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Enhanced Gas Recovery Techniques From Coalbed Methane Reservoirs 253

Figure 8.4 Gas transport in coal matrix and cleats. Modified after Busch, A., Gensterblum, Y., 2011.
CBM and CO 2 -ECBM related sorption processes in coal: a review. Int. J. Coal Geol. 87 (2), 49 71.

The desorbed gas that has diffused through the micropore and macropore media
permeates through the cleat system toward the production well, a flow which is
described by Darcy equation and is the third mechanism in gas flow in coal rocks.

8.5 COALBED METHANE PRODUCTIVITY AND RECOVERY
ENHANCEMENT

Increasing gas production from CBM reservoirs is viable through two discrete
approaches. The first method is productivity enhancement via hydraulic stimulation in
which increasing the rock permeability is the main goal. The second method, termed
ECBM recovery, is accomplished through injecting a foreign gas into the reservoir in
order to maintain the pressure of the reservoir and also, in the case of injecting a
more adsorbable gas than methane such as CO 2 , to facilitate methane desorption from
the rock surface.

8.5.1 Hydraulic Stimulation

One of the main challenges in gas production from CBM reservoirs is their low pro-
ductivity index. Therefore, applying stimulation techniques can improve gas produc-
tion from these type of reservoirs. Hydraulic fracturing and natural fracture
stimulation techniques are the main productivity enhancement techniques in CBM
reservoirs. In hydraulic-based stimulation techniques, high-pressure fluid is injected
into the wellbore. The injected fluid improves conductivity of the existing cleat