Effect of matrix shrinkage on permeability of coalbed methane reservoirs

The dynamic nature of coalbed methane reservoir permeability makes the continuous modeling of the flow process difficult. Knowledge of conventional reservoir modeling is of little value because the gas storage and flow mechanisms in coal are remarkably different. Field observations suggest that the gas productivity does not decline with time as expected. An increasing permeability is a possible explanation. This laboratory study is aimed at measuring the volumetric changes in coal matrix resulting from gas desorption, its impact on coal porosity and permeability, and an estimate of the stage when the effect is significant. Results using cylindrical samples of coal suggest that coal matrix shrinks with desorption of gas. The shrinkage is linearly proportional to the quantity of gas desorbed. Using the measured changes in matrix volume, variations in total and cleat porosity were estimated and found to increase significantly with desorption. These changes should, therefore, be used as input when simulating long-term gas production