Simplified approach for in-situ characterization of desorption properties of coal seams

A simplified analysis technique to determine the desorption characteristics of coal seams using pressure transient analysis is presented. The proposed technique is based on formulation of single phase gas flow in coal seams with unsteady state sorption/diffusion phenomena in coal matrix and laminar flow in cleat system. The matrix geometry is represented by using spherical elements. A line source solution which defines the pressure distribution in an infinite-acting radial-cylindrical coal seam is used in the development of the proposed analysis technique. The simplified analysis presented in this paper is first tested against a previously developed closed form solution which describes the pressure transient behavior of coal seams. The proposed solution shows excellent agreement with this more rigorous solution, and its versatility is demonstrated through a simple analysis procedure which does not require the use of computationally laborious functional groups. Furthermore, a pressure transient data generated by a coal seam degasification numerical model is analyzed successfully using the methodology presented here. As an example application of the proposed technique to the analysis of drawdown test data is also included. The proposed inverse solution procedure which is devised as a viable pressure transient analysis technique is applicable to coal seams and other unconventional gas reservoirs where adsorption/desorption phenomena are effective. The methodology introduced in this paper has the potential of providing the necessary tools that can be used in in-situ determination of the transport properties and the sorption characteristics of the coalbed methane reservoirs.