Most of the coal reservoirs in China are of low-permeability and low-pressure, and the injection/fall off test is usually used to correctly evaluate formation parameters. But because water molecules can't enter the matrix porosity, the adsorption and desorption characteristics of the matrix can't be reflected. Concerning this issue, a triple-porosity and dual-permeability mathematical model has been established. Accurate solution to this model under different outer boundary conditions(Infinite, no flow and constant pressure)is obtained by use of Laplace transformation and Stehfest numerical inversion. According to the analysis in Laplace domain, the type curves of dimensionless pressure and pressure derivative are graphically illustrated. The pressure decline performance under the effects of CBM reservoir properties are discussed. A theoretical basis for CBM well test has been provided.

coal bed methane  /  triple-porosity/dual permeability  /  desorption/adsorption  /  bottom hole pressure