A New Model of CBM Permeability and Sorption-Induced Strain Based on Open-System Geomechanics*

The sorption-induced porosity and permeability alteration of coal beds as pore pressure and in-situ stress are varied can be couched within the frameworks of ‘open thermodynamic system' and ‘open-system geomechanics' (OSG). Production from coal bed methane (CBM) reservoir changes not only the fluid mass (gas and water) but also solid mass as a result of methane desorption from rock matrix. The latter manifests itself as rock matrix shrinkage which often leads to permeability enhancement. This approach captures the fundamental process of mass changes in the CBM as a result of methane production. In this formulation, the use of fluid and solid mass as elementary variables is beneficial because they directly impact the variation of strain and stress as fluid and ‘solid' are removed from the reservoir. This new model provides a fundamental, fully coupled description of stress-strain, solid and fluid masses evolution instigated by sorption/desorption transition. It involves a new constitutive relationship which describes the evolution of sorption-induced strain and heat transfer equations. As a result, the OSG approach expresses explicitly the sorption-induced volumetric strain via the Skempton coefficient of coal beds. It elegantly leads to a new model of coal-bed methane (CBM) permeability description as a function of pore pressure and fundamental measureable properties such as Skempton coefficient, fluid (gas) and solid (coal matrix) bulk moduli, Biot's constant, and initial porosity. The prediction of CBM permeability using this new approach is compared to published experimental data and existing theoretical models. Initial investigation is encouraging and the comparison shows substantial improvement over previous models.