5 - Gas transportation in CBM reservoir

Abstract

Coalbed methane (CBM) reservoirs are often characterized as a dual porosity system because coal is characterized by two distinct systems, namely, the matrix and the cleats. Most of the gas in a CBM reservoir is stored in the low permeability matrix, which is dominated by pores in nanometers range. The cleats are a pervasive network of natural fractures in coal, which have a very low storage capacity but much higher permeability than the matrix. The gas transportation in the matrix is by diffusion toward the interface with the cleats. Upon reaching the interface, the gas desorbs from the matrix and enters the cleat in “Free State.” The gas will flow, along with water, through cleat system to the production well. Flow within the cleat system is described by Darcy's Law. The gas production from the commercially developed CBM reservoir is generally limited by permeability and the diffusion has a negligible impact on the gas production. Therefore, the cleat system absolute and relative permeability are the most important properties that affect gas and water production rates and economic success of CBM production. These properties are estimated from a combination of well test analysis and laboratory core analyses. Well test analysis is the only reliable way to estimate in situ cleat system absolute permeability. The relative permeability is determined from the analysis of production data. The absolute permeability of the cleat system is not constant but varies drastically during production due to the changes in stress, pressure, and gas desorption.