Chapter 2 - Gas Content of Coal and Reserve Estimates

Abstract

Gas content of a coal seam is perhaps the most important reservoir property. Techniques to measure the gas content are discussed. A gas “isotherm” for a coal seam is another important characteristic that shows the relationship between gas content and reservoir pressure. Mathematical derivation of this relationship (for indirect estimation of gas content) is done and gas isotherms with key parameters are derived for some prominent US coal seams. Influences of various factors, such as depth/reservoir pressure, rank of coal, temperature, moisture, and ash content of coal, are discussed. Reserve estimates of minable coal seams but particularly, the gas production from overlying and underlying coal seams is also presented. A procedure to estimate the gas reserve of deeper/nonminable coal seams is also presented. It involves coring, gas content measurement in the laboratory, and proximate analysis of coal. Finally, important properties of most components of coal bed methane, such as methane, ethane, propane, hydrogen, carbon dioxide, and nitrogen, are listed.