Page 87 - Principles of Applied Reservoir Simulation 2E

72 Principles of Applied Reservoir Simulation

and hydrogen [Mavor, et al., 1999]. Gas content can range from approximately
20 SCF gas per ton of coal in the Powder River Basin of Wyoming [Mavor, et
al., 1999] to 600 SCF/ton in the Appalachian Basin [Gaddy, 1999]. Gas recovery
begins with the desorption of gas from the internal surface to the coal matrix
and micropores. The gas then diffuses through the coal matrix and micropores
into the cleats. Finally, gas flows through the cleats to the production well. The
flow rate depends, in part, on the pressure gradient in the cleats and the density
and distribution of cleats. The controlling mechanisms for gas production from
coalbeds are the rate of desorption from the coal surface to the coal matrix, the
rate of diffusion from the coal matrix to the cleats, and the rate of flow of gas
through the cleats.
The production performance of a coalbed methane well typically exhibits
three stages. The reservoir dewaters and methane production increases during
the first stage of pressure depletion. Methane production peaks during the second
stage. The amount of water produced is relatively small compared to gas
production during the second stage because of gas-water relative permeability
effects, and desorption of natural gas provides a counterbalance to permeability
loss as a result of formation compaction. The third stage of production is similar
to conventional gas field production in which gas rate declines as reservoir
pressure declines.

Gas Hydrates
The entrapment of natural gas molecules in ice at very low temperatures
forms an ice-like solid. The ice-like solid substance is a metastable complex
called a gas hydrate. Gas hydrates are clathrates. A clathrate is a chemical com-
plex that is formed when one type of molecule completely encloses another type
of molecule in a lattice. In the case of gas hydrates, hydrogen-bonded water
molecules form a cage-like structure in which mobile molecules of gas are
absorbed or bound.
The presence of gas hydrates can complicate field operations. For
example, the existence of hydrates on the ocean floor can affect drilling
operations in deep water. The simultaneous flow of natural gas and water in
tubing and pipelines can result in the formation of gas hydrates that can impede