(193t) A New Algorithm for Multiphase Analysis of Gas/Water Mixture Adsorption On Coals

Coalbeds typically contain large amounts of water. Since a large proportion of (natural) gas in such reservoirs is in an adsorbed state, an adsorption equilibrium model is required to describe the adsorption/desorption behavior on coals. Traditionally, these models do not account explicitly for water as a separate adsorbed component in a gas mixture and only treat water as a “pacifier” of the coal adsorbent surface. Further, the conventional adsorption models (thus far) account for the presence of only two equilibrium phases - gas and adsorbed phases. This simplified modeling approach may lead to significant errors in simulations of enhanced coalbed methane recovery and CO₂ sequestration potential of coalbed reservoirs.

When water is one of the adsorbed components in a high-pressure gas adsorption system, as many as three phases may coexist at equilibrium. In this work, a new multiphase (three-phase) algorithm is presented and used to investigate the high-pressure adsorption behavior of CO₂/water gas mixtures on wet coals. The algorithm uses a phase-insertion technique, which involves formally inserting a third (liquid) phase and solving a series of three-phase flash problems, wherein the three phases are the adsorbed, bulk gas, and liquid phases. A Gibbs-energy-driven algorithm was used to conduct both fixed and variable feed multiphase analyses for CO₂/water mixture adsorption on wet coals. The simplified local density/Peng-Robinson (SLD-PR) adsorption model was used in the multiphase flash algorithm.

Results indicated that high-moisture containing coals form a third (liquid) phase at nearly all pressures in the range of measurements. Further, the predictions obtained utilizing a fixed, experimentally measured feed were qualitatively consistent with estimates obtained through variable-feed calculations. The analysis also indicates that low-moisture coals contain only two phases at equilibrium and do not form a free-water phase.