Page 31 - Handbook of Natural Gas Transmission and Processing Principles and Practices

Most of the gas (primarily methane) is adsorbed on the lamellar surfaces of the coal, which is an
excellent “storage medium.” The permeability of the coal decreases with increasing depth, meaning
that deeper wells have a lower production rate. As such, it is easy to drill and complete the coal

deposit at shallow depths (<1000 m) at relatively low costs. However, there is an initial period of
production where water must be unloaded from the well before gas production can commence.
A common characteristic of the different types of unconventional gas resources is that they
contain large quantities of natural gas, but it is usually more difficult to produce this gas as
compared to conventional reservoir rocks. In fact, unconventional gas is found in reservoirs with a

relatively low permeability (<1 mD) and therefore, cannot be extracted by conventional methods
(Speight, 2013). To extract natural gas at cost-effective rates, artificial pathways must be created. The
key extraction technologies are horizontal drilling and hydraulic fracturing as described previously.
Because of the relatively rapid decline rate of these wells, many more must be drilled compared to
the number of wells needed for a conventional reservoir.
A large proportion of the gas produced globally to date was by conventional means (Speight,
2013). However, with improving extraction technology, many of the unconventional gas resources
have either become viable or show promise to become economically viable in the future (Kelkar,
2008). In gas exporting regions, unconventional resources might complement conventional gas
production; however, in gas importing regions, unconventional gas production can lead to falling
gas prices and a reduction of gas imports (Korn, 2010).

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