Page 248 - Fundamentals of Enhanced Oil and Gas Recovery

236 Alireza Keshavarz et al.

while volatile and moisture constituents decrease. However, there is no clear-cut ver-
dict regarding the ash content of a coal rock based on its rank. Indeed, ash is an index
of purity level of coals and is dependent on the coal mineralization as well as burying
environment.

8.2.2 Macerals

The organic equivalent of inherent minerals in sandstones is referred to as “macerals”
in coals and is mainly composed of remaining fossilized plants in the coal. The mac-
erals are manifest under microscopes. The macerals content of coals could be catego-
rized into three groups: vitrinite, liptinite, and inertinite, which change independently
as the coal rock progresses toward coalification [14]. These three macerals group are
originated from different parts of a plant. Vitrinite group materials are originated
from woody plants (mainly lignin) and make up the major part of macerals in a coal.
Increasing vitrinite fraction in a coal macerals composition often correlates with the
sorption capacity of the rock [15]. On the other hand, the derivation of liptinite is
lipids as well as waxy plant substances, and inertinite group components are originated
during peat formation stage from oxidized plant materials. The macerals composition
of a coal might include all of the three types or might be accounted for by two or just
one of these groups. The macerals composition of the three groups vary toward the
same composition as the coal rank increases, and when the carbon content reaches
94% they become almost indistinguishable [16].
One of the approaches based on which the coal rank is determined is through
interpreting the maceral composition of the coal rock. For this purpose, the reflected
light (which is the reflection of a vertical ray of a prespecified wavelength) by the
vitrinite maceral from a polished sample is quantitatively measured. This measurement
is accomplished under oil and might vary up to 8% for a given sample based on the
orientation of the sample. Subsequent to examining this value for many times in dif-
ferent sample orientations, the maximum vitrinite reflectance is reported. In fact, this
measure increases with maturity of coal rocks; therefore, this value is normally consid-
ered among the criteria in determination of the coal rank.

8.2.3 Coal Porosity
Coals are among naturally fractured reservoirs for which a dual porosity system is con-
sidered, as they encompass a wide spectrum of pore sizes categorized into two distinct
groups: primary (micropore and mesopore) and secondary (macropore and cleats)
porosity systems. The primary porosity is the major sorption site for the gas and holds
the greatest fraction of the containing gas, while secondary porosity system serves as
the main conduits to transfer the fluid toward the production wellbore. In coals,