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 ( d 0 ), crushability indicator ( n ), crushing degree index (λ), and the median diameter ( d 50 ) 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, d 0 and d 50 decrease, and n and λ increase. However, parameters d 0 , d 50 , 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.

Dynamic characteristics and crack evolution laws of coal and rock under split Hopkinson pressure bar impact loading

“…Since the number of broken blocks is difficult to count, a screening test is carried out. To realize the quantitative expression of the degree of coal and rock fragmentation, Mandelbrot [74] and Zhao et al [75] established a mass-frequency relationship model based on the screening method, as shown in equation ( 6):…”

SDBS-Induced Dispersion Effect of High-Rank Coal Fines to Inhibit the Conductivity of Propping Fractures

“…Lyu et al and Yuan et al believed that natural fractures provide a direct flow channel for CBM and coalbed water, and it can be concluded that the distribution of fracture systems in the reservoir scale can significantly control the hydraulic fractures. , The tectonics of the CBM reservoir is complex, and the coal cementation is poor in China, especially the high brittleness of high-rank coal, which is easy to produce coal fines during the process of drilling, fracturing, and production (Figure ). , The sources of coal fines can be divided into five types: (1) primary coal fines from coal seams; (2) tectonic processes; (3) mechanical interference, such as hydraulic erosion, shear, and wear of the coal surface caused fluid during drilling and hydraulic fracturing; (4) the elastic self-regulation effect of coal seams when the stress of the coal matrix changes; and (5) flushing with gas and liquid during reverse discharge and production. − During fracturing fluid backflow, the generated coal fine particles will enter the wellbore with flowing water, thereby blocking the underground water pump . Additionally, coal fines may block or deposit coal seam cleats, pores, and cracks during migration, , reducing the permeability of propping fractures and leading to a significant decrease in gas productivity.…”