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Enhanced Gas Recovery Techniques From Coalbed Methane Reservoirs 245

Figure 8.2 Schematic illustration of gas-flow stages in coals.

desorption of the gas molecules from the sorption sites. The desorbed gas from the
micropores diffuses through the porous media (governed by Fick’s law) and enters the
cleat system in which the gas flows freely (Darcy flow). Therefore, these mechanisms
are discussed in this section, focusing on sorption and diffusion. Fig. 8.2 schematically
illustrates theses three steps associated with gas flow in coals.

8.4.1 Sorption
The main gas-storage mechanism in coals differs from that of conventional reservoirs
in which the compressed gas fills the void space (pores) due to imposed pressure from
overlying planes. In coals, a little amount of dissolved gas in water or free gas might
exist in cleats, and the bulk volume of gas is stored onto the surface of micropores, a
mechanism termed sorption or adsorption. Therefore, in order to extract gas from
CBM reservoirs, gas desorption is the first step, accomplished through pressure draw-
down in the reservoir, which is in turn viable by dewatering the reservoir. The
adsorbed gas is attached to the coal rock by van der Waal’s bonds between gaseous and
solid molecules. Therefore, the affinity of a given rock for gas adsorption differs for
different gases. CH 4 and CO 2 are two gases with a strong capability to bond with the
coal rock surface. It should also be mentioned that most of the existing gas in coals in
adsorbed state is at liquid-like density [42].
There is a critical point for the pressure in coal reservoirs, termed saturation pres-
sure, over which no gas molecule is desorbed from or adsorbed on the rock surface
during pressure alteration. However, for reservoir pressures lower than this point, any
reduction in pressure (during reservoir depletion) would lead to gas desorption. The
desorption rate is controlled by reservoir temperature, porosity distribution, gas com-
position, coal rank, and composition [43,44]. One of the most striking features of
coals is their abnormal large pore surface area, providing the site on the rock to store
enormous amount of gas in adsorbed state. A given reservoir volume of a coal might