How gas adsorption and swelling affects permeability of coal: A new modelling approach for analysing laboratory test data

• A new modelling approach for analysing laboratory permeability data is proposed.
• Applied to permeability data for different gases on two coals reported in literature
• Established an equation describing how swelling strain affects cleat permeability
• The equation allows for estimation of swelling strain partition ratio.

Permeability models developed for coalbed reservoirs are generally not suitable for describing permeability response under fixed confining pressure in the laboratory test conditions where the core samples are allowed to expand with adsorption of gas. This paper presents a novel modelling approach to analyse the laboratory permeability data by seeking to isolate the impact of swelling strain on the permeability, in the form of a swelling strain term, from the test data for different gases. Two sets of comprehensive permeability and swelling data for three pure gases on two different (low rank) coals reported in the literature were analysed and it was found that the swelling strain terms thus obtained for the two samples can be described using a common empirical equation of the free swelling strain. The empirical equation (with two fitting constants) was then used to predict permeability response of the same cores to the flow of a binary gas mixture of N2 and CO2 based on the measured free swelling strain data. Good agreement with the measured permeability data was achieved for one of cores over the entire test pressure range. Comparison between the empirical swelling strain equation and the corresponding theoretical strain equation under uniaxial strain conditions reveals that the ratio of the two constants determined for the two cores can be related to their elastic properties. It is further shown that the establishment of the empirical swelling strain equation allows for the estimation of the part of the swelling strain (and thus the partition ratio) that contributes to the reduction in the permeability under fixed confining pressure conditions.