Recently, considerable attention has been focused on the long term potential of natural gas. Studies have included resources that range from conventional gas in Stratigraphie traps to unconventional gas locked in hydrates. However, these studies have overlooked one large and potentially productive gas resource — the methane stored in deep coal seams.
Currently, the conventional wisdom is that coals below 5.0 feet will be extremely low in permeability and thus unproductive. This viewpoint stems from the early laboratory based tests of confining pressure (depth) versus permeability and from the initial field tests in the deeper portions of the Piceance Basin, U.S.A.
Recent basin studies show that the resource potential in deep coal basins is considerable, both in the U.S. and worldwide. For example, in the U.S. the bulk of the 84 Tcf of coalbed methane contained in the Piceance Basin is in deep coals, below 5.0 feet. Even larger amounts of gas exist in deep coal seams in Alberta, Canada and other deep coal basins of the world. Also, comparisons of laboratory based permeability tests on coal with field results indicate that the lab tests may be unduly pessimistic. While the evidence that deep coals can be economically productive is limited, some data is now available and discussed in this paper. For example:
An experimental completion of a Cretaceous age coal seam at 10,0 feet in the Deep Basin of Alberta, showed that deep coals contain substantial methane, that gas can be produced from such depths, but that the well completions are very challenging.
A recently drilled 6,000 foot deep well at Piny on Ridge (Northern Piceance Basin) had an initial gas flow of nearly a million cubic feet per day.
The purpose of this paper is to examine the resource and production potential of deep coal seams and thus add this resource to the list of long-term sources of natural gas.