Volume-Translated, Modified Peng-Robinson Equation of State for Coalbed Gas/Water Mixtures

Simulations of enhanced coalbed methane (CBM) recovery and CO₂ sequestration operations require reliable phase equilibrium models capable of describing accurately the vapor-liquid equilibria (VLE) behavior and the phase densities of coalbed gases (methane, nitrogen and CO₂) and water. A suitable model needs to be applicable over the range of conditions encountered in coalbed reservoirs, which are near-critical or supercritical. Our recent studies have indicated that the modified Peng-Robinson equation of state (MPR EOS) provides good representation of the phase behavior of these systems. Moreover, when equipped with volume translation and scaling-law behavior correction, the MPR EOS is capable of predicting volumetric properties of the individual CBM components within 2% absolute average deviation.

 In this work, the MPR EOS with volume translation and scaling-law behavior correction was extended to mixtures for the prediction of phase densities comprised of mixtures of methane, nitrogen, CO₂ and water. The volume-translation and scaling correction to the EOS has yielded satisfactory predictions for the mixtures. To further validate the MPR EOS, the phase densities of selected mixtures were measured experimentally on a volumetric apparatus and the EOS was tested with the newly acquired data.