Page 259 - Fundamentals of Enhanced Oil and Gas Recovery

Enhanced Gas Recovery Techniques From Coalbed Methane Reservoirs 247

for gas adsorption is reached. It should be noted that pressure increase occurs in labo-
ratory investigations for the purpose of sorption characteristic determination, and in
reservoir conditions during depletion, pressure decreases and desorption is the
expected phenomenon.
The adsorption/desorption behavior of gases on coals could be described with
type I isotherms. Among the most popular models presented to describe gas sorption
on rocks is Langmuir and Freudlich equation that fits the type I isotherm curve. This
equation closely matches the gas adsorption data on rocks in CBM reservoirs and is
widely used to predict the sorption behavior of gaseous phase on coals [2].

P
V 5 V L (8.12)
P 1 P L
where V is the gas content (in adsorbed state), V L is the Langmuir volume constant,
P is the pressure, P L is the Langmuir pressure constant.
Langmuir volume constant is actually the maximum capacity of a coal rock to
adsorb gas onto its surface area. Eq. (8.12) clearly implies that the higher the reservoir
pressure, the more gas adsorbed onto the coal, and at the infinite pressure, the maxi-
mum adsorption capacity of a coal is expected to be met. It also reveals that Langmuir
pressure constant represents a pressure at which half of the Langmuir volume constant
of the coal surface is covered with gas. Therefore, the Langmuir pressure constant
indicates the half-saturation pressure.
There are several factors affecting the maximum capacity of a coal rock to adsorb
gas. In coals, some of the potential gas adsorption sites might be preoccupied with ash
or moisture, both of which obstruct the access of gas to these sites, hence lowering vol-
ume constant at a specific pressure. Therefore, for reservoir engineering purposes, the
Langmuir volume constant needs to be replaced with the dry, ash-free (daf) Langmuir
volume constant in order to minimize the estimation errors. Particularly, in low-rank
coals where a considerable amount of water is available, and also the rock has higher
affinity to water than methane in some sorption spots, the moisture content would play
a significant role in determination of Langmuir volume constant. Therefore, isotherm
curves for coal samples must be drawn taking into account the moisture and ash con-
tents. Eq. (8.13) considers effects of moisture and ash content in Langmuir equation [2]:
V Lis
V Ldaf 5 (8.13)
ð1 2 a 2 wÞ
where V Ldaf is the dry, ash-free Langmuir volume constant, V Lis is the in situ
Langmuir volume constant, a is the ash mass fracture, w is the equilibrium moisture
mass fraction.
The sorption capacity of coal is also influenced by coal rank. Coals of higher ranks
are typically more tortuous rocks presenting more complicated pore structures.