Overview of Coal and Shale Gas Measurement: Field and Laboratory Procedures
Noel B. Waechter, George L. Hampton, III, and James C. Shipps Hampton, Waechter & Associates, LLC
Published in the Proceedings of the 2004 International Coalbed Methane Symposium, May 2004, The University of Alabama, Tuscaloosa, Alabama
ABSTRACT
Methods for measurement of gas in coals were initially developed to improve coal-mine safety. The Direct Method introduced by the U.S. Bureau of Mines for estimation of lost gas in the early 1970s has been updated to better estimate subsurface gas content of coals and shales. Proper sampling procedures and sample handling are critical factors in determining the in situ sorbed gas in coals and shales, especially with regard to the estimation of lost gas. Determining total gas content requires measurement of three components— lost gas, measured gas, and residual gas. Lost gas is estimated by projecting the first few hours of desorption measurements back to the time at which desorption begins. The desorption process may require two or more months to reach a point where the desorption rate becomes negligible. Because lost-gas volumes cannot be measured directly, careful measurement of sorbed gas and selection of an appropriate mathematical projection are essential in deriving an accurate estimate of lost-gas. Residual gas is not produced from the reservoir, but it is part of the total gas measured in adsorption isotherms. Analyses of coals usually include proximate analysis and bulk density measurements of all samples, adsorption isotherms of several representative samples, and sequential gas-compositional analyses. Other analytical work might include detailed coal descriptions and photography, maceral composition, cleat mineralogy, vitrinite reflectance, pyrolysis, gas isotope analysis, and measurement of residual gas. Shales are commonly analyzed for total organic carbon (TOC) in lieu of proximate analyses. The method used for collecting down-hole samples for desorption has a huge impact on the reliability of gas content data. Getting a large, unbroken sample of coal or shale to the surface in a short amount of time gives the best results. Continuous wireline-core-retrieval systems usually fulfill both of these needs, yielding better core recovery and shorter retrieval times than conventional coring. At shallow depths, conventional coring can provide good gas-content data and can be less costly if only one or two seams are to be cored. Rotary sidewall coring is a good option with deeper wells, particularly for coal sequences where multiple seams make conventional coring impractical. Desorption of conventional drill cuttings can provide a quick look at gas contents for exploratory wells, but the results are less reliable.
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