ESTIMATION OF CHANGES IN FRACTURE POROSITY OF COAL WITH GAS EMISSION

This paper discusses the results of a laboratory study aimed at measuring the changes in the coal matrix volume with release of gas, and estimating the resulting changes in the cleat porosity and permeability of coal. The volumetric strain of the matrix of coal samples held at constant external pressure was measured with decreasing concentration of methane. From the results and a simple geometry for fractured reservoirs, the variations in the cleat porosity and permeability with decreasing pressure were estimated. It is shown that the coal matrix volume decreases steadily as the methane pressure decreases. This matrix volumetric strain results in an increase in the cleat porosity of coal of as much as 80%, depending on the initial cleat porosity. This in turn results in as much as a sixfold increase in the cleat permeability of the coal. There is a linear relation between the coal matrix volumetric strain and the quantity of gas desorbed.