Page 100 - Fundamentals of Enhanced Oil and Gas Recovery

88 Ramin Moghadasi et al.

oil rate increased to 200 BPD. Overall, CO 2 injection resulted in 2.2% of OOIP
recovery.

3.3 CO 2 INJECTION METHODS
3.3.1 Injection Location

Depending on the location of gas injection wells, there are two types of injection
techniques, including crestal and pattern injections [57].

3.3.1.1 Crestal Injection
This type of injection is sometimes called external or gas cap injection. In this type,
gas is usually injected into higher structural positions in primary or secondary gas
caps. This method of injection is normally applied to reservoirs with thick oil columns
and good vertical permeability. Indeed, recovery is assisted by the gravity drainage
process. Crestal injection, if applicable, is superior to pattern injection due to its high-
er volumetric sweep efficiency [58].

3.3.1.2 Pattern Injection
Pattern injection is usually called the internal or dispersed injection method. Through
this type of injection, a geometric arrangement of injection wells is designed for the
purpose of uniform distribution of the injected gas throughout the oil-productive
portions of the reservoir. This type of injection is normally used for reservoirs with a
low vertical permeability and relatively homogeneous reservoirs with low permeabil-
ities. Well spacing could be regular (e.g., five spot) or irregular. Injection and produc-
ing wells are located in an area, thus there is no great benefit of gravity drainage
drive. Sweep efficiency decreases due to high potential of gas override and fingering.
Few pattern gas injection projects have been implemented in recent years because this
method is not very attractive economically [58].

3.3.2 Injection Mode
CO 2 injection could be implemented in different ways. Generally, CO 2 is injected
either as continuous or injected alternatively with water. When CO 2 is injected con-
tinuously into the reservoir, it contacts with crude oil to recover the oil through dif-
ferent mechanisms, depending on the degree of miscibility. Although mixing CO 2
with reservoir oil can reduce oil viscosity, mobility ratio in a continuous CO 2 injec-
tion is unfavorable due to very low viscosity of CO 2 . Such an unfavorable mobility
ratio will promote fingering, and as a result, CO 2 will break through soon, leaving a