Page 129 - Fundamentals of Enhanced Oil and Gas Recovery

Miscible Gas Injection Processes 117

volume remaining in reservoirs and also its economic benefits lead to production
enhancement via CO 2 for more than 40 years [58].
Heavy crude oils have a wide range of viscosities from about several hundred to
hundred thousand centipoises. Concerning mobility of oil under reservoir conditions
and reducing its value is an important factor in oil recovery. Carbon dioxide can be
used as an effective agent for reducing viscosity of the heavy oil which results in oil
mobility reduction [59].
Various mechanisms are observed during injection of carbon dioxide such as oil
viscosity reduction, oil swelling, and relative permeability hysteresis due to reduced
water saturation, wettability alternation, depressurization, diffusion, and IFT reduction
in the zone near the wellbore [12,42,60 63].
Some physical properties such as viscosity, density, and CO 2 solubility in heavy oils
are required to design and simulate a heavy oil recovery process. Determination of the
effects of CO 2 on the physical properties of heavy crude oils is the first step to design
an effective displacement process. Hence, different studies have focused on predictive
methods for properties of heavy oil/CO 2 mixtures. For instance, Simon and Graue
[64] published data for mixtures of CO 2 and nine different oils ranging from 11.9 to
33.3 API. Experimental conditions covered a range of temperatures from 38 to

121 C and pressures up to 15.9 MPa. In addition, they presented correlations for pre-

dicting solubility, swelling, and viscosity behavior of CO 2 /crude oil systems, which
are the principal correlations currently used in reservoir engineering. Miller and Jones
[65] investigated the properties of four dead heavy oils obtained from Cat Canyon
(10 API) and Wilmington (15 17 API) oil fields in California, and Densmore

(19.8 API) oil field in Kansas. Data were presented on solubility of CO 2 in the pre-
mentioned heavy oils, oil swelling by CO 2 , and effect of CO 2 on heavy oil viscosity.
Sankur et al. [66] also presented some data on properties of CO 2 /reservoir oil
mixture.
The abovementioned methods used to characterize CO 2 /reservoir oil mixture
apply mole fraction instead of volume fraction for indicating composition. Therefore,
determination of the MW of heavy crude is essential. For convenient application, cor-
relations developed in the work of Chung et al. [67] used CO 2 volume fractions in
the crude oil leading to minimum experimental work.

4.5.1 Vapor ExtractionsHeavy Oil

The recovery of heavy oil using thermal methods from reservoirs with low porosity,
low thermal conductivity, and high fractures and/or fissures can be problematic.
Moreover, the production from these reservoirs is not satisfactory from economic
aspects. Hence, the vapor extractions (VAPEX) process can be proposed as an alterna-
tive approach [68].