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Enhanced Gas Recovery Techniques From Coalbed Methane Reservoirs 243

Young’s modulus for coals is typically in the range of 7000 35,000 bars [39]. High
existence of fractures inside the coal rock decreases Young’s modulus [39]. Therefore,
since fissures inside samples are not authentic representatives of the fracture system in
the reservoirs, exact estimation of Young’s modulus in laboratory studies is much
more challenging. Typically, the modulus of high-rank coals such as anthracite is
higher compared to bituminous or lower rank coals [30]. The values of the Young
modulus of coals are much lower than that of conventional rocks, and also compared
to its surrounding rocks [30]. This difference in the modulus between a coal and adja-
cent rock, which could be up to an order of magnitude in some cases, contributes to
fracture confinement inside the coal [30].
Another important elastic property of coals to be taken into account for reservoir
engineering purposes is Poisson’s ratio, defined as the ratio of transverse (lateral) strain
to axial strain while subjected to uniaxial loads, or in other words, the measure of the
lateral expansion versus longitudinal shrinkage for a longitudinally imposed load,
Eq. (8.10) [40]. The range for Poisson’s ratio for all rocks is between 0 and 0.5, while
the typical range of this factor for coals is between 0.2 and 0.4, with its average being
somewhere around 0.3 [2].

dε lateral
ν 52 (8.10)
dε axial
where ν is the Poisson ratio, dimensionless, dε lateral is the lateral strain, dε axial is the
longitudinal (axial) strain.
Rogers suggested that Poisson’s ratio for the reservoir rock and the surrounding
rock affects the reservoir stress profile and the parameters defining fracture boundary
as well as orientation [30]. Additionally, Poisson’s ratio is among the influential para-
meters in cleat width determination [30]. In reservoir engineering studies, Young’s
modulus and Poisson’s ratio are often interpreted concurrently to evaluate the elastic
behavior of reservoir and cleat system in coals. Lu et al. suggested that during hydrau-
lic fracturing, a larger effective disturbance zone is expected in the condition of a low
Young’s modulus and high Poisson’s ratio. This was attributed to the better compress-
ibility of coal in such condition, which in turn would result in a greater shear stress to
facilitate disturbance zone expansion [41].

8.3 PRODUCTION PROFILE IN COALS

The production mechanism in coals is exclusively associated with this type of
reservoir and differs substantially from that of conventional reservoirs. The production
process in coals mostly initiates with extracting a considerable amount of water, being
the mobile water originally filling the cleats. This stage of coal production is referred