Deformation Behavior of Coal as a Composite Material and its Impacts on Permeability in Coalbed Gas Reservoir

Abstract

Coal is thought as a special engineering material which has been widely studied in mining engineering, specially its mechanical behavior. However a relatively recent but important development recognizes underground coal as coalbed methane (CBM) reservoirs providing potential reserves for CBM or CO sequestration enhanced CBM (ECBM) recovery. Success for processes of CBM/ECBM recovery from and CO sequestration into coal, in particular the deep coal, largely relies on the transport properties such as porosity and permeability which are dynamic and related to deformation behavior of coal, which has not yet been well-addressed. This paper describes an alternative approach to investigate the deformation behavior of coal under dynamic stress conditions through a model correlating the structural parameters of coal to its mechanical and transport properties. The model treats coal as a composite material using the major individual constituent components - cleats and matrix and their relative positions, aggregating these individual component properties to provide the coal material properties. With the model, the overall deformation behavior in terms of stress-strain correlation can be simulated numerically, which has been verified with experimental data obtained by triaxial compression tests of coals. Based on this approach, the overall deformation is attributed to deformations of cleats and matrix, providing the developed model with capability to evaluate impacts of cleats and matrix on transport properties of coal. As a result, the directional and deformation dependent permeability, one of the most important transport properties for coalbed methane reservoirs is further discussed