Page 84 - Fundamentals of Enhanced Oil and Gas Recovery

72 Ramin Moghadasi et al.

and they could increase oil recovery by improving oil mobility. The abovementioned
mechanisms assist the oil in flowing easily; however, immiscible gas injection has a
lower recovery factor compared with miscible flooding with the same operational
conditions [25,36].

3.2.2.1 CO 2 Solubility in Oil
CO 2 solubility in crude oil is mainly under control of saturation pressure, tempera-
ture, and oil gravity. Generally, CO 2 solubility increases with pressure and API gravity
and decreases with a rise in temperature. Oil composition and liquefaction pressure
are two other factors that affect CO 2 solubility at temperatures less than CO 2 critical
temperature. At this condition (i.e., subcritical CO 2 condition), CO 2 dissolves in oil
as a gas rather than as a liquid. As CO 2 dissolves in oil, it affects oil viscosity, density,
and IFT value, and it causes oil to swell. There are several correlations that can predict
CO 2 solubility in crude oil.

3.2.2.1.1 Simon and Graue [37]
In 1965, they developed a graphical correlation for dead oils with temperatures rang-

ing from 43.33 C to 121.1 C, pressures up to 15.86 MPa, and oil gravity from 12 to
33 API. They presented solubility of CO 2 (mole fraction of CO 2 in a mixture of

) as a function of fugacity, saturation pressure, and temperature at
CO 2 1 oil, x CO 2
Universal Oil Products Company (UOP) characterization factor (UOPK) equal to
11.7. For oils with different UOP characterization, they proposed a correction factor.
Simon and Graue [37] have reported an average deviation of 2.3% between their pre-
dictions and experimental data.

3.2.2.1.2 Mulliken and Sandler [38]
In 1980, they argued about the inconvenience of the Simon and Graue [37] graphical
method for reservoir simulation studies. They also stated that Simon’s method is
among the methods that are not applicable for impure CO 2 or mixed gases.
Considering such shortcomings, they tried to develop a theoretical basis for predicting
the CO 2 solubility in crude oils with a wide range of application. They applied
Peng Robinson’s (PR) equation-of-state (EOS), which is as follows:
RT a
P 5 2 (3.1)
V 2 b VV 1 bÞ 1 bðV 2 bÞ
ð
where for mixtures:
X X
a 5 x i x j ð1 2 δ ij Þða i a j Þ 1=2 (3.2)
i j
X
b 5 x i b i (3.3)