A simplified dynamic model for accelerated methane residual recovery from coals

Effective recovery of residual methane from coal seams after major extraction operations is an important but difficult task due to the small driving forces and low mass transfer coefficients. An obvious option to achieve nearly complete methane recovery is the depressurization technique. In this communication, we develop a model for the prediction of mass transfer dynamics between coal matrix and fractures under depressurization conditions. The fluid flow in fractures is modeled using Darcy's law described by partial differential equations, whereas the variations of gas and adsorbed phase concentrations within the coal matrix are represented by a lumped parameter model represented by ordinary differential equations. Key parameters are estimated using well-established methods from the general literature. The model is validated using a true tri-axial stress coal permeameter (TTSCP), which provides accurate dynamic measurements of system properties under a series of controlled pressure reductions. The model provides physical insights into the observed behavior. The relative importance of the convective and diffusive flows is quantified through numerical simulations, which is essential to the design of depressurization operations.