A workflow to build a model investigating coal cleat upscaling conditioned by the lithotype

Abstract

Cleats are natural macroscopic fractures in coalbeds. The distribution of cleats inside network patterns is an important attribute as it influences mechanical behaviour, effective permeability and gas production performance. Various publications on geological and geotechnical investigations show relationships between rank class, lithotype and cleat frequency, and it is well known that the block size of coalbeds affect coal strength. These attributes are commonly tested for and numerically modelled independently without regard for any natural sequence (e.g. dulling upward, brightening upward, randomly distributed) within the coalbed. This paper presents a developing workflow with a methodology to set up a numerical modelling using the coal lithotype information as its basis. The contribution of each lithotype to the whole behaviour of the coalbed under transient stress conditions, and then to the larger rock mass, is considered a critical component. The main issues observed include: reliable information exists for individual bright bands but not for mixed lithotypes; neither a correlation between the individual bands and the lithotypes, nor between the lithotype and the coal seam, have been proposed; derived geotechnical properties are not assumed to be conditioned by the individual lithotypes; and, upscaling coal geotechnical properties from laboratory tests remains uncertain. These matters, therefore, should be pursued to allow some quantification of the heterogeneous nature of coal and to capture the combined effects on coalbed strength. It is anticipated that the workflow still requires validation by running a set of numerical experiments for proposed scenarios.