Ongoing measurements are critical in assessing the
effects of Basin-wide water extraction from the Fruitland coalbeds. As water is produced from coalbeds down-dip
from the outcrop, the hydrostatic pressure is reduced most dramatically in the
vicinity of each well bore. The influence of coalbed water withdrawal, which is
manifested as a decrease in hydrostatic pressure within the various coal seams,
would be expected to diminish with increased distance from gas well(s). Hydrostatic pressure reduction on the
coalbeds in turn allows sorbed gas to be desorbed when the pressure is lessened
sufficiently.
The coal isotherm
(Appendix C: Charts 7b) is a
graphical representation of the relationship between the release of adsorbed
coalgas (in cubic centimeters per gram of coal) and the effects of pressure at
a specified temperature. By consulting
this chart, it is possible to predict the equilibrium adsorption/desorption
isotherm. This isotherm depicts the
amount of gas anticipated to be released/desorbed from the coal at prevailing
conditions of temperature and pressure.
Carbon dioxide has a greater affinity for coal than
does methane. Therefore, as reservoir
pressure is reduced, methane will be desorbed first. Thus, early stages of reservoir pressure reduction yield gas with
a methane component often in excess of 99 percent by volume. As reservoir pressure is further reduced,
the carbon dioxide component increases.
With each incremental pressure reduction, commensurately more gas will
be released, but the carbon dioxide component will account for an increasing
portion of the gas mixture. Coalgas production may be limited by economic factors
when the carbon dioxide component of the natural gas reaches a threshold
concentration at which the cost of carbon dioxide removal is prohibitive. At current gas prices, this threshold may be
breached when the carbon dioxide component of the produced gas stream exceeds
20% by volume.
This change in coalgas composition is well
illustrated by Appendix C: Chart 12, which
illustrates the changes observed in soil gas composition at the Valencia Canyon
Gap Collector. The methane component
has decreased from 91% in 1995 to 84%
in July1999. Conversely, the carbon
dioxide portion has increased from roughly 9% in 1996 to nearly 16% in July
1999. In October 1999 soil vapor tube
determinations at seven sites surrounding the collector yielded carbon dioxide
in concentrations from 15% to 18%. This
same phenomenon may be perpetuated along the entire Fruitland coal outcrop as
water saturation of the near-surface coalbeds decrease. Initial high methane concentrations would be
anticipated to decline with a commensurate increase in the carbon dioxide
component. Monitoring for the carbon
dioxide component of soil gas was initiated in October/November 1999, revealing
carbon dioxide concentrations as high as 36% at one site directly up-canyon
from a shallow gas well converted to monitoring status. Methane decreases may in part reflect the
release of greater percentages of carbon dioxide from the coal. Methylotrophic bacterial oxidation of
methane in near-surface environments also produces carbon dioxide (Bennett and
Lee, 1996). One third of the soil vapor
tubes showing significant change in methane concentration are decreasing. The
respective gas-to-coal affinity relationship combined with bacterial oxidation
effects may explain why some sites along the outcrop exhibit waning (or
fluctuating) soil vapor methane (LEL) concentrations.
While
de-watered Fruitland coal exposures may most readily manifest the influence of
low pressures predicted by the coalgas isotherm, similar responses would be
anticipated in coalgas wells where the reservoir pressure is reduced at
depth. Indeed, higher carbon dioxide
gas content (to 20%) is currently being observed in portions of the basin where
reservoir pressures have been reduced the most dramatically. Northern Basin infill gas wells drilled in
1999 still encounter virgin reservoir pressures and initially produce gas
characterized by greater than 99 percent methane and only several tenths of a
percent carbon dioxide (Zimmerman, 1999).