Reservoir Engineering in Coal Seams: Part 1—The Physical Process of Gas Storage and Movement in Coal Seams

TLDR

Coal seams differ from typical porous gas reservoirs in both storage mode—primarily sorption into the coal matrix—and permeability characteristics, which vary widely during production. The study examines how coal seam permeability, governed by cleat and joint structures, fluctuates during production due to competing effects of effective stress from fluid pressure reduction, which closes cleats, and coal shrinkage, which opens them, while gas storage occurs mainly through sorption.

Abstract

Summary This is the first of two papers concerning the movement of gas in coal seams. It deals directly with the physical behavior of the coal seam as a reservoir. Coal seams show considerable differences in behavior from normal porous gas reservoirs in both the mode of gas storage and permeability characteristics. Most of the storage of gas in coal is by sorption into the coal structure, while the coal permeability is cleat-(fracture-) or joint-controlled and may vary over a wide range during production. This permeability fluctuation is not solely a phase relative permeability effect, but is rather a result of the opposing effects of effective stress increase with fluid pressure reduction and shrinkage of the coal. Reducing fluid pressure tends to close the cleats, reducing permeability, while shrinkage tends to open them.