An improved productivity model in coal reservoir and its application during coalbed methane production

Permeability of coal reservoir is a key parameter affecting coalbed methane (CBM) productivity. The dynamic variation of permeability depends on the effect of effective stress, coal matrix shrinkage and gas slippage during CBM well drainage. On the basis of S-D stress-permeability model, a dynamic prediction model of coal reservoir permeability considering effective stress, coal matrix shrinkage and gas slippage effect (is called "three effects") is established. The dynamic change of permeability in CBM development is revealed. It shows that permeability initially drops and then rises. An improved CBM productivity model considering "three effects" is derived, and the influence law of "three effects" on CBM production during CBM well drainage is revealed. The results of the theoretical model are compared with the field data, and the reliability of the theoretical model is further verified. During CBM production, with the decrease of reservoir pressure, the effective stress increases. Before reservoir pressure reaches the critical desorption pressure, the effective stress effect is dominant and the permeability of coal reservoir declines; after the reservoir pressure declines to below the critical desorption pressure, the coal matrix shrinkage effect will play a dominant role in the permeability variation, and the permeability will start to increase at lower pressure. It is critical not only to avoid too much initial pressure drawdown, which decreases well productivity; but also to ensure matrix shrinkage effect improve the permeability and productivity of coal reservoirs in longer term.