Analytical Theory of Coalbed Methane Recovery by Gas Injection
- Jichun Zhu (Stanford U.) | Kristian Jessen (Stanford U.) | Anthony R. Kovscek (Stanford U.) | Franklin M. Orr Jr. (Stanford U.)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- December 2003
- Document Type
- Journal Paper
- 371 - 379
- 2003. Society of Petroleum Engineers
- 5.8.3 Coal Seam Gas, 4.1.2 Separation and Treating, 5.4 Enhanced Recovery, 4.6 Natural Gas, 4.1.5 Processing Equipment, 5.4.2 Gas Injection Methods, 4.1.4 Gas Processing, 5.3.1 Flow in Porous Media, 5.7.2 Recovery Factors, 5.2.2 Fluid Modeling, Equations of State, 5.2.1 Phase Behavior and PVT Measurements
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Injection of either carbon dioxide (CO2) or nitrogen (N2 ) enhances recovery of coalbed methane. In this paper, we provide new analytical solutions for the flow of ternary gas mixtures in coalbeds. The adsorption/desorption of gaseous components to/from the coalbed surface is approximated by an extended Langmuir isotherm, and the gas-phase behavior is predicted by the Peng- Robinson equation of state (EOS). Langmuir isotherm coefficients are used that represent a moist Fruitland coal sample from the San Juan basin (U.S.A.). In these calculations, mobile liquid is not considered. Given constant initial and injection compositions, a self-similar solution consisting of continuous waves and shocks is found. Mixtures of CH4, CO2, and N2 are used to represent coalbed and injection gases. We provide examples for systems where the initial gas is largely CH4, and binary mixtures of CO2 and N2 are injected. Injection of N2-CO2 mixtures rich in N2 leads to relatively fast initial recovery of CH4. Injection of mixtures rich in CO2 gives slower initial recovery, increases breakthrough time, and decreases the injectant needed to sweep out the coalbed. The solutions presented indicate that a coalbed can be used to separate N2 and CO2 chromatographically at the same time coalbed methane (CBM) is recovered.
Coalbeds have large internal surface area and strong affinity for certain gas species such as CH4 and CO2. In CBM reservoirs, most of the total gas exists in an adsorbed state at liquid-like density. Only a small amount of the total gas is in a free phase. Primary recovery using depressurization induces desorption of the CBM by lowering the overall pressure of the reservoir. Primary recovery factors are roughly 50%.1 On the other hand, enhanced recovery of coalbed methane (ECBM) by injecting a second gas maintains the overall reservoir pressure, while lowering the partial pressure of CBM in the free gas. Injectants also sweep desorbed gas through the reservoir. Nitrogen is a natural choice as an injection gas because of its availability. Carbon dioxide is also promising because of the additional benefit of greenhouse gas sequestration. When combusted, methane emits the least amount of CO2 per unit of energy released among all the fossil fuels. Therefore, there is a synergy between CO2 sequestration and production of methane that leads to greater utilization of coalbed resources for both their sequestration ability and energy content. The first application of ECBM by CO2 injection has been carried out in the San Juan Basin.1
One important aspect of ECBM is the adsorption and desorption behavior of gas mixtures on coalbeds. A significant amount of work has been invested on this issue as it is related to coal-mine safety.2-11 However, transport of multicomponent gas mixtures through coalbeds has not been examined in detail. Arri et al.12 studied the primary recovery of a single sorbing component and ECBM by nitrogen injection. In this paper, we extend the analysis to systems with three adsorbing components. Besides CH4, coalbeds may contain significant amounts of CO2, N2 , and other gas species. A coalbed gas representative of San Juan Basin conditions is composed of about 93% CH4, 3% CO2, 3% wet gases, and 1% N2.13 Further, when flue gas is used for ECBM and CO2 sequestration, the injection gas may contain more than two components. The exact composition of flue gas depends on the combustion temperature, oxygen content of the air supply, and fuel moisture content, among other factors.14 Accordingly, a complete spectrum of injection gas conditions is studied.
We solve the flow problem analytically using the method of characteristics. Similar approaches have been used for related problems. Rhee et al.15 modeled convective and adsorptive exchange processes from a chromatography point of view. Helfferich16 investigated a similar problem from a different standpoint. A set of rules for coherent waves was developed based on qualitative features of frontal displacement. The topology of the wave pattern was laid out in distance and time, and constructed in a step-bystep fashion.
The analyses of Rhee et al.15 and Helfferich16 are extended to the adsorption, desorption, and transport of CBM gases during the ECBM process with gas injection. In a work related to gas injection for enhanced oil recovery, Dindoruk17 coupled equilibrium phase behavior of a multicomponent mixture with multiphase fluid flow through porous media using the method of characteristics. The analysis included the effect of volume change upon mixing and complex phase behavior through an EOS. To develop an analytical model for ECBM, we adopt the analysis of Dindoruk to describe volume change on mixing and solve for flow with adsorption and desorption behavior. Gas properties as a function of composition are described by the Peng-Robinson EOS.18 The following assumptions were made:
Flow is one-dimensional (1D).
The adsorption and desorption of gas mixtures on coal is modeled with the extended Langmuir isotherm.19
Adsorbed gases occupy negligible volume on coalbed surfaces.
Flow is single-phase. Any water phase present is immobile as might occur after the coalbed is produced by a primary method.
The temperature remains constant.
For the purpose of calculating the adsorption and gas density, the pressure is assumed constant.
The porosity and permeability of the coalbed are constant and uniformly distributed.
Frontal advance is rapid enough that diffusion and dispersion along the axial direction are neglected.
The effects of gravity are negligible.
These assumptions simplify the problem while still allowing valuable insights into the subject processes. Importantly, they allow us to begin the formulation of a general analytical theory for multicomponent, two-phase flow in coalbeds.
In the following sections, we describe the extended Langmuir isotherm, establish a mathematical model based on material balances, and discuss the solution procedure for systems with up to three components. The analytical solution method is then applied to a few examples, in which we study the effect of injection gas composition on CBM recovery and CO2 storage.
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