Summary
The production rates of coalbed gas wells commonly vary significantly, even
in the same field with similar reservoir permeability and gas content. The
compositional variation in produced gas is also not everywhere predictable,
although in most fields produced gas becomes progressively enriched in CO2
through the production life of a reservoir, such as parts of the San Juan
basin. In contrast, it is generally observed that the ratio of CO2:CH4 declines
with time during field and laboratory desorption testing of coal cores. In this
study, we investigate numerically the importance of coal fabric, namely cleat
spacing and aperture width, on the performance of coalbed gas wells and gas
compositional shifts during production. Because of the cubic relationship
between fracture permeability and fracture aperture width (and thus fracture
porosity) for a given cleat permeability, the production profile of coal seams
varies depending on whether the permeability is distributed among closely
spaced fractures (cleat) with narrower apertures or more widely spaced
fractures (cleat) with wider apertures. There is a lower fracture porosity for
coal with widely spaced fractures than for coal with closely spaced fractures.
Therefore, the relative permeability to gas increases more rapidly for coals
with more widely spaced cleats as less dewatering from fractures is required,
assuming that the fractures are initially water saturated. Increase in cleat
spacing from 0.01 to 10 cm significantly enhances the peak gas production and
shortens the period to reach peak production.
The main stage of gas production is controlled by equilibrium desorption of
gas from coals due to the relatively slow changes in reservoir pressure. The
enrichment of CO2 in the production gas with time occurs because of the
stronger adsorption of coals for CO2 than CH4. However, during desorption of
coal cores, CO2 desorbs more rapidly than methane because desorption rate is
governed more by diffusion than by sorption affinity, and CO2 has much higher
effective diffusivity in microporous coals than CH4. Therefore, during canister
desorption, there is a rapid increase in CO2 concentration in the desorbed gas
followed by a steady decline in CO2 concentration, in contrast to the
progressive enrichment of CO2 in produced gas from wells.
Introduction
Coalbed methane wells even within the same field invariably have dissimilar
production rates, times to peak production, decline curves, and gas
compositional shifts. Such variations in production and gas composition have
been attributed to many factors in the literatures, including coal physical and
chemical properties; variable damage caused by drilling, cementation, and
inconsistent completion; and basin-scale geologic and hydrologic settings
(Kaiser et al. 1994, Scott 2002, Cui et al. 2004a). Numerous studies (Remner et
al. 1986, Reeves and Decker 1991, Reid et al. 1992, Young et al. 1992, Shi et
al. 2002, Roadifer et al. 2003) have investigated the effects of coal
properties and reservoir conditions on coalbed gas-well performance.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
16 September 2004
- Revised manuscript received:
15 September 2005
- Manuscript approved:
27 October 2005
- Version of record:
20 March 2006
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