Page 253 - Fundamentals of Enhanced Oil and Gas Recovery

Enhanced Gas Recovery Techniques From Coalbed Methane Reservoirs 241

k
ð
5 exp 23c f σ 2 σ i Þ (8.5)
k i
where σ is the effective horizontal stress normal to cleats, σ i is the initial effective hor-
izontal stress normal to cleats, c f is the cleat volume compressibility, P ε is the matrix
deformation Langmuir pressure, and ε l is the matrix-shrinkage coefficient.
Based on their model, Shi and Durucan claimed that the matrix-shrinkage term is
1.5 3 times stronger than Palmer and Mansoori model. Indeed, they postulated that
Palmer and Mansoori model has an extra multiplier in the matrix-shrinkage term
causing the model to underestimate matrix-shrinkage term [27].

8.2.5 Coal Density
Coal density is among the important parameters for reservoir engineering purposes
and is inserted as an input property in simulation studies. Coal density is typically less
than that of conventional reservoirs and differs from seam to seam based on the given
coal rank and purity [2]. The bulk density of a coal consists of the matrix and the
void space, with the latter being expectedly filled with water. The dry coal density,
however, is only made up of matrix system density. The coal density is supposed to
increase with coalification, implying that high-rank coals are denser than low-rank
coal rocks [16]. The bulk coal density was presented by Seidle as a function of the
densities of entailed ash, moisture, and the organic rock with their corresponding
weight percentage, with the assumption of no free or sorbed gas available in the cleat
system [2].
1
(8.6)
ρ 5
ð 1 2 a 2 wÞ=ρ 1 a=ρ 1 w=ρ
o a w
where ρ is the coal bulk density, ρ is the organic fraction density, ρ is the ash density,
a
o
ρ is the water density.
w
It should also be noted that o, a, and w represent the weight percentage of organic,
ash, and water content of the rock, respectively.
Eq. (8.6) shows that depending upon the organic and inorganic content of the
coal rock, the rock density could vary substantially. In lack of the laboratory measure-
ments for a given coal rock, the organic and ash densities are assumed 1.25 and
3
2.55 g/cm , respectively. These numbers allude that for a high-rank coal, being denser
in essence, with a low ash content, the density might be lower than a low-rank coal
with a vast presence of ash inside the coal rock. Additionally, the density of
organic fraction of coal rock is further dependent on maceral composition of that par-
ticular coal.