NMR-Based Investigation of Pore–Fracture Structure Heterogeneity in Deep Coals of Different Macrolithotypes in the Daning-Jixian Block, Ordos Basin

Wei Zhang, Zheng Zhang (), Liheng Bian, Rui Shi, Hewei Zhang and Jian Shen
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Wei Zhang: Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of Education of China, China University of Mining and Technology, Xuzhou 221116, China
Zheng Zhang: Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of Education of China, China University of Mining and Technology, Xuzhou 221116, China
Liheng Bian: China United Coalbed Methane National Engineering Research Center Co., Ltd., Beijing 100095, China
Rui Shi: Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of Education of China, China University of Mining and Technology, Xuzhou 221116, China
Hewei Zhang: Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of Education of China, China University of Mining and Technology, Xuzhou 221116, China
Jian Shen: Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of Education of China, China University of Mining and Technology, Xuzhou 221116, China

Energies, 2024, vol. 17, issue 23, 1-16

Abstract: Deep coalbed methane (CBM) demonstrates significant production potential, and a fervent exploration and development boom is currently underway in China. The permeability of coal reservoirs is heavily influenced by pore–fracture structure heterogeneity. Some researches have been conducted on deep coals’ pore–fracture structure; however, these studies mostly consider coal as a homogeneous material, neglecting the heterogeneity of the macrolithotypes within the coal. In this study, 33 deep coals with burial depths of more than 2000 m were obtained from the Daning-Jixian block of the Ordos Basin, covering all macrolithotypes: bright coal (BC), semi-bright coal (SBC), semi-dull coal (SDC), and dull coal (DC). These samples were subjected to three sets of NMR tests in dry, fully saturated, and irreducible water conditions, with the pore–fracture structure characteristics being analyzed. The results demonstrate that the sampled deep coals’ pore–fracture structure is highly heterogeneous, with transitional pores being dominant, followed by mesopores, “macropores and fractures”, and micropores. The NMR T 2C ranges from 0.61 to 2.44 ms, with an average of 1.19 ms; a higher T 2C value indicates more developed micropores. The ranges for producible water porosity ( φ pr ) and producible water saturation ( S pr ) are 0.31–7.24% (avg. 2.42%) and 6.97–71.47% (avg. 31.06%), respectively. Both of them exhibit a high positive correlation with the total volumes of “macropores and fractures” and mesopores. Compared to SDC and DC, the BC and SBC, especially the former, overall contain more “macropores and fractures” and mesopores, fewer transitional pores and micropores, and higher φ pr and S pr . These findings suggest that regions with abundant BC and SBC should be prioritized during deep CBM exploration and production due to the inherently superior permeability and gas extraction potential of BC and SBC, and these coals are likely to require less intensive stimulation to achieve higher recovery rates and could provide more sustainable gas production over time.

Keywords: deep coalbed methane; pore–fracture structure heterogeneity; macrolithotype; NMR; Ordos Basin (search for similar items in EconPapers)
JEL-codes: Q Q0 Q4 Q40 Q41 Q42 Q43 Q47 Q48 Q49 (search for similar items in EconPapers)
Date: 2024
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