Sorbed Gas Content of Coals and Gas Shales Utilizing Well Cuttings

K. David Newell.  Kansas Geological Survey, University of Kansas, Lawrence, KS 66047-3726  [email protected]

In the Western Interior Basin coal basin in eastern Kansas, drill cuttings from Pennsylvanian-age, high-volatile A, B, and C bituminous coals were experimentally used for desorption studies. It was determined that drill cuttings can indeed be used for desorption analyses but with more uncertainty than those done with cores. Gas-content measurements from drill cuttings are not recommended to replace those from cores, but cuttings can provide timely and economic supplementary data. The mixed lithologies in drill cuttings are the primary source of uncertainty in their analysis for gas content, for one has to apportion the gas generated from both the coal and the dark-colored shale that is commonly mixed in with the coal. Dark-colored shales with normal (~100 API units) gamma-ray levels appear to give off minimal amounts of gas on the order of less than 5 standard cubic feet per ton (scf/ton), and this represents a minor correction in gas-content determinations for coals. Shales with high gamma-ray values (>150 API units) may yield several times this amount of gas and present difficulties that may be overcome with mathematical and graphical techniques.

The uncertainty in desorption analysis of drill cuttings can be depicted graphically on a diagram identified as a "lithologic component sensitivity analysis diagram." Comparison of cuttings-desorption results from nearby wells on this diagram can sometimes yield a unique solution for the gas content of both a dark shale and a coal mixed in a cuttings sample. A mathematical solution, based on equating the dry, ash-free gas contents of the admixed coal and dark-colored shale, also yields results that are correlative to data from nearby cores.