Supersaturation effect and its characterization in gas-rich intervals of deep coal seams

Authors

ZHOU Lihong, China United Coalbed Methane National Engineering Research Center, Beijing 100095, China; PetroChina Coalbed Methane Co., Ltd., Beijing 100028, ChinaFollow
LI Yong, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, ChinaFollow
DING Rong, China United Coalbed Methane National Engineering Research Center, Beijing 100095, China; PetroChina Coalbed Methane Co., Ltd., Beijing 100028, China
LI Yun, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China
LI Yongzhou, China United Coalbed Methane National Engineering Research Center, Beijing 100095, China; PetroChina Coalbed Methane Co., Ltd., Beijing 100028, China
KONG Jintao, China United Coalbed Methane National Engineering Research Center, Beijing 100095, China; PetroChina Coalbed Methane Co., Ltd., Beijing 100028, China

Abstract

Objective and Method High free gas content serves as a key factor controlling the enrichment and high productivity of deep coalbed methane (CBM). However, its direct determination is challenging, making the accurate identification of free gas-rich coal seam intervals a core scientific issue for efficient exploration and recovery of CBM. This study investigated the Daning-Jixian block (also referred to as the Daji Block) within the Ordos Basin. Based on analyses and tests of the gas content, coal macrolithotype, physical properties, and log responses of cores collected from representative wells through pressure-retained coring, this study systematically assessed the vertical heterogeneity of the deep coal seams and identified the supersaturation effect and its influential factors. Results and Conclusions Deep coal seams in the Daji Block exhibit the supersaturation effect, where total gas content exceeds the maximum adsorbed gas content under in-situ geological conditions, establishing the deep coal seams as dominant gas-bearing intervals and gas pay zones. The gas content index (GCI; defined as the ratio of actual gas content to the Langmuir volume measured under in-situ geological conditions) was proposed as an indicator of free gas enrichment. GCI values of < 0.8, 0.8–1.2, and > 1.2 correspond to predominance of adsorbed gas, coexistence of adsorbed and free gas, and free gas enrichment, respectively. The coal seam intervals exhibiting the supersaturation effect (GCI > 0.8) are dominated by bright and semi-bright coals, characterized by favorable reservoir physical properties and high gas content, as well as low natural gamma-ray (GR) values (≤ 50 API), low density (≤ 1.5 g/cm³), and high sonic interval transit time (≥ 350 μs/m) in terms of log parameters. The correlation between the GCI values and log data was determined as follows: (1) GCI values > 1.2 correspond to peak total hydrocarbon values of > 50% and GR values while drilling of < 70 API; (2) 0.8 < GCI values < 1.2 are associated with peak total hydrocarbon values of 20–50% and GR values while drilling of 70–100 API; and (3) GCI values < 0.8 correspond to peak total hydrocarbon values of < 20% and GR values while drilling of > 100 API. Based on the vertical combinations of bright + semi-bright coals, semi-dull + dull coals, and gangue-bearing intervals, deep coal seams in the Daji Block are categorized into three types: thickly layered box type, sandwich type, and micro-sawtooth-shaped thickly layered types, with coal seams of the first type serving as the dominant intervals for the supersaturation effect due to its high gas content. By characterizing the gas-bearing properties of coal seams using the proposed GCI, this study determined the log parameters for identifying the supersaturation effect. This study holds great theoretical and practical significance for investigating the mechanisms behind gas enrichment and predicting sweet spot intervals for deep coal seams.

Keywords

deep coalbed methane (CBM), gas content, free gas, adsorbed gas, high-yield interval, Daning–Jixian block, Ordos Basin

DOI

10.12363/issn.1001-1986.25.07.0517

Recommended Citation

ZHOU Lihong, LI Yong, DING Rong, et al. (2025) "Supersaturation effect and its characterization in gas-rich intervals of deep coal seams," Coal Geology & Exploration: Vol. 53: Iss. 12, Article 14.
DOI: 10.12363/issn.1001-1986.25.07.0517
Available at: https://cge.researchcommons.org/journal/vol53/iss12/14

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