Abstract
Gas diffusivities of coal are not measured directly but are normally regressed by fitting mathematical diffusion models to fractional sorption data measured in sorption experiments. This paper firstly measured three fractional adsorption curves at various equilibrium pressures with the manometric method. The measured fractional adsorption curves were then modeled with one single-fitting-parameter (SFP) model and three triple-fitting-parameter (TFP) models. The modeling results showed that the TFP models were phenomenologically over-parameterized due to the usage of three fitting parameters, which may be excessive for curve fitting. The phenomenological over-parameterization resulted in multiple pressure-dependences of gas diffusivity for the TFP models. The TFP models should thus be carefully used. On the other hand, the dual-fitting-parameter (DFP) models also have excellent performance in curve fitting and can produce interpretable modeling results. The DFP models can be used as an alternative to the TFP model in the future.
Highlights
Coalbed methane (CBM) recovery and carbon dioxide (CO2 )-enhanced CBM production have flourished in many countries [1]
This paper presents the experimental measurements of three fractional adsorption curves at various equilibrium pressures
Similar results have been reported in literature that the TFP models normally had better performance in curve fitting than the UP model did [13,14]
Summary
Coalbed methane (CBM) recovery and carbon dioxide (CO2 )-enhanced CBM production have flourished in many countries [1]. The development of these projects requires detailed and reliable information on gas sorption and flow [2]. Since gas sorption is normally assumed instantaneous [6,7,8], CBM production and CO2 injection rates are mainly determined by gas flow processes. Gas flow in coal is often considered as a two-stage process: laminar flow in cleats and diffusion in coal matrices [9,10,11]. This paper focuses on gas diffusion and does not include the laminar flow in cleats
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