Measurement of parameters impacting methane recovery from coal seams

Summary

This paper describes the behaviour of coalbeds as gas reservoirs and discusses the results of a study carried out to establish the effect of release of methane on gas flow behaviour of coal. Experimental work consisted of microscopy, establishing adsorption/desorption isotherms, and monitoring changes in the volume of coal matrix with increasing and decreasing gas pressure. Micrographs obtained using small pieces of coal indicated that coal is made up of blocks, containing matrix and pores, separated by microfractures. This confirms the dual porosity model of coal structure with a primary porosity, and a fracture/cleat porosity-physical model used in coalbed methane simulators developed recently. Isotherms suggested that for the samples tested, a major part of the gas is released only after pressure falls below 600 psi, and this is primarily due to desorbing gas. Results of the volumetric strain experiments indicated that there is an increase in matrix volume with increase in gas pressure, in spite of matrix compressibility. Adsorption, therefore, induces swelling of the matrix. With decrease in gas pressure from 1000 psi to atmospheric, the matrix volume shrunk by 0.5%. These experimental results were inputted in a reservoir model and simulation runs made to determine the effect of pore volume and matrix shrinkage compressibilities on gas production. Over a five year period 60% more gas was produced when matrix shrinkage was used as an input parameter.