Recovery of Coalbed Methane by Gas Injection

Injection of either carbon dioxide (CO₂) or nitrogen (N₂) can serve as an effective method for enhanced recovery of coalbed methane. In this paper, we provide new analytical solutions for flow of ternary gas mixtures. The adsorption/desorption of the components to/from the coalbed surface is approximated by an extended Langmuir isotherm, and the gas phase behavior is predicted by the Peng-Robinson EOS. Langmuir isotherm coefficients are used that represent a moist Fruitland coal sample from the San Juan Basin of Colorado. In these calculations, mobile liquid is not considered. Given constant initial and injection compositions, a self-similar solution that consists of continuous waves and shocks is found. Mixtures of CH₄, CO₂, and N₂ are used to represent coalbed and injection gases. We provide examples for systems in which the initial gas has a high CH₄ content, and binary mixtures of CO₂ and N₂ are injected. Injection of N₂-CO₂ mixtures rich in N₂ leads to relatively fast initial recovery of CH₄. Injection of mixtures rich in CO₂ gives slower initial recovery, increases breakthrough time and decreases the injectant needed to sweep out the coalbed. The solutions presented indicate that a coalbed can be used to separate N₂ and CO₂ chromatographically at the same time CBM is recovered.