Summary
The recent advances in numerical simulation for primary coalbed methane
(CBM) recovery and enhanced coalbed-methane recovery (ECBMR) processes are
reviewed, primarily focusing on the progress that has occurred since the late
1980s. Two major issues regarding the numerical modeling will be discussed in
this review: first, multicomponent gas transport in in-situ bulk coal and,
second, changes of coal properties during methane (CH4) production. For the
former issues, a detailed review of more recent advances in modeling gas and
water transport within a coal matrix is presented. Further, various factors
influencing gas diffusion through the coal matrix will be highlighted as well,
such as pore structure, concentration and pressure, and water effects. An
ongoing bottleneck for evaluating total mass transport rate is developing a
reasonable representation of multiscale pore space that considers coal type and
rank. Moreover, few efforts have been concerned with modeling water-flow
behavior in the coal matrix and its effects on CH4 production and on the
exchange of carbon dioxide (CO2) and CH4. As for the second issue, theoretical
coupled fluid-flow and geomechanical models have been proposed to describe the
evolution of pore structure during CH4 production, instead of traditional
empirical equations. However, there is currently no effective coupled model for
engineering applications. Finally, perspectives on developing suitable
simulation models for CBM production and for predicting CO2-sequestration ECBMR
are suggested.
Introduction
CBM has been recognized as a significant natural gas resource for a long
time. Recently, CO2 sequestration in coalbeds for ECBMR has been attracting
growing attention because of greater concerns about the effects of greenhouse
gases and the emerging commercial significance of CBM.
Reservoir-simulation technology, as a useful tool of reservoir development,
has the capability to provide us with an economic means to solve complex
reservoir-engineering problems with efficiency. The development of a numerical
simulator for CBM-reservoir simulation before the late 1980s has been reviewed
by King and Ertekin (1989a, 1989b). This paper will further present the recent
advances in this highly active area, focusing on modeling the CBM and ECBMR
processes.
Pore Structure and Gas-Diffusion Mechanisms. The pore structure of coal is
highly heterogeneous, and the heterogeneity of the pores depends on the coal
type and rank (Unsworth et al. 1988, Laxminarayana and Crosdale 1999). Recent
studies (Smith and Williams 1984; Gan et al. 1972; Clarkson and Bustin 1999a)
suggest that lower-rank coals usually exhibit a bidisperse or multimodal pore
structure, with significant fractions of the total pore volume being larger
than 30 nm and smaller than 1.2 nm. The commonly assumed pores in coals can be
divided into three categories: micropores (<2 nm), mesopores (between 2 and
50 nm), and macropores (>50 nm) (Shi and Durucan 2003a, Cui et al.
2004).
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
13 January 2005
- Meeting paper published:
5 April 2005
- Revised manuscript received:
14 March 2007
- Manuscript approved:
18 March 2007
- Version of record:
20 December 2007
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