Spatial topological configurations of cleats and geometrical quantitative characterization of matrix blocks in deep coal seams

Authors

• 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
• LI Wei, 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
• TIAN Yongjing, CNOOC Research Institute Co., Ltd., Beijing 100028, 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
• ZHANG Hewei, 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
• 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

Abstract

Objective Cleats in deep coal seams are characterized by strong heterogeneity, and their topological configurations, as well as their topological relationships with matrix blocks, remain poorly understood. On the other hand, the assumptions regarding the size and geometry of matrix blocks adopted in representative reservoir structure models and numerical simulations do not fully reflect actual reservoir conditions. Methods Deep coal cores collected from the northeastern Ordos Basin were investigated in this study. Using CT scanning and topological theory, a multi-dimensional topological parameter system was established to characterize cleats in deep coal seams. Furthermore, four topological configurations of cleats in deep coal reservoirs were identified, and corresponding discriminant models were developed. Results Cleats in the deep coal cores exhibited the synergistic development of I-, Y-, and X-type nodes, accounting for average proportions of 59.7%, 33.8%, and 6.5%, respectively, in the same sample. Despite representing the lowest proportion, X nodes played a key role in driving the evolution of cleats from isolated structures to complex, interconnected networks, thereby enhancing their global connectivity. Based on the clustering of multidimensional topological parameters and convex tessellation theory, four topological configurations of cleats in deep coal seams were identified, primarily controlled by the combined effects of cleat development and network connectivity. Among these, the dense reticulation type (Type I) accounted for 4.18%. This topological configuration was governed by Platonic attractors and exhibited extremely developed fracture networks, with the matrix partitioned into highly interconnected, tiny rectangular blocks. The complex branch type (Type Ⅱ) accounted for 23.34%. It was characterized by dense branches accompanied by a large number of ineffective blind ends, with matrix blocks transitioning toward irregular hexagons. The branch transition type (Type Ⅲ) accounted for 55.06%. This topological configuration exhibited fracture networks with a moderate scale but efficient connectivity and approached Voronoi attractors, with matrix blocks approximating regular hexagons. The sparse isolation type (Type Ⅳ) accounted for 17.42% and was characterized by a limited number of effectively connected cleats. Accordingly, the matrix remained largely intact, with matrix blocks represented by large circles. The geometrical quantitative characterization of the matrix in deep coal seams indicates that with an increase in the cleat development, the matrix blocks evolved from Type Ⅳ [S_med = 2320.09 mm²; median diffusion distance L_med = 27.18 mm] toward types Ⅲ (S_med = 210.76 mm²; L_med = 7.80‒9.01 mm) and Ⅱ (S_med = 16.87 mm²; L_med = 2.21‒2.55 mm), ultimately broking into tiny rectangles (Type I; S_med = 4.75 mm²; L_med = 1.09‒1.54 mm). This transition in topological configuration resulted in dramatic changes in gas desorption and diffusion radii of the matrix. Conclusions The results of this study refine the understanding of the topological configurations of cleats in deep coal reservoirs, along with their control over the geometric characteristics and gas diffusion scales of the matrix. The discriminant diagrams developed in this study provide a practical tool for coal reservoir classification and the selection of optimal intervals. Furthermore, the qualitified sizes and gas diffusion radii of matrix blocks provide a basis for numerical simulation and reservoir stimulation design.

Keywords

deep coal seam, matrix-cleat, topological structure, convex tessellation theory, gas diffusion, northeastern Ordos Basin

DOI

10.12363/issn.1001-1986.26.06.0388

Recommended Citation

SHEN Jian, LI Wei, TIAN Yongjing, et al. (2026) "Spatial topological configurations of cleats and geometrical quantitative characterization of matrix blocks in deep coal seams," Coal Geology & Exploration: Vol. 54: Iss. 8, Article 8.
DOI: 10.12363/issn.1001-1986.26.06.0388
Available at: https://cge.researchcommons.org/journal/vol54/iss8/8

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