Multicomponent Sorption Modeling in ECBM Displacement Calculations

Abstract

AbstractEnhanced coalbed methane (ECBM) recovery by injection of CO2 or by injection of mixtures of CO2 and N2 is an attractive method to recover additional natural gas resources while at the same time sequestering CO2 in the subsurface. The dynamics of ECBM recovery processes are determined in large part by the sorption behavior of mixtures of CH4, CO2 and N2 on the coal surface. Conventional simulation tools use the extended Langmuir model for predicting the sorption behavior of the gas mixtures that form during a displacement process. In previous work, we demonstrated the applicability of this approach for binary CH4/CO2 and CH4/N2 displacements by comparison to lab-scale experiments. The extended Langmuir model was, however, unable to describe the behavior of ternary CH4/CO2/N2 displacements. Adsorption hysteresis is a complicating factor also. This paper investigates the accuracy of ternary gas displacement calculations. We find that a sorption model more sophisticated than the extended Langmuir model is needed to represent the dynamics of multicomponent systems. We use the Ideal Adsorbate Solution model (IAS) and compare the predicted behavior to standard calculations and experimental results.Initially, we describe the implementation of the IAS model into our dual-porosity simulator. The IAS model requires an iterative scheme as opposed to the explicit calculation from the extended Langmuir model. Accordingly, the IAS model is more computationally expensive than traditional approaches. The predicted displacement behavior substantially agrees with the experimental observations in comparison to the inaccurate predictions from the traditional approach for a series of ternary CH4/CO2/N2 displacements.Accurate tools for prediction of displacement performance in ECBM processes are instrumental in design and implementation of enhanced gas recovery schemes. The results and analysis presented in this paper, demonstrate that more sophisticated thermodynamic models must be used in ECBM simulators when mixtures of CO2 and N2 are used to displace CH4 from coalbeds.