Characteristics and generation mechanisms of coal fines in coalbed methane wells in the southern Qinshui Basin, China

Abstract

Coal fines produced in coalbed methane (CBM) wells from various geologic settings at different well production stages from coal seam #3 in the southern Qinshui Basin were investigated. Particle size measurements, micro-morphology and mineralogy using grain size analysis, environmental scanning electronic microscopy (ESEM), X-ray diffraction (XRD), and a coal fines transport experiment were performed. By comparing grain size compositions of the coal fines from the underground coal mines and drill cuttings, the genetic types and mechanisms of coal fines from different production stages of CBM wells were described, and their significance to CBM production was discussed. The results indicate significant differences in particle size, micro-morphology, mineralogy, and genetic type of coal fines produced during different stages of CBM wells. During the water production stage, coal fines in CBM wells are primarily produced by washing the fracture surfaces with the fracturing proppant. Coal fines of this genetic type are relatively small and have sheet and flat block shapes because of grinding by the proppant. Brittle minerals, such as quartz and pyrite, are the major minerals in these coal fines because of the intermixing of quartz from the proppant and the preferential mineral transport and enrichment during pulverization; additionally, the brittle minerals content exceeds that in the raw coal by a factor of five or more. During the gas production stage, coal fines from CBM wells primarily consist of coal that was originally crushed by tectonic stress, generated between partings and the coal floor or in a fault zone. Brittle minerals are formed by shearing along the fault or via coal bed fragmentation in the floor rock during folding. In this case, the coal fines are relatively large and have block or thick, flat block shapes, suggesting that the coal fines have not experienced strong shearing after their formation. Similar to the coal fines from the water production stage, the minerals in the coal fines are mostly brittle, although their degree of enrichment is relatively low. Overall, the characteristics and genetic mechanisms of coal fines determine their grain size, total quantity, and the extent to which they can be transported by fluids. Coal fines play an important role in determining the coal structure, the development of the coal deposit, and the parameters of methane production management and control.