Multicomponent Sorption Modeling in ECBM Displacement Calculations

Abstract

Abstract Enhanced 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. Introduction The increasing concentration of CO2 in the earth's atmosphere has motivated a wide range of initiatives aimed at reducing greenhouse gas emissions and developing less carbon intensive energy sources. Storage of CO2 in coal seams is a potentially attractive carbon sequestration technology for at least two reasons. First, injection of CO2 or mixtures of CO2 and N2, enhances methane production from coalbeds. Only about one-half of the gas originally in place in a coalbed is estimated to be recovered following primary operations. This leaves a large target in place for enhanced recovery operations. Second, coals of various ranks all generally adsorb a greater amount of CO2 in comparison to CH4 and thus geological sequestration of CO2 in coalbeds has the potential to be carbon neutral or perhaps a carbon sink. In short, CO2 is sequestered, while CH4 recovery from coal is enhanced, thus costs are either partially or completely offset while increasing production of CH4. Gas flow in coal, however, is a result of a delicate interplay among pressure gradients, coal permeability, and adsorption phenomena. Although some gas within coal exists as bulk phase within pores and cleats (fractures), most gas is on the surface of coal in an adsorbed state at liquid-like density. Because of adsorption, a coalbed may contain more methane than a conventional gas reservoir of comparable size, pressure, and temperature. Effective methods, however, to release fully methane from a coalbed have yet to be developed. Nitrogen is one choice for ECBM due to its availability. Nitrogen serves to reduce the partial pressure of methane in the free gas within pores and this causes CH4 to desorb from coal surfaces. CO2 also has advantages as an injection gas. CO2 is a more effective displacement agent as it adsorbs more strongly to coal surfaces than either CH4 or N2. The adsorption properties of CO2 may also help to limit premature breakthrough at production wells. In practice, injection gases for ECBM may be mixtures of CO2 and N2. N2 generally leads to a more rapid production response than does CO2, but CO2 leads to more complete displacement. Another advantage of gas mixtures is that changes in coalbed permeability are mitigated by inclusion of some N2 with injected CO2. Our experiments reported elsewhere [1], show that a small fraction of N2, 10 to 20% by mole, helps to preserve coalbed permeability significantly.