Pressure Transient Analysis of Coal Seams in the Presence of Multi-Mechanistic Flow and Sorption Phenomena

Abstract

Abstract Coal seam reservoirs are characterized by their dualporosity nature. This dual-porosity reservoirs are composed of well-defined macropore and micropore structures. In contrast with conventional natural gas reservoirs where gas is stored in the internal pore structure of the reservoir rock in free state, methane gas in coal is subject to sorption phenomena. This mechanism of storage results in an adsorbed monolayer film on the coal grain surface. The dual-porosity characteristics of coal seams together with time dependent sorption phenomena make the mathematical description of methane flow in coal seams a relatively complex problem. Accordingly, analytical solution to the coalbed methane transport problem is also challenging. The development of a pressure transient analysis technique for coalbed methane is the primary focus of this paper. As a first step toward solving this problem, a single-phase mathematical model in radial-cylindrical coordinates is formulated to describe both unsteady state sorption phenomena in the coal matrix and laminar gas transport in the fracture network. The analytical diffusion expression which is obtained from the bounded matrix cylinder is imposed as a non-uniformly distributed source function over the macropore formulation. The subsequent solution of the macropore formulation, then, represents the solution to the composite macropore/micropore coal seam reservoir. The constant terminal rate solution is first obtained in Laplacian space for an infinite acting coal seam reservoir. This solution is, then, brought to the real time domain with the aid of a numerical Laplace inverter. The proposed semi-analytical solution is validated by collapsing the solution to continuous point source solution which is applicable for conventional natural gas reservoirs. Also, comparisons are made against available numerical solutions. A series of example problems is presented to further demonstrate the validity of the proposed solution. Along these lines, field data compiled from Mary Lee/Blue Creek Seam in Warrior Basin, Alabama and from Deep Coal Seams in Piceance Basin, Colorado are analyzed using the solution procedure presented in the paper, and good matches have been obtained.