Page 250 - Fundamentals of Enhanced Oil and Gas Recovery

238 Alireza Keshavarz et al.

Table 8.1 Relationship Between Coal Rank and Pore Size
Pore sizes Coal rank (ASTM Designation D388-98a)
Micropores d , 2 nm High-volatile bituminous coal A and higher
Mesopores 2 nm , d , 50 nm High-volatile bituminous coal (C 1 B)
Macropores d . 50 nm Lignites 1 subbituminous
Source: After Rodrigues, C., De Sousa, M.L., 2002. The measurement of coal porosity with different gases. Int. J. Coal
Geol. 48 (3), 245 251.

8.2.4 Coal Permeability
Coal permeability is among the most significant parameters in economic and technical
viability of a CBM reservoir, and also it is a key factor in predicting CBM behavior
in the state of natural depletion as well as ECBM [26 29]. In coals, the matrix system
is considered to be impermeable, and the fluid flow is expected to occur through cleat
system; thus, cleats serve as the permeability path for Darcy flow of gas and water
toward the production well. Theoretically, the development of cleat system permeabil-
ity is supposed to be dependent on the rank, grade, and type of the concerned coal. It
is also affected by in situ stresses, matrix shrinkage during gas desorption, the fre-
quency of natural fractures as well as their interconnection level, degree of fissure
aperture opening, face and butt cleats’ direction, reservoir depth, and initial water sat-
uration [30]. The permeability value of a coal seam changes during the production
profile of the reservoir and is one of the most difficult parameters to be estimated
accurately. The gas relative permeability is further challenging to obtain during the
reservoir depletion that roots in the changing of cleat aperture measure and the corre-
sponding water saturation inside the cleats within the production of the reservoir. The
typical coal seam permeability range is from impermeable to over 100 mD [30].
The permeability of coals could be defined as a function of cleat porosity and spac-
ing when permeability is in millidarcies and cleat spacing is in millimeter square:
5 3 2
k 5 1:0555ð10Þ φ a (8.1)
Considering the assumption of a stiff coal matrix, in which the cleat spacing dur-
ing depletion changes negligibly, the permeability ratio is cubically related to the
porosity ratio:

3
k φ
5 (8.2)
k i φ
i
However, this relationship between permeability and porosity is more comprehen-
sively applicable to many naturally fractured reservoirs [2,31].
One of the most striking features of coal permeability, being unique to CBM reser-
voirs, is its variation mechanism within the production profile of the reservoir.