Earlier numerical models for coal seam gas reservoirs assume that at initial reservoir conditions coal is either on or above the sorption isotherm, i.e., either equilibrium or saturated conditions prevail. Therefore, when undersaturated conditions exist, implementation of these models does not capture the physics of the problem accurately. In this paper, simple algorithmic procedures are proposed which allow the model to cross the isotherm between saturated and undersaturated regions during computations.

Several examples which demonstrate the successful application of the proposed algorithms are presented. Implementation of the switching mechanisms to existing coalbed simulators is straightforward and should significantly enhance their applications.

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