Controls on coal cleat spacing

Abstract

This study was undertaken to determine the relationship between cleat spacing, cleat height and coal banding texture for Queensland Permian age coals of different rank, four of which are presented here. Whereas relationships between cleat frequency and rank, and with coal type or grade, have been reported in the past, relationships between the spacing and height among the different kinds of cleats are not quantitatively established. For other layered sedimentary rocks, joint or fracture spacing relates directly to both bed thickness and rock strength. Coal is similar to other layered rocks. Four major classes of cleats were distinguished, which were separate data populations when cleat spacing was plotted against cleat height; master cleats, single vitrain layer cleats, multiple vitrain layer package cleats, and durain (dull coal) cleats. Understanding the relationship between cleat height and spacing for specific coals, and the specific kinds of cleats within those coals, will lead to more accurate predictions of cleat density and hence coal permeability. This can improve modelling and prediction of methane gas deliverability in coal seams. In the Australian Permian coals studied, narrowly spaced cleats exist at all ranks, but the distribution of cleat spacing with cleat height is what varies for specific cleat classes. Cleat spacing was found to be directly proportional to cleat height in most cases.