Adsorption of pure and multi-component gases of importance to enhanced coalbed methane recovery: Measurements and simplified local density modeling

Abstract

Scope and Method of Study:

The Simplified Local Density (SLD) theory was investigated to facilitate precise representations and accurate predictions for high-pressure, supercritical adsorption isotherms encountered in coalbed methane (CBM) recovery and CO2 sequestration. Specifically, the ability of the SLD model to describe pure and mixed-gas adsorption was assessed using a modified Peng-Robinson (PR) equation of state (EOS). High-pressure adsorption measurements acquired in this study and data from the literature were used in this evaluation. In addition, a new pure-fluid EOS was developed to accommodate the future needs of high-pressure adsorption modeling.

Findings and Conclusions:

Precise gas adsorption measurements were completed using a constant-pressure, volumetric technique at temperatures between 318 and 328 K (113-131�F) and pressures to 13.8 MPa (2000 psia). These measurements included the pure-gas adsorption of methane, nitrogen, ethane, and CO2 on eight coals and one activated carbon (wet Fruitland, wet Lower Basin Fruitland coal, wet / dry Illinois #6, wet Tiffany, dry Beulah Zap, dry Wyodak, dry Upper Freeport, dry Pocahontas, and dry Calgon Filtrasorb 400). Mixed-gas measurements for methane, nitrogen and CO2 were conducted on the activated carbon and wet Tiffany coal.

Analysis of the SLD-PR model indicated that the limiting high-pressure adsorption behavior is governed by the high-density limit of the EOS, which is determined by the EOS covolume. Consequently, modification of the EOS covolume allowed for precise representation of pure-gas high-pressure adsorption and the reliable prediction of binary and ternary gas mixture adsorption. Specifically, the model can (a) represent adsorption on activated carbon and coals within their expected experimental uncertainties, and (b) provide generalized binary and ternary predictions, within two to three times the experimental uncertainties, based on regressed parameters from pure-gas adsorption data.

A new pure-fluid EOS capable of accurate representation of high-density behavior was developed. This EOS, which covers a wide range of phase conditions, utilizes an accurate hard-sphere repulsive term. Evaluation results for 19 fluids, including coalbed gases (CO2, methane, and nitrogen) and water, indicated that the new EOS can represent precisely the volumetric behavior and the saturated vapor-liquid equilibrium properties of these pure fluids with average errors of 1%.