Adsorption of Pure Methane, Nitrogen, and Carbon Dioxide and Their Binary Mixtures on Wet Fruitland Coal

Abstract

Abstract A detailed experimental study has been made of the adsorption of methane, nitrtheir binary mixtures, on a wet Fruitland coal at 115°F. Mixture measurements were typically made at nominal gas-phase compositions of 20, 40, 60, and 80 mole percent and pressues to approximatey 1,800 psia. These data elucidate clearly the competitive adsorption behavior of the individual components in these mixtures. The experimental information was used to test predictive methods for describing the adsorption behavior of the pure and mixed gases. Models included various two-dimensional equations of state, as well as more traditional methods, such as the Langmuir and loading ratio correlations and the ideal adsorbed solution (IAS) model. The relative merits of the various models are described. In general, all models perform well for pure-gas adsorption; however, results are less satisfactory for mixtures. The errors in mixture predictions increase as the individual components become more dissimilar in their adsorption behaviors. Greatest (percentage) errors occur for the less-adsorbed component and, in the worst case (nitrogen+carbon dioxide), the predictions for nitrogen show errors of more than 100% at some conditions. Overall, the two-dimensional equation-of-state and IAS models perform comparably, and they are more accurate than the Langmuir model. The data and modeling results should be of interest in coalbed methane production and, especially, in evaluating potential enhanced recovery operations based on nitrogen and/or carbon dioxide injection into coalbeds. INTRODUCTION Large quantities of naturul gas (methane) are stored in coal deposits. The amount of gas currently in coalbeds in the U.S. is estimated to be 400 Tcf(trillion cubic feet) of which about 95 Tcf is recoverable under current technology [1]*. For comparison, estimates put the U.S. domestic natural gas resources at less than 200 Tcf m. This conventional gas can supply our needs for only about 25 years at current consumption rates. Thus, coaled methane represents a valuable addition to the nation's energy reserve. Development of the coalbed resource is, however, in its infancy; coalbed methane acounted for 3% of domestic gas production in 1992 [3]. Further, the current state of scientific and engineering knowledge on coalbed methane is inadequate to develop optimum strategies for its recovery.