Volumetric shrinkage due to methane desorption from the coal matrix has significant influence on the stress regime and fracture permeability in coal. Permeability changes (due to primamry depletion) up to two orders of magnitude have been reported from the Fairway region in the San Juan Basin. Several analytical models have been developed to predict changes in fracture permeability in coal as a function of stress and sorption induced coal swelling/shrinkage.
Coal can be considered as a highly crosslinked macromolecular network structure, where aromatic clusters of graphite like complexes are connected by aliphatic chains. All sorption processes in coal are characterized by relaxational mechanisms of such crosslinked complexes. Any relaxational mechanism is indicative of swelling related stresses in coal. Variations in the effective horizontal stresses under uniaxial strain conditions are effected as a function of the reservoir pressure reduction during primary drawdown, which includes a cleat compression term and a matrix shrinkage/swelling term that have opposite effects on fracture permeability.
The matrix shrinkage/swelling term have been revisited a theory proposed to explain the coupling of swelling stress to porosity change in coal as a function of the change in diffusion coefficient, the Langmuir constants, cleat compressibility and material properties.