Abstract

During injection of carbon dioxide in deep coal seams, or during production of methane from those same seams, very significant variations of permeability are observed. Coal contains micropores (i.e., pores with a diameter smaller than 2 nm), in which pore fluid is adsorbed, i.e., in which most fluid molecules are in intermolecular interaction with the atoms of the coal solid. Variations of permeability of the seam are a consequence of this adsorption in sub-nanometer pores: adsorption induces an expansion of the coal matrix, and hence a closure of the cleat system (i.e., a set of fractures naturally present in the coal bed), which in turn leads to a decrease of permeability of the seam. In terms of modeling, we present an extension of the poromechanical approach to microporous solids and adsorption effects. One originality of the model is that, rather than focusing on strains induced by adsorption, we focus on the mechanical stresses this adsorption induces. Experimentally, we show on intact coal cores (i.e., in the lab) that adsorption can induce mechanical stresses of several dozen MPa and variations of permeability of more than two orders of magnitude (as observed in the field), and that desorption can even lead to mechanical failure of the coal sample.