Page 106 - Fundamentals of Enhanced Oil and Gas Recovery

94 Ramin Moghadasi et al.

3.8 ENVIRONMENTAL ASPECTS OF CO 2 INJECTION

CO 2 has been known as a byproduct of industrial and domestic operations,
which is undesirable due to its harmful impacts on the world’s climate. Since the
Industrial Revolution, a rise of about 30% has been reported in released CO 2 content
in the atmosphere [85]. Many research studies have been conducted to achieve proper
ways for mitigating CO 2 . Recently, and to a greater extent of innovation, geological
sequestration has been known to be a promising option that may lead to significant
reduction of CO 2 . Beside this sequestering opportunity, underground CO 2 injection
serves as an opportunity for increased oil recovery. Indeed, nowadays CO 2 EOR is
being studied along with the opportunities of CO 2 sequestration. This is because oil
reservoir has such characteristics that could bring the opportunity of CO 2 sequestra-
tion. Generally, there are three major mechanisms by which CO 2 could be seques-
tered within the oil reservoirs. The first is hydrodynamic trapping, in which CO 2 gas
would be trapped beneath a cap rock. The next is related to CO 2 solubility in water
and oil phases, which is known as solubility trapping. Lastly, CO 2 can react with res-
ervoir rock and organic matter, directly or indirectly, and can be converted into the
solid phase. However, each of these mechanisms can alter the injection and produc-
tion efficiency toward reduction of injection and production efficiency. For instance,
when CO 2 is converted into a solid phase, it may lead to permeability impairment.
As a result, the injectivity is reduced [11,86].
To measure the storage capacity of a reservoir, specific capacity of the rock could
be used. Indeed, this value could be used to differentiate sequestration potential
among reservoirs. Specific storage capacity is expressed by Eq. (3.8) as follows [87]:

ð
C 5 ρ 1 2 S or 2 S wir Þ[1 S wir [C s (3.26)
where ρ is CO 2 density, which is a function of pressure and temperature; S or indicates
the residual oil saturation; S wir stands for the irreducible water saturation; [ shows the
rock porosity; and C s symbolizes the mass of CO 2 dissolved per unit volume of water.
Based on this equation, a reservoir that is deep and has a sizeable porosity in which a
large fraction of moveable fluids is contained would lead to maximum CO 2 sequestra-
tion potential.
Eq. (3.26) describes the total possible amount of the CO 2 that could be stored
within a reservoir. In real conditions, this capacity depends on many factors, including
reservoir heterogeneity, aquifer availability, reservoir boundaries, and geophysical
aspects. Reservoirs with a high degree of heterogeneity have less potential for com-
bined efficient storage and EOR purposes. This is because CO 2 breakthrough occurs
at early stages when there is high permeability variation within the reservoir (i.e.,
high degree of heterogeneity). Aquifers are categorized as bottom water or edge water