Influence of Matrix Shrinkage and Compressibility on Gas Production From Coalbed Methane Reservoirs

Abstract

Abstract The effect of methane desorption on gas flow through coal was investigated. Adsorption/desorption isotherms were established for coal samples. Permeability of cylindrical samples to different gases was measured at simulated in-situ conditions in a triaxial stress field. Gas pressure-permeability-desorption relationship was established for helium, methane and carbon dioxide. Related experiments measured coal compressibility, and volumetric changes of coal matrix with methane adsorption and desorption. The hypothesis that desorption of methane shrinks coal matrix, and results in increased permeability was confirmed. The increase in matrix permeability with reduction in pressure from 1000 psi to atmospheric was highest for carbon dioxide (the most adsorptive gas), on the order of a factor of five for methane, and non-existent for helium. The corresponding volumetric shrinkage of coal matrix on desorption of methane was 0.6%. When measured values of compressibility and shrinkage were used as input parameters into a reservoir simulation model, 60% more gas was produced over a five year period.