Modelling permeability for coal reservoirs: A review of analytical models and testing data

As with other reservoir types permeability is a key controlling factor for gas migration in coalbed methane reservoirs. The absolute permeability of coal reservoirs changes significantly during gas production, often initially decreasing but then increasing as the reservoir pressure and gas content is drawn down. It has also been observed to decrease markedly during CO2 injection to enhance coalbed methane recovery. In order to predict gas migration models for coal permeability must represent the mechanisms leading to these observed behaviours. The permeability of coal reservoirs behaves in a similar fashion to other fractured reservoirs with respect to effective stress, decreasing exponentially as the effective stress increases. However a unique effect of coal is that it shrinks with gas desorption and swells with adsorption. Within the reservoir this swelling/shrinkage strain leads to a geomechanical response changing the effective stress and thus the permeability. Modelling coal permeability incorporating the impacts from both effective stress and coal swelling/shrinkage dates back about 25 years. Since then a number of permeability models have been developed. In recent years this topic has seen a great deal of activity with a growing body of research on coal permeability behaviour and model development. This article presents a review of coal permeability and the approaches to modelling its behaviour. As an important part of this, the field and laboratory data used to test the models are reviewed in detail. This article also aims to identify some potential areas for future work.