With depletion of coalbed methane (CBM) wells in the San Juan Basin, permeability rises by 10 to 100 times. Similar permeability increases are found in other CBM basins around the world. Models previously advanced to explain this behavior combine a compaction mechanism (which closes cleats) with matrix shrinkage (which opens cleats). In this paper, the original model (Palmer-Mansoori model) is extended to include the transversely isotropic elastic response found in vertically cleated coals. This new version of the model has been calibrated by laboratory measurements on San Juan Basin coal loaded under uniaxial-strain conditions to simulate reservoir depletion. The result is a predictive model characterized by two measured Young's moduli and three measured Poisson's ratios.
To our knowledge, a rigorous geomechanical analysis of anisotropic coal has not before been incorporated into efforts to match large permeability increases observed in the field. We have been able to match the permeability increases in CBM wells using rock and reservoir parameters that are viable and consistent with independent constraints. To illustrate the results, permeability increases with depletion from a few wells in one area of the San Juan basin have been matched by the new model. The new model should allow better forecasting of gas rates by reservoir simulators, as well as improved predictions of gas production in fields yet to be developed.
While there exist many models that seek to explain the huge permeability increases in coalbed methane wells, the significance of this logical extension to the Palmer-Mansoori model (i.e., the addition of coal anisotropy) is that it has the ability to predict permeability increases in wells in the field using measurable rock moduli and other quantities, thus reducing the need for empirically-derived field-matching parameters. Note that this new model confirms an empirical constant that was invoked in a previous version of the Palmer-Mansoori model to account for cleat anisotropy. The rigorous derivation, for anisotropic coal, of permeability increase during depletion provides a missing link (and a significant one) that will save modelers from invoking more esoteric (and unnecessary) factors to try to get a match to field data.