Permeability changes in coal: Analytical modeling

Permeability changes can be very large during depletion of coalbed methane wells: up to 100 times in the San Juan basin. Analytic models of permeability increase during depletion are accessible, easy to use, and practical. They are more transparent, and less complicated, than coupled numerical models. However, there are minor differences between the analytic models, and these are reviewed here. There are also discrepancies in attempting to match field data, which is the final test of a model, and these are discussed. Two different sets of San Juan data have been modeled, after suppressing the pressure-dependent permeability, with reasonably consistent reservoir parameters. The matching parameters appear to be justifiable, the small porosities do not seem to be ruled out by reservoir experts, and the changes of elastic parameters (including pore–volume compressibility) with depletion do not seem unreasonable. The analytic models are used in reservoir simulators to model and predict injection of greenhouse gases in coal seams. Only when we can match the permeability increase of coalbed methane wells during depletion, will we have the confidence to use the analytic models in reservoir simulators to match or predict sequestration of greenhouse gases such as CO2.