Gas flow in coal
seam consists of laminar flow through coal cleat
and diffusion through pores of coal matrix. Previous studies on the
prediction of CBM production mostly focused on the impact of permeability
change while the gas exchange between matrix and cleat was assumed
to obey unipore diffusion assumption with a single diffusion coefficient.
However, numerous scholars have found that a single diffusion coefficient
cannot reproduce the sorption kinetic data precisely for a lot of
coals, while bidisperse diffusion with fast and slow diffusion coefficients
can represent the diffusion process well. Until now, attempts on studying
the impact of bidisperse diffusion on CBM production are very limited
and mathematical model describing the gas flow with bidisperse diffusion
is unavailable. In this study, we propose a fully coupled coal seam
gas flow model with consideration of bidisperse diffusion and the
interaction between bidisperse diffusion, adsorption strain and geomechanical
response of coal. A series of experiments were carried out to understand
the characteristics of Sydney Basin required by the gas flow model.
The sorption kinetic data was matched by unipore and bidisperse diffusion
models, results show that bidisperse diffusion can describe the diffusion
process much better than unipore diffusion. Based on the developed
gas flow model, the difference of CBM production rates between applying
the two diffusion assumptions was studied by using the determined
bidisperse diffusion coefficients and the approximated unipore diffusion
coefficient. Results show apparent deviations of the predicted production
rates, the difference is reduced with decreasing cleat spacing while
can still be observed with decreasing initial permeability. From the
experimental and modeling results, we believe the assumption of bidisperse
diffusion cannot be replaced by unipore diffusion if diffusion is
a constraint of gas production. For history matching of field CBM
production data, careful examination with consideration of bidisperse
diffusion is also recommended to gain a better understanding of in
situ permeability change.
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