Low-btu gas in the US Midcontinent: A challenge for geologists and engineers

Abstract

Several low-btu gas plays can be defined by mapping gas quality by geological horizon in the Midcontinent. Some of the more inviting plays include Permian strata west of the Central Kansas uplift and on the eastern flank of Hugoton field and Mississippi chat and other pays that subcrop beneath (and directly overlie) the basal Pennsylvanian angular unconformity at the southern end of the Central Kansas uplift. Successful development of these plays will require the cooperation of reservoir geologists and process engineers so that the gas can be economically upgraded and sold at a nominal pipeline quality of 950 btu/scf or greater. Nitrogen is the major noncombustible contaminant in these gas fields, and various processes can be utilized to separate it from the hydrocarbon gases. Helium, which is usually found in percentages corresponding to nitrogen, is a possible ancillary sales product in this region. Its separation from the nitrogen, of course, requires additional processing. The engineering solution for low-btu gas depends on the rates, volumes, and chemistry of the gas needing upgrading. Cryogenic methods of nitrogen removal are classically used for larger feed volumes, but smaller feed volumes characteristic of isolated, low-pressure gas fields can now be handled by available small-scale PSA technologies. Operations of these PSA plants are now downscaled for upgrading stripper well gas production. Any nitrogen separation process should be sized, within reason, to match the anticipated flow rate. If the reservoir rock surprises to the upside, the modularity of the upgrading units is critical, for they can be stacked to meet higher volumes. If a reservoir disappoints (and some will), modularity allows the asset to be moved to another site without breaking the bank.