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Coalbed Methane Compared to Traditional Natural Gas
The
primary component of commercial natural gas is methane. Methane
can also be found in coal deposits, as it is created by the same
biological and geological forces that transform organic material
into coal. Methane is stored in coal seams in four different ways:
- as
freely trapped gas within the pore spaces and natural fractures
of the coal;
- as dissolved
gas in the water within the coal seam;
- as adsorbed
gas on the surface of the coal; and
- as adsorbed
gas held within the molecular structure of the coal itself.
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Methane
stored in coal deposits by all four of these methods is released
upon the removal of water from coal seams. The removal of water
reduces the amount of pressure on free and dissolved gas in the
coal allowing it to be produced. As a result, coalbed methane
wells typically produce significant amounts of water when they
are first drilled, often for the first one or two years of a generally
projected eight to twelve year well life. During this de-watering
phase, water production typically decreases while gas production
typically increases. After this initial production phase, gas
production typically declines over the remaining producing life
of the wells.
While traditional natural gas wells and coalbed methane wells
require largely the same infrastructure and produce the same end
product, coalbed methane production differs from traditional natural
gas production in the following ways:
- Other
than dehydration and compression, coalbed methane typically
needs no other processing after extraction prior to entering
a pipeline, reducing production costs;
- Although
certain structural features such as fractures enhance production
of coalbed methane, such structural features are generally not
necessary for production, making the discovery of coalbed methane
reserves less expensive;
- Methane
bearing coals exist at much shallower depths than the formations
that traditionally contain natural gas, allowing coalbed methane
to be produced from shallower wells using more readily available
equipment, such as water well rigs, thereby reducing drilling
costs; and
- Since the
location of coal seams is typically known through prior mining
activity or from data provided by existing wells drilled to
deeper formations, extensive geophysical or seismic data is
not required to drill a coalbed methane well.
It
should be noted that coalbed methane reservoirs require a cleat
system to be productive. Cleats are formed during the coalification
process and provide the path for the methane to travel to the
wellbore. The size and number of the cleats determines the permeability
and productubility of the coalbed reservoir.
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