Coal Structure Characteristics in the Northern Qinshui Basin and Their Discrimination Method Based on the Particle Size of Drilling Cuttings.

Abstract

Coal structure, including primary coal, cataclastic coal, granulated coal, and mylonitized coal, is one of the key factors controlling coal and gas outburst, and it also determines the efficiency of coalbed methane (CBM) extraction. Therefore, it is significant to identify the characteristics of coal structures and to predict them in advance. In this work, the spatial distribution, mechanical properties, and microscopic morphology of the four coal structures from the No. 3 coal seam of the Xinyuan Mine in the northern part of the Qinshui Basin were investigated through the in situ observation in the roadway, the hardness coefficient (f) test, and the scanning electron microscope analysis. Moreover, the drilling cuttings from the gas pressure releasing holes were sampled and sieved, and then, the correspondence between different coal structures and the particle size of the cuttings was analyzed quantitatively based on the Rosin–Rammler model. The result shows that the spatial distribution of the coal structure has strong heterogeneity in the vertical and lateral directions. The f value decreases successively with the increase in coal structure deformation, which indicate that f can directly characterize the coal structure. Furthermore, the relations between f and drilling cutting average particle size (d0), crushability indicator (n), crushing degree index (λ), and the median diameter (d50) were established. Specifically, the coal deformation degree is positively correlated with the mass fraction of large particles in the cuttings under the same drilling parameters. Overall, as f increases, d0 and d50 decrease, and n and λ increase. However, parameters d0, d50, and λ of granulated coal are inconsistent with other coal structures, and mylonitized coal is inconsistent with other coal structures in n, as a result of the coal structure broken characteristics itself and the difference in the stress state between the coal and the drill bit during the rock breaking process. Ultimately, the coal structures determined by the surface CBM well logging curve and the cuttings particle size method were compared, and they have a high degree of coincidence in the distinction between primary coal and tectonic coal.