Page 249 - Fundamentals of Enhanced Oil and Gas Recovery

Enhanced Gas Recovery Techniques From Coalbed Methane Reservoirs 237

micropore and mesopore sites are considered to be impermeable, and the gas flow
occurs by diffusion through the porous media toward the cleats. The flow gradient in
the primary porosity system, on the contrary to conventional reservoirs, is controlled
by gas concentration gradient. The cleat porosity has been suggested to be dependent
on the coal composition and rank [17]. Through X-ray CT scanning, Karacan and
Mitchell recognized that coal microlithotypes determine the coal porosity [18].
Similarly, Mukhopadhyay and Hatcher suggested that coal porosity is related to both
coal type and coal rank [19].
Coal porosity refers to the total void space in a coal rock. However, in some reser-
voir engineering studies, the mobile water porosity is considered instead of the total
porosity. The mobile water porosity is defined as the space filled with water, which
will be produced in dewatering stage (the first CBM production stage). It is obvious
that in the latter porosity concept, the void space containing gas or immobile mois-
ture is excluded from the porosity, and the coal porosity consists of only the mobile
water space inside the cleats, macropore, and some mesopore porosities. In fact, the
mobile water porosity is the major conduit of fluid flow toward the wellbore, for
which the Darcy flow is applicable. The fracture (cleat) porosity inside a coal is the
same as that in typical naturally fractured reservoirs, at about 1% or lower [20]. The
estimation of matrix and cleat porosities differs in methodology; matrix porosities are
estimated through laboratory experiments, while in order to determine the coal cleat
porosity, conceptual models or simulation history matches are reliable tools. However,
an experimental method termed “miscible tracer technique” in which the displacing
fluid contains a traceable component is also an experimental approach for cleat poros-
ity estimation using cores [21].
It is noteworthy that in coals, the porosity distribution is related to the fixed car-
bon content of the rock, such that the increase in carbon content would result in a
greater fraction of coal porosity to be composed of micropores, which in turn roots in
the consolidation of the coal rock as it progresses through coalification process.
Therefore, as it is expected, in high-rank coals with the fixed carbon content of over
85%, the coal porosity is mostly accounted for by micropore structures [22]. There is
a widely agreed classification of the coal pores with regard to coal rank [23]. This clas-
sification is resulted from high-resolution electron microscopy and is illustrated in
Table 8.1 [24,25]. As is observed on the table, an increase in the coal rank (from lig-
nite and subbituminous to the highest rank of bituminous and anthracite coals) results
in the reduction in the pore size, and also the dominant pore structure changes from
macropore in lignite to micropores in the highest ranked coals. Furthermore, it alludes
that while in high-rank coal, adsorption is the main storage mechanism due to a high
specific surface fractal dimension, low-rank coals might encompass considerable
amount of free gas compressed in their void space, in the macropores.