An improved brittleness characterization method for deep coalbed methane reservoirs and its application to well logs

Authors

• HUANG An’er, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, ChinaFollow
• GONG Fei, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China; State Key Laboratory for Fine Exploration and Intelligent Development of Coal Resources, Beijing 100083, ChinaFollow
• PENG Suping, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China; State Key Laboratory for Fine Exploration and Intelligent Development of Coal Resources, Beijing 100083, China
• ZOU Guangui, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China; State Key Laboratory for Fine Exploration and Intelligent Development of Coal Resources, Beijing 100083, China
• ZHANG Zhaoji, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China
• WU Yiquan, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China
• RONG Zhihong, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China
• LI Qingrun, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China

Abstract

Background Deep coalbed methane (CBM) reservoirs are characterized by high pressure, low porosity, and low permeability. Therefore, large-scale volume fracturing is required for their commercial development. As an important engineering sweet spot parameter for selecting optimal fracturing targets, the brittleness index can reflect the difficulty in generating complex fracture networks during fracturing. However, existing brittleness characterization methods are largely applicable to strata with high brittle mineral content, such as shales and sandstones, while methods for characterizing the brittleness of deep CBM reservoirs with high organic matter content and low brittle mineral content are lacking. Methods Rock physics experiments on 20 primary coal samples reveal that the elastic parameters of the samples exhibited significant directional dependence and correlated negatively with reservoir porosity. Based on the rock physical properties of primary coals, this study proposed an improved brittleness index ($ I_{\text{B}}^{*} $) tailored to deep CBM reservoirs. Furthermore, the sensitivities of the improved brittleness index were analyzed based on experimental results and theoretical modeling. Results The analytical results indicate that compared to commonly used brittleness indices, the improved brittleness index demonstrated higher sensitivity to the physical properties of coals, delivering distinct advantages in identifying the high-brittleness areas of deep gas-bearing coal seams with relatively developed matrix pores and fractures. Lamé constant λ was identified as the dominant factor governing the responses of the improved brittleness index to reservoir porosity and fluids, while Poisson's ratio further enhanced the sensitivity of the index. Additionally, Lamé constant μ exerted the most significant influence on the responses of the improved brittleness index to the fracture density of coal reservoirs. Finally, the improved brittleness index was applied to well logs for reservoir assessment. The application results reveal that the improved brittleness index outperformed other brittleness indices in identifying local brittleness extrema of coal seams. Conclusion The improved brittleness index exhibits high applicability to deep CBM reservoirs. It enables a fine-scale characterization of the relative distribution characteristics of reservoir brittleness, holding great positive significance for the exploration and exploitation of deep CBM.

Keywords

deep coalbed methane (CBM), anisotropy, elastic property, rock physics model, brittleness index

DOI

10.12363/issn.1001-1986.26.04.0191

Recommended Citation

HUANG An’er, GONG Fei, PENG Suping, et al. (2026) "An improved brittleness characterization method for deep coalbed methane reservoirs and its application to well logs," Coal Geology & Exploration: Vol. 54: Iss. 9, Article 12.
DOI: 10.12363/issn.1001-1986.26.04.0191
Available at: https://cge.researchcommons.org/journal/vol54/iss9/12

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