Modeling gas-adsorption-induced swelling and permeability changes in coals

作者：Chareonsuppanimit Pongtorn; Mohammad Sayeed A; Robinson Robert L Jr; Gasem Khaled A M^*

来源：International Journal of Coal Geology, 2014, 121: 98-109.

DOI：10.1016/j.coal.2013.11.009

• Coal swelling
• Gas adsorption isotherm
• Local-density model
• Generalization
• Surface potential
• CO2 sequestration

The swelling of a coal matrix as the result of gas adsorption can have important implications in operations related to the production of coalbed gases and the sequestration of greenhouse gases in coalbeds. In view of this, we undertook a modeling study to describe the relationships among gas adsorption on coals, coal swelling and permeability changes. Specifically, we incorporated the simplified-local-density (SLD) adsorption model within the theory-based swelling model by Pan and Connell (PC). The resultant, internally-consistent SLD-PC model was used to investigate the swelling behavior caused by adsorption of methane, nitrogen and CO2 on several coals, using data from the literature. The SLD-PC model was found capable of representing both the gas adsorption and the adsorption-induced swelling data on these coals.
The PC swelling model relates the linear strain or adsorption-induced swelling in coals to the surface potential of the coal, which herein is calculated by the SLD adsorption model. Two model parameterization scenarios were considered for describing the quantitative relationship between swelling and adsorption surface potential. Results indicate that the SLD-PC approach provides lower errors in representing swelling behavior than the original PC model utilizing the Langmuir adsorption model. This improvement in representing swelling behavior with the SLD-PC model, which was especially true for CO2, is attributed to a combination of two factors: (1) a more accurate description of surface potential and (2) the non-linear relation between the surface potential and strain that is accounted for in the SLD-PC approach.
In cases where swelling data were reported without the corresponding gas adsorption data, we utilized our previously-developed generalized model to predict gas adsorption on coals. The predicted adsorption data were then used successfully in the SLD-PC model for systems lacking experimental adsorption data. The efficacy of this approach was verified using an additional test system from the literature. Further, we also tested the hypothesis by Pan and Connell that coal swelling is more dependent on the molar amount of gas adsorbed than on the particular gas being adsorbed. Current results confirm that the linear strains induced in coals are similar when compared at equal levels of adsorption of different gases.
Lastly, we utilized adsorption-induced strain information obtained from the SW-PC approach to model normalized permeability changes in coal. Our results suggest that the SLD-PC approach combined with the Pan and Connell permeability model may be capable of providing useful description of the adsorption-induced normalized permeability changes in coal. The development of completely predictive models for coal swelling and permeability changes, however, will require additional experimental data and further testing.

• 出版日期2014-1-1