Mass Transfer in Coal Seams for CO 2 Sequestration F. Y. Wang, Z. H. Zhu, P. Massarotto, and V. Rudolph Division of Chemical Engineering, School of Engineering, The University of Queensland, Brisbane, Qld. 4072, Australia DOI 10.1002/aic.11115 Published online February 27, 2007 in Wiley InterScience (www.interscience.wiley.com). CO 2 sequestration in coal seams is a relatively new technique to simultaneously achieve enhanced coal bed methane production and reduced CO 2 emission. In this ar- ticle, we integrate understandings in individual research fields to provide improved insight into the nature of this complex process. Our current overall model constructed from a number of sub-models consists of mass transfer in four pore types, namely, fractures, micro-, meso-, and macro-pores, all having pore size dependent characteris- tics. Key parameters are estimated using well established methods from the general lit- erature. Three mechanisms of coal swelling leading to permeability variations during adsorption are proposed based on molecular simulations. The macroscopic level model is validated using a true tri-axial stress coal permeameter, which provides previously unpublished, accurate dynamic measurements of systems properties in three orthogonal directions including changes to the coal matrix volume. The integrated model provides a more complete and flexible representation for this complex system. Ó 2007 American Institute of Chemical Engineers AIChE J, 53: 1028–1049, 2007 Keywords: complex fluids, diffusion (microporous), mathematical modeling, porous media, simulation, process Introduction CO 2 sequestration in coal seams is a relatively new tech- nique to simultaneously achieve enhanced coal bed methane (ECBM) production and reduced CO 2 emission. The signifi- cance and current status of this research area have been com- prehensively reviewed in two recent review papers. 1,2 In any particular practical case feasibility, economics, and risk eval- uation require the determination of a number of operational conditions. Both theoretical analysis and experimental inves- tigation are essential for the development of effective ECBM strategies. A multidisciplinary approach is required, covering a number of challenging scientific areas such as multi-com- ponent transport in porous media, coal characterization, geo- physics, and geochemistry. In spite of an extensive recent lit- erature in relevant areas, significant gaps in knowledge can still be identified because of the complexity and broad scien- tific coverage of the field, which are identified as follows. 1. The multiscale nature, which ranges from molecular inter- actions in the length scale of nanometers and time scale of micro seconds all the way to coal seam operations with lengths of kilometers and performance analyzed in hours, days, and even weeks. Significant research advances have been achieved in both microscopic level using molecular simulations, and macroscopic level studies through com- mercialized software packages, for coal bed methane (CBM) simulations. However, coupling these two scales remains elusive as evidenced by the fact that the recent con- ceptual advances achieved from fundamental studies are scarcely incorporated into the large scale simulators. For example, it can be shown that neglecting micro-pore size distributions in CBM models leads to considerable errors. It is necessary to develop adequate transition procedures to bridge the micro- and macro-scale formulations. 2. A process can be described by many models with differ- ent complexity and accuracy. We adopt the position of Box and Draper 3 that: ‘‘Essentially, all models are wrong, Correspondence concerning this article should be addressed to F. Y. Wang at f.wang@eng.uq.edu.au. Ó 2007 American Institute of Chemical Engineers AIChE Journal April 2007 Vol. 53, No. 4 1028