Page 260 - Fundamentals of Enhanced Oil and Gas Recovery

248 Alireza Keshavarz et al.

Consequently, high-rank coals provide larger surface area and more sorption sites
compared to low-rank coals, thereby increasing the maximum adsorption capacity.
Another parameter that could alter the sorption capacity of a given coal rock at a spe-
cific pressure is temperature. There is a negative correlation between the maximum
sorption capacity and temperature of a given coal at a specific pressure. Although con-
siderable variation in reservoir temperature occurs quite seldom in practice, this evalu-
ation might be accomplished for some especial purposes such as extrapolation of the
data obtained from a shallower taken sample in the exploration stage of a basin, to a
deeper basin section that is higher in temperature [2].
In order to make sure regarding the accuracy of the sorption isotherm considered in
reservoir engineering evaluations for a given coal seam or basin, a number of isotherms
should be calculated and compared to find the most reliable and precise isotherm curve
to characterize the reservoir. Seidle suggested that the number of required sorption iso-
therms depends on the scale of the reservoir section considered; therefore, this number
is different for a single seam, multiple seams in a single well, a project, and a basin [2].
While the justifiable number of required sorption isotherms varies from three isotherms
in good consistency for a single coal seam, more than a dozen isotherms are required to
characterize the sorption behavior of a basin [2]. This is quite obvious that this number
increases with complexity degree of coals. It should also be mentioned that the simplis-
tic consideration of a single isotherm for a given seam or well might provide substan-
tially erroneous results, and this assumption would be much worse for a given basin.
The initial gas content in coals is mainly accounted for by methane, while consid-
erable amount of other gases such as CO 2 , ethane, and nitrogen might also exist in
coal seams. The sorption isotherm of a given coal differs for distinct gases, and the
coals’ affinity degree toward various gases is related to their boiling point [47].
Typically, coals have more affinity to adsorb CO 2 than methane and have the least
affinity to nitrogen among these three gases [47]. Consequently, for a given coal, the
Langmuir volume constant for CO 2 is higher and for nitrogen is lower than methane.
Correspondingly, among the three gases, the Langmuir pressure constant is the lowest
for CO 2 and the highest for nitrogen. Therefore, coal seams are potential sites for
CO 2 sequestration, a process which is also aimed at enhancing methane recovery from
CBM reservoirs.
For sorption characterization of a coal in the state of entailing a mixture of gases,
the Langmuir equation should be modified to be capable of describing multicompo-
nent sorption behavior. The modified Langmuir equation is referred to as extended
Langmuir isotherm equation [2]:

py j =p Lj
V j 5 V Lj P nc (8.14)
1 1
k51 py k =p Lk