & Reservoir Formation Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, China; School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China

Abstract

Background Deep coalbed methane (CBM) holds an important strategic position in China's unconventional natural gas system. However, concerning deep CBM occurrence, there remains a lack of systematic quantitative understanding of the underlying mechanisms and the controlling effects of pore structure. Methods This study investigated high-rank coal samples collected at a burial depth of approximately 2000 m from the No. 8 coal seam in the Daning-Jixian block along the eastern margin of the Ordos Basin. Using multiscale pore characterization techniques, including low-temperature liquid nitrogen adsorption, high-pressure mercury injection, and low-temperature CO₂ adsorption, the full-scale pore size distribution of the coal samples was determined. In combination with computed tomography (CT) scanning and gas content measurement through pressure-retaining coring, this study conducted a quantitative analysis of the mechanisms underlying the coexistence of multiphase gas in deep coal reservoirs. Results and Conclusions The deep coal reservoirs in the study area showed a highly heterogeneous pore structure, characterized by a dumbbell-shaped pore volume distribution. Specifically, micropores (<2 nm) and macropores (>50 nm) constituted the primary reservoir spaces, while mesopores were underdeveloped. The specific surface area of pores exhibited a highly concentrated, unimodal distribution pattern, with over 99% attributed to micropores. This finding highlights the dominant role of adsorbed gas in the coal reservoirs. Pore structure is identified as the fundamental factor controlling the binary occurrence pattern of adsorbed and free gas. Micropores store adsorbed methane due to the high potential energy of adsorption, while macropores and partial mesopores provide reservoir spaces and migration pathways for free gas. Consequently, adsorbed and free gas jointly form a stable synergistic equilibrium system under deep high-pressure conditions. Based on the theory of volume filling of micropores (TVFM) and the gas equation of state, a quantitative calculation method for gas content was developed. The calculation results are highly consistent with the measured data, validating the reliability and applicability of the method. The proposed method, grounded in the CBM occurrence mechanisms controlled by pore structure, is primarily affected by the differential pore structure of coal seams. This method enables the theoretical assessment and analysis of gas content in deep coal reservoirs, thereby providing a theoretical basis and technical support for the resource potential assessment, optimal development zone selection, and production system optimization of deep CBM.

Keywords

deep coalbed methane (CBM), pore structure, adsorbed gas, free gas, coexistence, Ordos Basin

DOI

10.12363/issn.1001-1986.26.01.0026

Recommended Citation

BIAN Liheng, SHEN Jian, SHI Rui, et al. (2026) "Occurrence pattern of adsorbed and free gas in deep coal seams under the control of pore structure: A case study of the No. 8 coal seam in the Daning-Jixian block along the eastern margin of the Ordos Basin," Coal Geology & Exploration: Vol. 54: Iss. 9, Article 6.
DOI: 10.12363/issn.1001-1986.26.01.0026
Available at: https://cge.researchcommons.org/journal/vol54/iss9/6

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