Page 133 - Fundamentals of Enhanced Oil and Gas Recovery

Miscible Gas Injection Processes 121

4.6 HYDROCARBON: LPG, ENRICHED GAS, AND LEAN GAS

This process involves the continuous injection of high-pressure methane, ethane,
nitrogen, or flue gas into the reservoir. The lean gas process, similar to enriched gas,
involves multiple contacts between reservoir oil and lean gas before forming a miscible
bank. But, there is a difference in the enriched gas process where light components
condense out of the injected gas and into the oil, then intermediate hydrocarbon frac-
tions (C 2 C 6 ) are stripped from the oil into the lean gas phase.
In a reservoir with initial reservoir pressure of 6425 psi, the swelling test was simu-
lated by various proportions of injection gas mixed with original reservoir oil. The
bubble point is calculated as 2302 psi. A comparison of the effects of CO 2 and lean
gas injection on the saturation pressure change is reported in the relevant literature
[79]. It is observed that the saturation pressure can be reduced by increasing the per-
centage of injected CO 2 , whereas the saturation pressure increases as the percentage
of injected lean gas increases. Swelling factor shows the same pattern in injection of
CO 2 or lean gas.
The effect of injected gas mole fraction on the relative volume is reported in the
relevant literature [79]. The relative volume goes up by increase in gas mole fraction.
The reason lies in the fact that more gas can evolve from the oil when pressure goes
below the bubble point pressure [80].

4.7 RESERVOIR SCREENING

It should be noted that CO 2 EOR and storage do not lead to efficient results in
all oil reservoirs due to different technical and economic reasons. Hence, selecting a
screening procedure seems to be vital. Shaw and Bachu [81] established a number of
suggestions for doing basic evaluations on some selected oil reservoirs for conducting
simultaneous CO 2 EOR and CO 2 storage before considering other economic criteria.
These criteria are as follows [81]:
• Screening for EOR and storage suitability,
• Technical ranking of suitable reservoir,
• Improved oil recovery (IOR) and CO 2 storage capacity predictions.
A series of criteria were recommended by various authors for the technical screen-
ing of CO 2 EOR by miscible flood [8,83 85]. These criteria are based on the opti-
mization of reservoir performance to have better IOR efficiency. These criteria allow
a rapid screening and evaluation of oil reservoirs which are suitable for CO 2 EOR based
on general reservoir characteristics and oil properties. Rivas et al. [82] studied the impact