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Enhanced Gas Recovery Techniques From Coalbed Methane Reservoirs 255

the shear stress perturbation and improve flow passes by offsetting the two frac-
ture’s rough surfaces [78]. Therefore, application of this stimulation method in
CBM reservoirs can improve CBM productivity index because of mismatches and
asperities in the cleat system created due to relocation of cleat’s walls from their
original place [76]. This is a long-term fracture stimulation as created asperities on
the rough fracture walls resist against sliding back to their original locations after
withdrawal of the injected fluid.
2. Improving conductivity of preexisting fracture network by increasing the average of fractures’
aperture due to high-pressure fluid injection.
In stress-dependent fractured reservoirs such as CBM reservoirs, the average of
fractures’ aperture in preexisting fracture network is proportional to the reservoir
pressure. Therefore, increasing reservoir pressure can improve the conductivity of
the fracture system by increasing fracture’s aperture. In this stimulation technique,
although injecting high-pressure fluid opens up preexisting fractures and improves
fracture conductivity and connectivity, the opened fractures may get closed after
withdrawal of the injected fluid. Therefore, the main challenge is to keep the frac-
tures open after pressure decline.

8.5.1.3 Proppant Placement
The aim of hydraulic stimulation techniques, either hydraulic fracturing or natural
fracture stimulation, is to improve productivity index by creating high conductive
flow conduit from the reservoir toward the wellbore. This can be achieved by inject-
ing high-pressure fluids into the reservoir. However, in order to keep the fractures
open subsequent to pressure decline, injected fluids should be accompanied by rigid
small propping agents (proppant) to be placed inside the open fractures in order to
lessen the influence of pressure dissipation on fracture conductivity, due to withdrawal
of the injected fluid.
Induced fractures are filled by multilayers of proppants in hydraulic fracturing
treatments. The pack of proppants creates an artificial conductive porous media inside
the fracture resisting against fracture closure in the production stage. Darin and Huitt
reported that larger conductivity is achieved where a hydraulic fracture is filled by a
partial monolayer of large-sized proppant particles rather than fully packing by multi-
layers of small-sized proppant particles [81]. The partial monolayer proppant place-
ment technique can also be used to prop up natural fracture system using microsized
proppant particles [82 84]. Two main parameters influencing fracture conductivity
are confining stress and proppant concentration [82,85 88]. The higher the concen-
tration of placed proppants in the fracture, the more barrier against flow conductivity
in the fracture. The lower the concentration of placed proppants in the fracture, the
higher the risk of fracture deformation and conductivity decline, as observed on