Recently, considerable attention has been focused on the long term potential of natural gas. Studies have included resources that range from conventional gas in stratigraphic traps to unconventional gas locked in hydrates. However, these studies have overlooked one large and potentially productive gas resources - the methane stored in deep coal seams.
Currently, the conventional wisdom is that coals below 5,000 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 5,000 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 10,000 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 Pinyon 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.
A major portion of the coalbed methane potential of the world resides in deep coals, coals buried at depths below 5,000 feet (1,500 meters). Industry has been reluctant to pursue these "deep coals" for four reasons:
limited downhole data;
higher drilling costs;
assumed unfavorable desorption characteristics; and
concerns about low permeability.
Of the four reasons for avoiding deep coals, the loss of permeability with increasing depths is the one most often mentioned. The low permeabilities encountered at GRI s Deep Coal Seam Test Site (Red Mountain, Piceance Basin), and the unfavorable depth versus permeability charts presented in various technical papers and reports (McKee and others, 1986; 1988; Koenig and others, 1989) are generally cited as supporting evidence for these concerns over the lack of permeability in deep coals.