ABSTRACT

This paper, second of a two-part series, discusses the results of the experimental work conducted to estimate the different compressibilities of coal under both unconstrained and constrained conditions. Under unconstrained conditions, the shrinkage or swelling compressibility (Cm) was measured, which was found with certainty to be a pressure-dependent parameter. The model proposed to estimate Cm was able to effectively predict the variation trend, although the modeled values were larger than those calculated using experimental results. The pore-volume compressibility (Cp) under uniaxial strain conditions for helium depletion was found to be a constant positive value. The value of Cp for methane depletion, however, was found negative, indicating that the pore volume (cleat) increases with depletion. Moreover, its absolute value increased with decreasing methane pressure. Consistent with field permeability observations, the permeability increases with methane depletion, and the rate of increase at lower pressures is higher than at high pressures. The proposed pore-volume compressibility model was well able to predict the variation.

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