Page 97 - Fundamentals of Enhanced Oil and Gas Recovery

85
Enhanced Oil Recovery Using CO 2

reported an average deviation of 3.5% between measured and calculated viscosities for
429 data points.

3.2.2.2.11 Emera and Sarma [41]
Emera and Sarma [41] developed an empirical model based on the CO 2 solubility,
initial oil viscosity, saturation pressure, temperature, and oil-specific gravity as follows:

ð
Sol mole fractionÞ
μ 5 y 3 μ 1 A (3.24)
i
μ
i
where
y 5 x B
ðγ 3 SolÞ
x 5 ðC 3 μ ðP s =1:8T132ÞÞDÞ
i
A 52 9:5; B 52 0:732; C 5 3:14129; D 5 0:23
P s is saturation pressure in psi, T is temperature in F, Sol is CO 2 solubility in oil

in mole fraction, and γ is oil-specific density.

3.2.2.2.12 IFT Reduction
IFT determines the mixing potential between two fluids. In an immiscible process,
the IFT between CO 2 and oil is not very close to zero. However, when CO 2 dissolves
in the oil, the IFT between CO 2 and the oil phase reduces. It is not only the IFT
between CO 2 and the oil that decreases during an immiscible process, but also the
IFT between the oil and water decreases. Such a drop in IFT value will result in an
increase oil recovery when water is considered to be flooded alternatively/simulta-
neously with CO 2 .
To make a clear picture of IFT importance and its effects on oil recovery, a dimen-
sionless number is defined as the capillary number (N ca ). In essence, the capillary
number is the ratio of viscous versus capillary forces. Commonly, capillary number is
formulated as follows [12]:
Viscous force v 3 μ
N ca 5 5 (3.25)
Capillary force σ
where v is the velocity, μ denotes the viscosity, and σ indicates the IFT.
In practice, it is preferred to have a viscous-dominated flow regime. In another
words, a large capillary number leads to less residual trapped oil saturation. Looking at
Eq. (3.25), it is assumed that any technique raising the product of velocity and viscos-
ity does increase oil recovery, as it increases the capillary number. Owing to this fact
that the prementioned product is proportional to the pressure drop; thereby, the injec-
tion pressure is limited to fracture pressure. Therefore it is of great interest to make a