Quantitative characterization of fracture structure in coal based on image processing and multifractal theory

Abstract

The characterization of fracture structures is key for evaluating the permeability of coalbed methane (CBM) reservoirs, and an interconnected fracture network is essential for the production of CBM. Taking the Upper Permian coal in the eastern Yunnan and western Guizhou region as an example, we systemically studied the characteristics of coal deformation and fracture development through direct observation of coal faces and hand specimens. An image processing technique was used to obtain high-resolution fracture images (with an actual resolution of approximately 2 μm), followed by the statistics of the porosity parameters such as micro-fracture density, intersection point density, and plane areal porosity. The multifractal theory was applied to study the micro-fracture structures of the coal, and a practical algorithm and corresponding technical procedure was proposed to characterize the multifractal characteristics of coal fractures. The quantitative characterization of the heterogeneity of the whole and partial fracture structure was then performed, and the indicating significance of the generalized dimension spectrum, the singularity spectrum, and the calculated characteristic parameters was described. Our results suggest that the micro-fracture structures of coal are multifractal, which can be measured by combining image processing and multifractal methods. The fracture development and the distribution heterogeneity in the target coal seams show significant differences, both vertically and laterally. Coal seam groups with well-developed fracture systems and high connectivity were identified by combining macro- and micro-scale fracture observations and multifractal characterization, including the 8–12 coal seam group in the Tucheng mining area, the 3–7 group of the Yueliangtian mine in the Pan'guan mining area, the 8–12 group in the Pan'guan mining area, the 13–18 group in the Enhong mining area, and the 3–7 group in the Laochang mining area.