(672c) Modeling High-Pressure Phase Equilibria of Coalbed Gases and Water with a Modified Peng- Robinson Equation of State

Simulations of enhanced coalbed methane recovery and CO2 sequestration operations require reliable phase equilibrium models that can describe accurately the phase behavior of mixtures of coalbed gases (methane, nitrogen and CO2) and water. Typically, the phase conditions in a coalbed reservoir are either near-critical or supercritical with reference to the coalbed gases. Further, during the course of gas production, the reservoir can exhibit varying degrees of saturation with water. Therefore, an equation-of-state (EOS) model that is capable of describing accurately the vapor-liquid equilibria (VLE) behavior and the phase densities of these systems over the range of conditions encountered in coalbed reservoirs is required.

To facilitate reservoir simulations for coalbed methane recovery and CO2 sequestration, a modified Peng-Robinson equation of state (MPR EOS) is applied in this work. The quality of PR EOS density predictions, including the near-critical region, has been enhanced for the systems considered through volume translation and a scaling-law density correction. Case studies highlighting the efficacy of this approach are discussed.