Page 127 - Fundamentals of Enhanced Oil and Gas Recovery

Miscible Gas Injection Processes 115

over waterflooding are the potential for poor sweep due to its adverse mobility ratio
and gravity override due to higher contrast between oil and gas gravities [46].

4.3.4.3 Jay Little Escambia Creek Nitrogen Flood
The Jay field near the Alabama Florida border is one of the few nitrogen floods ever
conducted. The reservoir is in the Smackover carbonate at a depth of 15,000 ft.
Nitrogen is a good miscible gas in this reservoir because of its very light sour crude
(50 API) and high reservoir pressure around 7850 psia. The average formation perme-

ability is about 20 mD. A significant advantage of nitrogen is that it is readily available
via separation from the air, is relatively cheap, and does not cause corrosion unlike
CO 2 . Nitrogen was injected using a WAG ratio near 4.0, which is greater than typi-
cal. The overall recovery at Jay is expected to be near 60% OOIP. Incremental recov-
ery beyond waterflood recovery from miscible nitrogen injection is forecast to be
around 10% OOIP. The high primary and secondary recovery of around 50% OOIP
is likely the result of low vertical permeability coupled with good horizontal perme-
ability in the dolomite.
Miscible nitrogen injection is expected to give an incremental recovery of 10%
OOIP over waterflooding alone [47]. Low vertical permeability caused by shale
lenses or in this case, cemented zones associated with thin stylolites is an ideal can-
didate for both water and gas flooding as fluids are less able to segregate vertically
so that gravity override is reduced. This is especially true in this flood since nitro-
gen has very low density compared to the resident fluids and would likely have
gone to the top of the reservoir otherwise.

4.3.4.4 Overview of Field Experience
Gas flooding technology is well developed and has demonstrated good recoveries in
the field [43]. Recoveries from both immiscible and miscible gas flooding vary from
around 5% to 20% OOIP, with an average of around 10% incremental OOIP for mis-
cible gas floods [44]. Tertiary immiscible gas flooding recoveries are less on average,
around 6% OOIP. Although recovery by gas flooding is very economic at these levels,
55% OOIP still remains on average post miscible gas flooding assuming 65% OOIP
prior to gas flooding. The significant amount of oil that remains is largely the result of
gas channeling through the formation owing to large gas mobility, reservoir heteroge-
neity, dispersion (mixing), and gravity effects. Channeling also results in early break-
through of the gas (gas), typically around the same time that oil is produced. This is in
contrast to surfactant/polymer flooding, which almost always exhibits an oil bank
prior to surfactant breakthrough. Poor volumetric sweep is not as much of a problem
for surfactant polymer floods or waterfloods, which have more favorable mobility
ratios. Nevertheless, miscible flooding is generally very economic and less complex
than chemical flooding, especially for deeper reservoirs that are more technically chal-
lenging for surfactant/polymer floods.