Injection of CO2 into deep unminable coal seams is an option for
geological storage of CO2. Moreover, injection of CO2 may
enhance the recovery of CH4 in these systems, making coal reservoirs
interesting candidates for sequestration.
New analytical solutions are presented for two-phase, three- and
four-component flow with volume change on mixing in adsorbing systems. We
analyze the simultaneous flow of water and gas containing multiple adsorbing
components. The displacement problem is solved by the method of
characteristics. Mixtures of N2, CH4, CO2, and
H2O are used to represent enhanced coalbed-methane (ECBM) recovery
processes. The displacement behavior is demonstrated to be strongly dependent
on the relative adsorption strength of the gas components.
In ternary systems, two types of solutions result. When a gas rich in
CO2 displaces a less strongly adsorbing gas (such as
CH4), a shock solution is obtained. As the injected gas propagates
through the system, CO2 is removed from the mobile phase by
adsorption, while desorbed gas propagates ahead of the CO2 front.
The adsorption of CO2 reduces the flow velocity of the injected gas,
delaying breakthrough and allowing for more CO2 to be sequestered
per volume of CH4 produced. For injection gases rich in
N2, a decrease in partial pressure is required to displace the
preferentially adsorbed CH4 and a rarefaction solution results.
In quaternary displacements with injection-gas mixtures of CO2
and N2, the relative adsorption strength of the components results
in solutions that exhibit features of both the N2-rich and
CO2-rich ternary displacements.
Analytical solutions for ECBM recovery processes provide insight into the
complex interplay of adsorption, phase behavior, and convection. Improved
understanding of the physics of these displacements will aid in developing more
efficient and physically accurate techniques for predicting the fate of
injected CO2 in the subsurface.
Atmospheric concentrations of CO2 have increased significantly
from preindustrial levels of 280 ppm to current concentrations of 385 ppm
(Carbon Dioxide Information Analysis Center 2003). This increase is attributed
to human activity, the majority of which is ascribed to fossil-fuel combustion,
and is believed to be responsible for current global warming trends (Metz et
al. 2005). ECBM recovery is a promising technology that could contribute to
reduction of greenhouse-gas emissions (Stevens et al. 1998, Stevens 2001).
Simultaneous recovery of CH4 while CO2 is sequestered in
coal seams is an attractive option because it addresses the issue of increasing
atmospheric CO2 concentrations while offsetting some of the costs of
capture, compression, transportation, and storage of CO2 by
production of CH4, the fossil fuel that has the lowest
CO2 emissions per unit of energy made available for conversion.
In this paper, we use the method of characteristics to obtain new analytical
solutions for two-phase flow in ternary and quaternary systems with adsorption.
A number of researchers have applied this technique to solve problems relevant
to the oil industry (Isaacson 1980; Monroe et al. 1990; Dindoruk et al. 1992;
Johns and Orr 1996; Wang 1998; Orr 2007). Related problems for multicomponent
flow of incompressible fluids with adsorption have been investigated by
Johansen and Winther (1988, 1989), Dahl et al. (1992), and Shapiro et al.
(2004). Zhu et al. (2003) applied this method to model single-phase gas flows
with adsorption to investigate ECBM in a dry coal.
We consider isothermal, 1D flow in a homogeneous porous medium. The effects
of gravity and capillarity are neglected to isolate the interplay of sorption
and flow. We use an extended Langmuir isotherm (Yang 1987) to describe the
adsorption and desorption of gas species on the coal surface and assume that
water does not adsorb on the coal surface. In this analysis, no chemical
reactions occur between the injected CO2 and in-situ elements of the
coalbed reservoir system.
The analytical solutions presented are used to delineate the interactions of
convection, phase behavior, and adsorption and desorption of gas components
from the coal surface as gas mixtures propagate through the system. These
solutions are also used to predict the effect of mixed-gas injection on
CH4 recovery. They provide a guide to optimizing injection-gas
composition, depending on the objectives of the gas-injection scheme.
© 2009. Society of Petroleum Engineers
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- Original manuscript received:
25 October 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
29 September 2008
- Manuscript approved:
1 October 2008
- Published online:
16 March 2009
- Version of record:
1 March 2009